111 MORGAN ST. CHICAGO, IL

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1 111 MORGAN ST. CHICAGO, IL Ryan W. Friis Structural Option Spring 2003 Senior Thesis Penn State University Department of Architectural Engineering

2 111 MORGAN ST. CHICAGO, IL Owner: GC: Architect: Structural: Mechanical: Civil: 111 South Morgan LLC Walsh Construction Fitzgerald Associates Architects Somartano & Co. Ketchmark & Associates, Inc. McClier Engineering General New condominium complex located at the corner of Adams & Morgan St. 166 Units on 7 Floors 274,500 Sq. Ft. Total Cost: $29,125,000 Architectural 9 Story Residential Building 3 Floors of indoor parking (1 below grade) Brick & glass façade Fireplace & outdoor patio included in every unit Structural 2 Way concrete structural slab system 56-0 Deep caissons supporting the superstructure Concrete core acts as lateral support system Typical rectangular column grid Penn State University Architectural Engineering MEP Individual furnace and condenser in each unit Condo units & garage fully sprinklered 480V three phase 4000A electrical service Ryan W. Friis Structural Option

3 Executive Summary 111 Morgan St. is a 9 story residential condominium building located in Chicago, Illinois. It has 166 condominium units located on 7 floors and provides 3 levels of parking. The building s structural system consists of an 8 ½ two-way concrete slab with a typical bay size of 25-8 x The building is being marketed as a luxury condominium complex, however it s exposed concrete frame, exposed mechanical ductwork and exposed sprinkler systems all detract from the overall appearance of the condominiums. Therefore, the building s structural system was redesigned using a composite steel framing system for the residential units while keeping the parking garage in concrete. Also, two floors of residential units were added as well as one level of parking in order to fulfill the initial design intentions for the building. The main benefits of the redesigned system are shown below Improved appearance on the building s interior o Finished ceiling provided at a height of 9-6 o Concealed mechanical and sprinkler systems o Replaced large exposed concrete columns o Increased available floor space by using smaller columns Added 32 additional condos on 2 new floors Added 50 parking spaces on 1 new floor Did not affect current architectural layout of any of the condominium units The new design for 111 Morgan St. was accomplished while maintaining a per floor cost comparable to the existing system and without having to add any new lateral force resisting elements. The new system also maintains the STC and IIC ratings between floors to ensure that none of the luxury aspects from the existing system are lost. A preliminary look at construction shows that all of the above can be accomplished with only a small delay in the completion schedule for the building. 1

4 Project Team Owner : Architects: Structural Engineer: Mechanical Engineer: Civil Engineer: General Contractor: 111 S. Morgan, LLC Payne St., Evanston, IL Fitzgerald Associates Architects 912 W., Lake St., Samartano & Co. 221 N. Lasalle St., Ketchmark & Associates, Inc. 100 Tower Dr., Burr Ridge, IL McClier Engineers 401 East Illinois, Walsh Construction Company of Illinois 929 W. Adams St.,

5 Introduction The following report is a study for an alternative structural design for the 111 Morgan St. building located in Chicago, Illinois. This design study was performed assuming the building was still in its design-development stage and maintains the original design parameters of the building. The process for developing an alternative structural system involved three stages. The first stage investigated the existing building in order to establish the original design parameters. The second stage established design criteria by which a new structural system was designed. The final stage looked at the impact of changing the structural system on several of the building s non-structural characteristics. 3

6 Existing Conditions In early 2000 Bob Horner and Ibrahim Shihadeh both of Winthrop properties began to show interest in creating new luxury condominiums on the west side of Chicago. Soon afterwards Fitzgerald and Associates Architects were hired to prepare a preliminary set of scope drawings for the proposed building which was to be built at the intersection of Morgan St. and Adams St. on Chicago s west side. The scope set of drawings was presented to the Walsh Construction Company of Illinois, and a price of $29,125,000 was agreed upon. With building contract in hand, Fitzgerald Associates turned to other design firms in the city for assistance in preparing a complete set of construction drawings for the building. Somartano & Company was hired to perform the structural design of the building, Ketchmark & Associates was responsible for designing the mechanical and electrical systems, and McClier Engineering provided the necessary civil work. The result of their work was a 9 story cast- inplace concrete structure which housed 166 condominium units on 7 floors and provided 3 levels of parking. Late in August of 2000, drawings were issued for building permits and ground breaking took place in November of 2001 with substantial completion expected in February of Architectural 111 Morgan contains 166 luxury condominium units on 7 levels totaling 180,000 sq. ft. On each of the first 6 levels there are 25 condo units per floor each with 1 or 2 bedrooms. The penthouse floor contains 16 units all of which are 2 and 3 bedroom condominiums. All units contain numerous luxury amenities such as 10 ceilings, hardwood floors, granite counter tops, and walk out balconies. In addition, each unit has a gas fire place, stainless steel appliances, and an available parking space in the heated garage below. A typical residential floor plan is shown in Figure 1. (Note: Larger versions of every figure in this report included in Appendix A) The parking garage is located below the condominium units and is comprised of one subterranean level and two levels above grade. In total, 184 parking spaces are provided in the three levels along with 2 storage rooms for personal belongings and 2 rooms for bicycles. Also contained in the parking garage are the building s mechanical and plumbing rooms, the 4

7 electrical distribution equipment, and a small commercial area located on the first floor in the southwest corner of the building. Fig. 1- Typical residential floor plan The building s façade is comprised of 2 colors of brick combined with silver metal panels. The overall look of the building is very conservative and is well integrated with the architecture of the surrounding buildings. Entrance to the condominium building is located on the west side of the building and can be seen in Figure 2 (under the canopy halfway back on the building). The commercial space and entrance can also be seen in the photo, located in the lower right hand corner. Fig. 2 - West side of 111 Morgan looking north 5

8 Architectural Interiors As stated earlier, the condominium units in 111 Morgan St. feature many luxury amenities, however they also have a few unattractive features. The primary eyesores are the building s exposed concrete structure, the cast iron sprinkler piping and the exposed mechanical ductwork. As can be seen in the photo to the left both the ceiling and the columns remain unpainted and give the impression that the condominium unit is unfinished. Fig. 3 Typical condominium interior 6 Architectural - Building Codes The building code used in the design of 111 Morgan Street was the 1999 edition of the Chicago Building Code. As implied by the name, the code is unique to the city of Chicago and is the required building code for all buildings within the city s limits. The Chicago Building Code has provisions for the design of all a building s major systems the same as any of the more common codes such as IBC 2000 or BOCA does. However, the Chicago Code does have a few significant differences. One difference which is of particular importance for this report is the unique requirements the Chicago Building Code has for the structural loading of buildings. The Chicago building code dictates that only lateral loads due to wind need to be considered when designing a building s lateral system. Unfortunately the Chicago Building Code was not available for use in this report, and IBC 2000 was used to determine the required live loads on each part of the building as well as the wind loading for use in the analysis and design of the lateral system. Building Chicago Building Code 1999 Structural Chicago Building Code 1999 ACI Mechanical Chicago Code Electrical Chicago Electrical Code Fire NFPA 13/ Chicago Code Table 1 Building and design codes used

9 Structural Loads The existing structural system was designed using the Chicago Building Code of Unfortunately, a copy of this code was not available for use in this report, and therefore, IBC 2000 was used as a substitute. IBC 2000 was used in lieu of the Chicago Building Code for determining the design live loads as well as the building s lateral loading. An important difference between the two codes is that the Chicago Building Code does not require seismic design for any buildings, instead it dictates that wind loading shall control for all cases. Since this is such a notable difference between the two codes, it was decided that wind loading from IBC 2000 would also control and earthquake would not be considered in order to give a fair comparison between the existing system and the work completed for this report. As stated earlier the gravity loads were taken from IBC 2000 and can be seen in Table 3. Live Load: General 40 psf Corridor 40 psf Parking Garage 50 psf Roof 20 psf Snow 25 psf + drifting Balconies 100 psf Table 3 Live loadings per IBC 2000 Lateral loads were also calculated using IBC 2000 and the results can be seen in Figure 8. (note: all values shown include windward and leeward pressures). Fig. 8 Wind loading per IBC

10 Structural - Slabs The building s floor system consists of an 8 ½ 2-way flat slab of 5,000 psi concrete in the residential floors. The typical slab s column strip uses a bottom layer of #4@12 in the E-W direction and the N-S direction. The typical top layer in the column strip uses 16-#6 bars in both directions. The middle strips in the slab use 12-#4 bars in the bottom layer running both directions. The floor system in the parking garage is very similar except that the slabs are only 8 thick, and the parking ramps which connect the different levels are made of a 7 thick one way slab. At these locations, the beams span perpendicular to the direction of the ramps length. In both the residential floors and the parking garage, drop panels are used to resist the punching shear forces which occur at the columns. The two most common drop panel sizes used were a 4-0 x 4-0 x 4 panel in the residential floors and an 8-8 x 7-4 x 4 drop in the parking garage.. Floor plans for both the parking garage and residential floors can be seen in Figures 4 & 5. Figures 6 & 7 show the building longitudinal and transverse sections Fig. 4 Parking garage floor plan 8

11 CENTER OF RIGIDITY BUILDING CENTER Fig. 5 Typical residential floor plan Fig. 6 Longitudinal building section Fig. 7 Transverse building section 9

12 Structural - Columns The interior columns in the building are 20 in diameter and are spaced at 25-8 in the N-S direction and in the E-W direction the spacing varies from 22-0 to The concrete used for the columns varies in strength from 4,000 psi to 8,000 psi depending upon the location as can be seen in Table 2. The exterior columns are rectangular in shape and are primarily used on the residential levels where a normal column layout can not be maintained due to the windows and doors in the building s façade. Floor Concrete Strength (psi) Reinforcemnt B # #8 7-R #7 Table 2 Column concrete strengths by floor Structural Lateral Force Resisting System The lateral force resisting system of the building is composed of shear walls in both the N-S and the E-W direction. All of the shear walls in the building are 8 thick and are composed of the same strength of concrete as used for the columns on that level (see Table 2). The shear walls on the interior of the building serve as shaft walls for the stair towers and elevator shafts, and extend to the full building height of There are also two shear walls located on the transverse face of the building s exterior (the shear walls are shown in green in Figures 4 & 5). The two shear walls located on the transverse face of the building are the two stiffest shear walls in the building. However, the southern wall has numerous windows in it, which significantly reduce the wall s stiffness when compared to the northern wall. As a result, the building has a large eccentricity about its center of mass. This causes problems when the building is subjected to lateral loads in the east-west direction because very large torsional forces are generated. Since this building has a glass and brick façade it is very important to limit the amount of drift in order to keep the mortar and bricks from cracking. A typical drift limit for this type of building is H/600, which would allow for 2 of drift at the roof level. 111 Morgan St. is very long and narrow and therefore has no problem 10

13 resisting forces in the N-S direction (parallel to its length) where total drift is less than ¼ ; however, in the E-W direction the building does not perform as well. In the E-W direction the building has more shear walls to resist force, but most of them are small compared to the walls in the N-S direction. This problem is made worse by the fact that the building s surface area is almost four times greater in the E-W direction, creating much larger loads. The result of this is a building drift of 1.5. Although this still meets drift criteria it is obvious that this direction controlled the design of the lateral system. Structural - Foundation 111 Morgan St., along with most buildings in Chicago, is built upon very sand and silty soil which has low bearing capacity. Therefore, the building rests on caissons which range in depth from 63 to 67 and have a diameter of 30. These caissons do not extend down to bedrock, but do reach a soil stratum of suitable bearing capacity. A caisson is provided for each column and grade beams were poured below the building s shear walls to transfer the loads into the caissons. 11

14 Mechanical Since 111 Morgan St. is a residential building there is no single mechanical system which services the entire building. Instead, each unit has its own mechanical system which is controlled by the tenant. The setup is very similar to what you would find in a typical single family home. Each condominium unit has its own furnace located inside the condo, and a condensing unit is located on the roof to provide cool air in the summer. It is also important to note that since the mechanical ductwork is exposed, care was taken in order to minimize its visual impact. This was done by using only one duct and running it along the entire perimeter of the apartment. While this does help to minimize the ductwork s impact it also creates a few awkward looking situations where holes have been cut into the drywall in order to allow the duct work to pass through. The attempt to minimize the visual impact of the mechanical ductwork could have been more successful if the ducts had been painted, however they still would have stuck out against the bare concrete ceiling. Also, since this system is tenant controlled, there is no requirement for the amount of fresh air which is to be delivered to the units at all times. Therefore, a ventilation requirement exists which dictates the amount of surface area that is required for natural light and natural ventilation to the outdoors based upon the square footage of each condominium unit. This requirement is met through windows and patio doors on the exterior face of each condominium unit. 12

15 Proposal for New Design Prior to the redesign of the structural system several criteria were established in order to maintain the building s original features. The following criteria were applied to all of the possible redesigns and had the most influence in choosing the best design solution for the problem. Add 2 Additional Floors of Residential Units During the initial design development phase, 111 Morgan St. was intended to be a 198 unit condominium complex; however, the final product house only 166 condominium units. The 32 units removed were located on two floors which were both identical to the 9 th floor of the existing building. Therefore, any new structural system considered would have to accommodate the addition of two floors of residential units and another level of parking to accommodate the new units (approximately 50 spaces). The overall goal of this project is to add the two floors of residential units while adhering to the other criteria set forth below. Maintain 9-6 Ceiling Height While Providing a Finished Ceiling The first criterion established required that the ceiling height be at least 9-6. In the original building 10 ceiling heights were achieved, however these ceiling had exposed mechanical systems which hung from the ceiling and subtracted from the true ceiling height. Since this building is being marketed as luxury condominium units, high ceiling heights were determined to be crucial in maintaining the luxury image and in creating a space that looks larger than it actually is. Another goal of the redesign was to provide a finished ceiling in the condominium units. In the existing system there is no ceiling and this greatly detracts from the otherwise upscale appearance of the condos. Therefore, every system evaluated must accommodate the installation of a ceiling at 9-6 and provide enough space for the mechanical ductwork and the sprinkler system in the plenum. Keep Building Costs Down Although cost is not always the most important factor in selecting a structural system, it usually has a significant influence. As a result, it will be desirable to select an alternative system which has a cost comparable to the existing system. It is important to note that the 13

16 cost of the structural system itself is not the only item which needs to be considered. Changing the structural system will have an effect on many of the building s other systems. For example, the requirement to provide a finished ceiling will increase the floor to floor height and require taller walls and increase the area of building façade, both of which will add to the overall cost of the system. Minimize infringement on floor plan As previously stated, the existing structural system uses 20 diameter round columns throughout the building s interior. In redesigning the structural system an attempt will be made to reduce the size of the columns for two reasons. The first reason is that most of the columns are exposed to view rather than hidden within walls or located in corners where they would not be noticed. This combined with their large size and unpainted finish makes them an eyesore in almost every room. The second reason for reducing the column size is to free up more floor space. The size of a condominium unit is its most important feature. It is usually a key element in comparing different condos and the more space the better. Therefore, by reducing the column size you can not only improve the aesthetics of each unit, but also increase the amount of sellable space. Keep Current Architecture Intact As originally designed 111 Morgan St. is, in general, a visually pleasing building both inside and out. For that reason, care must be taken in the redesign in order to ensure that the overall appearance of the building is not dramatically changed. This is especially important in regard to the building s exterior façade. Any changes to the building s exterior could violate the natural lighting and ventilation requirements set forth by the Chicago Building Code. Therefore, every attempt will be made to guarantee that any changes to the building, inside or out, will only have a positive impact on the original design. 14

17 Structural Redesign Brainstorming The structural redesign process began in the same way any design process begins. Several ideas were considered for a new floor system and then each one was looked at to determine if it should be discarded or studied further. Some of the systems which were looked at and discarded include a one-way concrete slab, open web steel joists, and hollow core pre-cast concrete planks. In the end, a standard steel system using wide flange shapes was selected to be investigated for this report. In the new system the residential floors would be constructed of steel and the parking garage would remain in concrete as originally designed. The parking garage was left in concrete because the original design was a very efficient way of meeting the requirements of the space. Most of the reasons for changing the residential units to steel do not apply to the parking garage and therefore there is no incentive to change the parking garage. The reasons steel was chosen for the residential units are numerous. First, steel is a comparably light building material. This is important because three floors (2 residential and 1 parking) are being added to this building and a lighter building material would minimize the impact on the building s foundation. Second, in a steel system the beams can be spaced at greater distances than in open web joists or one-way concrete. The only real limitation on how far beams can be spaced comes from the ability of the decking to span the distance. Larger beam spacing is beneficial because it will minimize the intrusion of the structural system in the plenum space and makes placing the ductwork and sprinkler system easier. Another benefit of the steel system is it can achieve the 2 hour fire rating necessary between floors without having to use a fire rated ceiling. This fact had a large influence on the decision to use steel because once the building is turned over to its owner there is no way of knowing what the tenants might do to the ceiling. If they decide to change a light fixture or add a ceiling fan they could be destroying the ceiling s fire rating without even knowing it. 15

18 Structural Steel Redesign Pre-Design Once structural steel was chosen for the floor system a few decisions had to be made before design began. The first of these decisions was how big to make the bays. Larger bays would make a more efficient use of the steel; however, they would also require deeper members which would intrude into the plenum space. Also, any change in bay size would affect the architectural layout of the floor plan, and most likely intrude even more than the existing system. Therefore, a decision was made to use the column grid of the existing system. Using the existing grid provided a typical bay of 25-8 x 22-0 and did not change the architectural layout of the existing floor plan. Although this bay size is slightly on the small size for a steel system, it did allow the steel beams to be relatively shallow and spaced farther apart. The next decision to be made was which way to orient the beams. Running the beams in the N-S direction would make the most efficient use of the steel since the girders would be running in the short direction. Unfortunately, running the beams in this direction creates significant problems. First, framing the balconies would be extremely difficult since the girders would have to cantilever off of the columns in order to support the balcony slab. Given the high loading of the balconies, a very large moment connection would have to be used which would be both costly and difficult to erect. Another problem in running the beams in the N-S direction is they could not be uniformly spaced because the length of the bay in the E-W direction varies. Running the beams in the E-W direction allowed the beams to be cantilevered over the girder in order to frame the balconies and they could also be evenly spaced at o. c. to provide a uniform framing grid. Once the determination of the framing plan had been completed there remained one major design decision. The question of whether or not to use composite design for the steel was a complicated matter. Using composite design would yield smaller members, but any cost savings which might be seen could quickly be removed by the cost of installing the shear studs. Another issue with the composite design was floor vibration problems. It is not uncommon for the occupants in lightly framed steel buildings to experience discomfort due to the vibration of the floor. Although it was originally believed that a composite design would be the best system there was no way of knowing whether composite design would save 16

19 money or if it would have vibration problem without first designing the structural system. Therefore, a composite system was designed initially and compared to a non-composite design later on when more information was available. Before design could begin the type of concrete to be used in the slabs had to be chosen as well as the type of decking, and the overall thickness of the slab. The type of concrete to use was an easy choice since lightweight concrete will reduce the dead load imposed on the beams and has better fire resistance than normal weight concrete. Although lightweight concrete is not as strong as normal weight, this was not an issue since the purpose of the concrete in this case was to provide a finished floor. The type of decking to use was also an easy decision since the only criteria involved was the ability to span the beams. From the United Steel Deck manual, 3 Lok-Floor decking with lightweight concrete would be required to make the necessary span. Even though the slab thickness could be arbitrarily chosen, a little research proved to be very valuable. In order to determine the slab thickness, the U.L. manual was consulted. It was discovered that 3 Lok-Floor with 3 ¼ of lightweight concrete cover could achieve a 2 hour fire rating with out any fireproofing on the underside of the decking. With this knowledge in hand, a 6 ¼ lightweight concrete deck was chosen to be poured over 18 gage 3 Lok-Floor. 17

20 Structural Steel Redesign Gravity Design Setup With a general framing plan in mind the computer software RAM Steel was used to perform the detailed design of each floor member. In RAM Steel each floor of the building was modeled using the general framing plan described above. The typical floor framing model used can be seen in Figure 9. Fig. 9 Typical floor framing model used in RAM Steel In the new framing model, it was possible to eliminate several columns along the east and west side of the building. This was possible because the steel beams are able to span further than the two-way concrete system. Another change in the steel system from the concrete was in how the balconies were formed. In the concrete system the balconies were one-way slabs cantilevered off of the main building slab. However, in the steel system the slabs were not as thick and it was no longer possible to form the balconies in the same way. Instead, the balconies were formed by cantilevering the beams over the girders. In order to do this the girders would have to be lowered into the plenum space and possibly into the wall if necessary. The downside of lowering the girders in this manner is they lose the lateral bracing provided by the attachment of the slab, and can no longer be designed as composite members. RAM Steel is a very useful computer program; however the results obtained by using it are only as good as the information that is put into it. Once a preliminary framing model had been established loads were put into RAM Steel so that an initial design could be 18

21 obtained. As previously stated, the live loads used for the redesign of 111 Morgan St. were obtained using IBC 2000, and can be seen in Table 4. Live Load: General 40 psf Corridor 40 psf Roof 20 psf* Snow 25 psf + drifting* Balconies 100 psf* * - Denotes an unreducible live load Table 4 Live loadings per IBC 2000 The dead loads used in the design were either calculated based upon a known quantity or a conservative estimate was made in order to arrive at a final design. For example, the 3 Lok-Floor decking had a weight of 3 psf, and the concrete in the slabs weighed 45.5 psf. Other loads which had to be estimated include the partition load and the ceiling load. It is extremely important to note that RAM Steel does not include member self weights in its design. Instead, the user is required to provide an estimate of the self weight and through an iterative process the value can be more closely approximated. The final dead loads used in the design can be seen in Table 5. Dead Load: Concrete Slab 46 psf Decking 3 psf Partitions 15 psf Ceiling 5 psf Superimposed 5 psf Building Façade 300 plf Steel Self Weight 4 psf* * - Denotes a load determined through an iterative process Table 5 Dead loads used for design 19

22 When using a computer program such as RAM it is important to know how the program is arriving at its designs. Fortunately RAM allows the user to specify many of criteria which will determine how the computer chooses the beam sections. First, RAM was setup to perform all designs using LRFD based upon the Second Edition of the AISC Manual of Steel Construction, Load and Resistance Factor Design. Second, all wide flange shapes were to be of A992 steel (50 ksi) since there is no premium for specifying high strength steel in Chicago. Next, the composite design was configured so that unshored construction could be used while the slabs were being poured. This is significant because the steel members need to be able to support the load of the wet concrete without the aid of the composite action since it will not be developed until after the concrete has cured. RAM Steel was also configured to allow camber in the beams and girders, as well as limit member deflections. The beams and girders were all cambered to eliminate the deflection caused by the wet concrete on the beams. This was done so that when the concrete cures the floor is relatively flat before any additional load is applied. If the beams had not been cambered the floor would have high and low points due to the initial deflection. It is beneficial to avoid this situation since there is no easy way to correct this problem. Care must also be taken not to over camber the beams since it is possible that the beam will not see the entire load originally intended; this would result in the floor bowing upward even after the concrete has cured. Member deflections were also limited in RAM to a live load deflection of L/360. A deflection of L/360 was chosen in order to ensure that the floor does not deflect excessively under full live loading and to avoid any problems in the non structural building elements. For example, an excessive deflection in the floor could cause the drywall ceiling on the level below to crack or tear. 20

23 Structural Steel Redesign Gravity Design Results After all of the proper information had been input into RAM Steel a design for the gravity system was finally obtained. The typical beam size was a W12x19 with ¾ camber and 7 shear studs per beam. These beams frame into W16x31 girders with ¾ camber and 12 shear studs along their length. A detail of a typical bay can be seen in Figure 10 (Figure 10 in the appendix shows a complete floor map). Fig. 10 Typical Bay Other members worth noting are the cantilevers which form the balconies on the building s east and west faces. The cantilevers on the building s east side use W14x30 beams with 11 shear studs and no camber. The cantilevers on the west side use W10x12 with 5 shear studs and ½ of camber. It is important to remember that these beams cantilever over the girder which was lowered in order to accommodate the beams above them. As it turns out the girders, which vary in size, will fit within the ceiling plenum even with the beams above them. Although this is not a crucial detail, it does save the trouble of having to frame the girder inside the wall. Overall, the members vary in depth from W8x10 to W16x57. The total steel tonnage for all 9 floors plus the roof is 421 tons and uses shear studs. The non-composite design uses 113 more tons of steel and has members which are up to 24 in depth. Not only would the non-composite system require very tall floor to floor 21

24 heights, it would also be more expensive. A conservative estimate of 1 stud having a cost equivalent to 10 lbs. of steel yields a 60 ton savings with the composite system. The only remaining benefit of the non-composite system would be its greater resistance to vibration problems. However, a vibration analysis on a typical floor framing member in the composite design showed that for normal human walking activity the peak acceleration was 0.43% of g which is less than 0.5% and would be judged as satisfactory by most people. Therefore, as originally expected the composite design steel system is a better choice. Given the maximum depth of the floor system at approximately 16 it was decided that the floor to floor height should be With a 9-6 ceiling height and a 6 ¼ deep slab there would still be 12 of plenum space to install mechanical ductwork, sprinkler piping, and a finished ceiling even under the deepest girders. This design yielded columns which range in size from W8x24 up to W10x77. In total there are 69 columns per floor and each column is 3 floors tall. A 3 floor column height was chosen in order to minimize the number of column splices and limit the amount of time spent plumbing the columns. One of the original goals of the redesign was to minimize the impact of the structural system on the floor plan. This goal was achieved since the columns required for the redesign are significantly smaller in size than the original 20 diameter columns. The largest columns required in the steel system are W10x77 which measure 10.5 x Even after these columns are fireproofed and framed in they will still take up 35% less floor space compared to the round concrete columns. This is a significant savings in floor space especially when you consider that the largest column is 35% smaller than the original. Most of the other columns in the building are significantly smaller. Another benefit of this redesigned steel system is the entire building is lighter than it was before. Even with the addition of the two floors of residential units and the extra level of parking the building is still 35% lighter than the concrete building it replaces. This weight savings allows for the use of 6,000 psi concrete in the parking garage instead of 8,000. It also means that the existing caisson system will be adequate to support the new structure without having to be redesigned. 22

25 Structural Steel Redesign Lateral Loads Once the gravity system had been designed the information necessary for determining the building s lateral loads could be obtained. The information necessary was the overall building height and the floor to floor height so that the loads could be distributed on a per floor basis. Once again wind loads were calculated using IBC The criteria used to determine the wind loading and the results can be seen in Table 6. It should be noted that since the building height increased to over 100 gust factor effects had to be considered in determining the wind loading. Exposure Category B, Case 2 Wind Design Speed 90 mph Use Group II Building Frequency 1.21 Hz (rigid in both directions) Windward Pressure Leeward Pressure E-W N-S 12.0 * K z psf 8.2 psf 5.0 psf Table 6 Determination of Wind Pressures per IBC 2000 It is important to remember again that since the original building was not designed for earthquake loadings, the redesigned building will also assume that the wind loading is the controlling case for lateral loads. Using the data above the following diagram was constructed to show the wind loading on the on the western face of the building. Fig. 11 Wind Pressures in E-W direction per IBC 2000 (windward + leeward pressures shown) 23

26 Using the pressures shown above the lateral load on the building was determined per floor based upon tributary area. The resulting forces are shown in Table 7. Floor Slab E-W Direction N-S Direction Roof Kips 9.6 Kips Table 7 Wind forces per floor 24

27 Structural Steel Redesign Alternative Lateral Force Resisting Systems The original lateral system in 111 Morgan St. utilized the building s cores by making them into shear walls. Together these cores were able to resist the lateral forces imposed upon them, and keep the total building drift within acceptable limits. However, in the E-W direction the drift did approach its design limit, which signifies that the existing lateral system will no longer be adequate to satisfy drift criteria if the loads were increased or if the building were to become taller. In the redesigned structural system both of these situations occurred. The addition of the 2 residential floors and the extra floor of parking increased the building height by almost 30. Also, in the redesign the floor to floor height was increased from 10-9 to 12-6, the accumulation of this increase is 17. Overall, the building height increased by 45 which is a 45% increase in building height over the old design. By inspection, the existing lateral system will no longer be adequate to resist the forces on the redesigned building. Three alternative lateral systems were considered for the redesigned building, however only two of the choices were deemed feasible for application in 111 Morgan St. The system eliminated first was a braced frame system. Due to the open layout of the columns in 111 Morgan St. there are very few places to install the braces. Most of the columns in the building are located in rooms, and not concealed in the buildings walls. This means the only locations where braces could be installed would be the north and south faces of the building. From experience it was determined that two braced frames would be inadequate to resist the loads and therefore this system was eliminated as an option. The next system considered was a moment frame system. This system seemed viable since the inner two bays of the building are symmetric in both directions, and a moment frame system would make use of the beams and columns which were already there. The frames shown in Figure 12 do not represent the amount of frames necessary, but rather they show the locations of where the frames could be placed. Beams which could be part of the moment frames are shown in green and columns are shown in magenta. 25

28 Fig. 12 Potential moment frame layout The final system considered involved modifying the existing shear walls. Ideally, more shear walls would have been placed in the building, however this was not possible for the same reasons braced frames could not be used. The only possible way to use the existing shear wall system would be to increase the thickness of the walls. By increasing the thickness of the shear walls they could be stiffened enough to bring the building drift under control. Figures 4 & 5 show the locations of the shear walls proposed for this system. After some preliminary calculations it was decided that the shear wall system would be a much better choice. This conclusion was reached based upon the consideration of three different factors. First, since moment frames are not as stiff as shear walls a large number of frames would have to be used and the size of both the columns and the beams would be significantly increased. The increase in beam size would further cut into the plenum space and leave virtually no room to run the mechanical ducts, which would mean the floor to floor height would have to be increased again to accommodate the deeper members. This was undesirable because increasing the floor to floor height increases the cost of numerous other systems in the building. The second reason for choosing the shear walls is they are a simple and relatively inexpensive lateral system compared to moment frames. Moment frames are one of the more expensive options for resisting lateral forces and typically require additional welding and detailing which can slow down the steel erection process. The final reason for choosing the shear wall system is the fire rating they provide. Many of the shear walls in the existing building also serve as fire rated walls for the stair towers. If the walls were not 26

29 constructed of concrete they would have to be made of CMU blocks or shaftwall. The basic reasoning was if they are they you might as well take advantage of them. Structural Steel Redesign Lateral Force Resisting System Design The design of the shear walls was based upon the assumption that each wall would receive a fraction of the lateral load proportional to its stiffness. Under this assumption a proportional amount of the total lateral load was distributed to each element based upon its stiffness compared to the total stiffness in that direction. The stiffness of each member was determined by the amount of deflection under a unit load. The deflection of each member was determined using the assumption that each wall acted as a cantilever with a point load applied at its end. The justification for modeling the shear wall as a cantilever beam is it will be acting like a deep beam in flexure since its depth (length of wall ) is large when compared to its length (height of wall). Once the deflections were obtained, the stiffness could be found by taking the inverse of the deflection. It is important to note that although the shear walls do not carry a great deal of the gravity load, it was assumed that the walls do carry enough load to prevent cracking. Another assumption was made in the analysis of the south shear wall along the exterior of the building. Since this wall has so many openings in it the wall was modeled as two walls acting as a coupled shear wall. After the stiffness of each member had been determined it was possible to determine the building s center of rigidity. The center of rigidity was calculated by multiplying the stiffness of each member by its distance from an arbitrary point (always the same point) and then dividing by the total stiffness of all members in that direction. The resulting value is the distance between the center of rigidity and the arbitrary point chosen. The center of rigidity for the upper floors of 111 Morgan St. is located 102 south of column line 1 and 49 west of column line A (between column lines 5& 6 and C& D). The center of rigidity can be seen in Figure 5 at the beginning of this report or in Appendix A. The center of rigidity is close to the center of the building in the transverse direction, but is 50 from the center in the longitudinal direction. Thus, significant torsional forces will be developed when force is applied parallel to the transverse direction (E-W). The torsional force created is a moment which is equal to the applied load times the distance from the center of rigidity to the point where the load is applied. This moment is resisted by all the shear elements on each level (in both directions) 27

30 and the force is distributed proportionally to each of the shear walls based upon each element s stiffness and distance from the center of rigidity. Also, it is important to note that torsional forces which came out negative were ignored since torsional forces are not allowed to reduce the amount of direct load that must be resisted by the shear wall at each level. A drift analysis was performed using the direct lateral loads (unfactored) in addition to the torsional forces that would be developed in each member as a result of the direct force. The method for determining drift was identical to the one used when stiffness was calculated, except when drift was calculated the appropriate force was applied at each level of the wall. Since these calculations are extremely repetitive an excel spreadsheet was constructed to carry out these tasks. A copy of this spreadsheet is included in Appendix B. Structural Steel Redesign Lateral Force Resisting System Results With the help of the excel spreadsheet it was determined that the thickness of the shear walls in the residential floors needed to be increased to 12 in the E-W direction. In the N-S direction, no modifications were necessary and the original 8 thick walls could be continued up to the new building height. There were also no modifications necessary to the shear walls in the parking garage. Using the same drift criteria as established for the old system (H/600) the redesigned structure had an allowable drift of 2.8. The maximum actual building drift occurred, as expected, in the E-W direction and had a value of 2.2. The drift in the N-S direction increased from 0.1 in the old design to 0.35 in the new design. It is worth noting that since the shear walls are no longer being poured with the columns, as was the case in the original design, there is very little incentive for using higher strength concrete and therefore all of the shear walls use 4,000 psi normal weight concrete in the redesign. Along with the increase in thickness a few other modifications had to be made to the shear walls. For the shear walls in the E-W direction the horizontal and vertical reinforcement had to be increased from two mats of 16 in both directions to two mats of 8 in both directions. The flexural reinforcement also had to be increased slightly in the E-W walls, however not to the extent that the horizontal and vertical reinforcement had to be. The increase in flexural reinforcement varies for each wall and depends upon the height at which the wall is being analyzed, but there is no problem fitting the extra reinforcement into the wall since it is 50% thicker than in the original design. 28

31 Structural Steel Redesign Summary The redesign of 111 Morgan St. using composite steel instead of a two-way concrete slab accomplished many things. However, what is most important is it accomplished everything it set out to do in the beginning. With the new structural system a plenum space has been provided along with room for a finished ceiling, and the two extra floors of residential units have been accommodated without affecting the building s parking garage or the foundation. The column sizes in every condominium unit have been reduced and the redesign of the entire structural system was accomplished with no major changes to the building s architecture. However, it remains to be seen whether the redesigned system is better than the original system. This decision can not be made until the full impact of the new structural steel system has been investigated. The remaining portion of this report will look at several of these issues. 29

32 Multidisciplinary Architectural Engineering Studies Cost Analysis The change in 111 Morgan Street s structural system has numerous effects. The most obvious effect, and perhaps the most important, deals with the difference in building cost due to the redesign of the structural system. When comparing the existing structural system to the new steel system it is important to know not only the cost of each structural system, but also the cost of other items that will be affected by the change. These items include fire proofing, providing a finished ceiling, and increases in partition height or building façade area. All of these factors and many more will play a role in determining which structural system is the best choice. Therefore, a brief comparison was made for the costs, direct and indirect, of each structural system. All of the cost estimates for this report were obtained using R.S. Means cost data. The estimates have all been adjusted to 2001 dollars since this is the time when construction began and all of the contracts for the project were finalized. The estimates have also been amended to reflect the differences in labor and material costs in Chicago compared to the national average on which R.S. Means is based. All of the costs shown include material cost, labor, and overhead & profit. Furthermore, equipment costs are also included in the estimates shown. An example of these items would include a crane to erect the steel or the use of a concrete bucket to pour slabs in the concrete system. Also, since the redesign of the structural system added three floors to the building, the costs being analyzed are on a per floor basis. Since the two buildings are essentially the same in the parking garage, the main focus of this estimate is on the residential floors. Comparing costs in this manner will give a better estimate of the cost of each system since it is pointless to compare the cost of a 9 story building to that of a 12 story building. The estimates for both the existing and redesigned structural systems were performed using a detailed cost estimate analysis. This required that take-offs be completed for all of the major structural elements that would go into erecting either structural system. Once the takeoffs were performed prices were determined for each individual element and a total cost was arrived at by summing the costs of the parts. For the non-structural elements such as building 30

33 façade and ceiling costs, assemblies estimates were performed based upon the square footage required. Also, it is necessary to know that if an element existed in both systems, only the price for the extra amount needed is shown. For example, in the steel system the floor to floor height is 1.75 greater than that of the concrete system and therefore an additional 1.75 sq. ft. of brick façade is required for every linear foot of building perimeter. The results of the cost analysis for the two structural systems are shown in Tables 8 and 9. More detailed information for both estimates can be found in Appendix B. Item Total Cost Item Total Cost Columns $49367 Slabs $ Shear Walls $44714 Total Cost per Floor $ Table 8 Cost analysis for existing two - way concrete system Structural Steel $ Shear Studs $2442 Steel Decking $64757 Concrete & Pumping $55190 Welded Wire Fabric $11184 Fire Proofing $27731 Shear Walls $52714 Extra Façade Costs $27429 Extra Wall Costs $8446 Total Cost per Floor $ Ceiling Costs $85533 True Total Cost $ Table 9 Cost analysis for composite steel system The values shown in Table 8 reflect the results of the detailed cost estimate for the existing concrete system. The costs shown for each structural element reflect the cost of the formwork, the reinforcing steel, the concrete, placing the concrete, and all of the labor necessary to perform the job. The values shown in Table 9 reflect the results of the detailed cost element for the composite steel system, with the exception of the façade, wall, and ceiling costs for which assemblies estimates were performed. 31

34 Cost Analysis Summary As the tables clearly show the costs of the two structural systems is very close. The difference in the costs per floor of the structural systems is about $6000, which is very small for a project of this magnitude. In fact, if the steel building did not have three additional stories on it the shear wall costs would be the same as the costs for the concrete building and the steel system would actually be slightly cheaper. Providing the finished ceiling is what makes the composite steel system more expensive than the concrete. However, these results are expected and adding the ceiling was an architectural improvement over the existing system for which it was known there would be a cost increase. If the cost of the ceiling was distributed evenly among all the condominium units on each floor the average cost increase would be $3420 per unit. This is not an extraordinary cost increase when you consider that most of the condominium units are selling at prices well over $200,000. Therefore, regardless of which structural system is used it may be worthwhile to install the ceiling if the tenants are willing to pay for it. 32

35 Acoustics Another important effect of changing the structural system is how the building s acoustics are affected. Since 111 Morgan St. is intended to be luxury condominiums it would be very unsettling if an occupant was constantly being disturbed by his or her neighbor. Therefore, the acoustical properties of the composite steel system including the ceiling were compared to the existing two-way slab system. The two properties which were of particular interest for this report were the sound transmission coefficient (STC) and the impact isolation class (IIC). A sound transmission coefficient is a rating assigned to a particular assembly based upon how well that system reduces sound transmission from one side of the assembly to the other over a frequency range of 125 to 4000 Hz. A simpler way of explaining an STC rating is with an example. Say for instance, someone in the next room has his stereo blasting at full volume. Obviously, the music would be louder in the room where the stereo is located, and would be quieter in the room you were seated in. The ratio between how loud you hear the music compared to how loud it is in the room with the stereo would give you the sound transmission loss between the two rooms. An STC rating is a single number which quantifies how well the wall between the two rooms reduces sound transmission over a variety of frequencies, the higher the STC the greater the noise reduction. An IIC rating is similar to an STC rating except that IIC ratings deal with the sound transmitted by impacts such as footsteps or bouncing a basketball. Typically, STC ratings are given for all types of assemblies (floors and walls) where as IIC ratings are usually only associated with floorceiling systems. The acoustical comparison made for this report used STC and IIC ratings of each floor-ceiling system as a general indication of the acoustical performance of each structural system. This should be a fairly accurate comparison since the rest of the room will not be affected by the choice of structural system. Regardless of what system is used, the wall will still be constructed the same and will still run from the floor to the ceiling. Therefore, there is no reason to be concerned about the change in acoustical performance between adjacent rooms on the same floor because there should be no change. The only concern is the sound transmission which will occur between rooms on adjacent floors. 33

36 As stated earlier, STC and IIC ratings are assigned to building assemblies and can not be calculated by adding the STC or IIC of the individual components. Therefore, flooring manufacturers typically provide samples of their products to be tested in several different floor assemblies. The floor assembly in the existing building consisted of a 5/16 hardwood floor over a felt and foam underlayment which was placed directly on the 8 ½ concrete slab. The underlayment used in the existing system is a product called Quiet-Cor and its main function is to increase the STC and IIC ratings of floor systems. This particular assembly had an STC rating of 65 and an IIC rating of 60. The floor assembly in the composite steel system would also have 5/16 hardwood flooring and would use the Quiet-Cor underlayment, but the slab thickness in the steel system is decreased. In the steel system the slab is only 6 ¼ and it rests on 3 decking which means there is only an average of 4 ¾ of concrete. However, in the steel system a 5/8 gypsum board ceiling would also be installed which greatly helps both the STC and IIC ratings of the assembly. According to the manufacturer of Quiet-Cor, a composite steel floor system similar to the one being proposed for 111 Morgan St. achieves an STC rating of 62 and IIC rating of 60. The difference between the system tested and the proposed system was the tested system used a 6 concrete slab and ½ drywall. These small differences should not have a significant effect on the outcome of the STC and IIC, and therefore the differences in the system will be neglected. Acoustics Summary Overall, the acoustical performance of the two structural systems is nearly identical. According to Architectural Acoustics by David Egan luxury apartments should have an STC rating of at least 55 between adjacent areas and have an IIC rating of at least 57 between floors. Both the existing and the proposed structural system meet these requirements and therefore any concerns about a loss in acoustical performance by changing to the steel system can be put to rest. 34

37 Fireproofing Another issue in changing the building s structural system is determining how the building will meet fire resistance requirements. According to IBC Morgan St. qualifies as a building type R-2 and uses type I-B construction. This means that the building must have a 2- hour fire rating between all adjacent condominium units and also have a 2 hour rating between floors. The building must also have a 3 hour fire rating between the parking garage and the residential floors, but in both structural systems the separation is still a twoway concrete slab which provides a 3 hour rating. Fortunately, in type I-B construction both steel and concrete systems are required to provide the same rating. Therefore, none of the building s fire rating requirement will be altered by the change to a composite steel system. This is very advantageous since fireproofing can become very expensive, especially if you are forced to meet a high requirement such as a 3 or 4 hour rating between adjacent areas. However, just because the requirements did not change it does not mean that the composite steel design will automatically fulfill its fire rating requirement. In fact, a considerable amount of effort will have to be put forth in order to make the steel system comply with fire code. The first issue to look at is the fire rating between floors. Originally, the deck and slab thickness were chosen so that fire proofing would not be required on the underside of the deck (UL design D907), and therefore the two hour floor to floor rating was achieved through careful planning. However, a two hour fire rating is still required on all structural frame members. Therefore, all of the beams and columns in the entire building will have to be covered with 1 1/8 of spray applied fire resistive material. Several manufacturers of the required spray on fireproofing material are listed under the UL Design #D907. The final fire rating which must be achieved under the new design is the rating between adjacent units. As was the case in the concrete structural system the two hour rating is achieved by using a 12 wall between units which runs from the floor all the way up to the bottom of the slab above. The 12 wall is comprised of a double layer of 5/8 drywall on both sides with 3 layers of 3 sound blanketing between them. The difference between the concrete system and steel system is that when the wall is running perpendicular to the decking there will be voids between the top of the wall and the underside of the deck. These voids are created because the decking is fluted and is not perfectly flat on its underside like the concrete slab was in the 35

38 existing structural system. As a result, these voids must be filled with a fire resistive material like those used to fireproof penetrations in fire rated walls. Hundreds of these types of products are available from numerous manufacturers, but care needs to exercised to select a product which is approved for this type of use and which has an approved UL designation. It is important to note that not every wall in the building has to be dealt with in this manner; only wall which separate condominium units from each other or from the corridor are required to run from floor slab to floor slab. If all of these requirements are met the proposed composite steel structural system can achieve the same fire rating as the concrete system. 36

39 Construction Issues The final implication of changing the building s structural system involves construction issues. From a construction management standpoint, changing structural systems is a big deal, and creates several problems which must be dealt with. The major issues involved in transforming the building from concrete to steel are erection sequencing, schedule impact, and site planning. The original erection sequence called for the building to be erected one floor at a time working on the north section first and then moving to the south side. This same sequence could be used if it was modified to allow for the concrete shear walls to be poured before steel erection began on that portion of the building. For example, steel could be erected on the south side of the building while shear walls were being poured on the north. Once the shear walls were poured on the north side steel erection could begin on that side while the shear walls were being poured on the south side. This process could continue back and forth until the building was completely erected. In the case of 111 Morgan St., changing the structural system from concrete to steel would have a minimal impact on the overall schedule because of the large amount of time the project spent in the design development phase. During this time the initial steel order could have been placed and by the time the parking garage was completed enough steel would have been fabricated to stay ahead erectors. The only other major scheduling issue would be a slowdown of one or two weeks immediately after the steel erection began. This time loss would occur because of the changing in building crews. One crew would be trying to get started while another finished up and the two crews would end up getting in each others way. Also, the first floor of steel would take longer to erect because there would not be enough work to utilize all of the crews. Once the first floor was completed the steel workers would have more space to work and more crews could be brought in to deck and detail the steal that had been erected. The erection of the remaining floors should run smoothly after that and should proceed at a pace at least as fast as the concrete system, if not faster. The final construction issue which needs attention is site planning. Since 111 Morgan St. is located in an urban area, there is very little space to stage construction materials. Therefore, a tower crane would have to be used, just as it was in the concrete system, to pick steel off of trucks which would arrive daily using a just in time delivery schedule. Another issue regarding site planning would be trying to pour the decks. With the steel system the decks would have to be 37

40 pumped and place would have to be set aside for the pump truck to sit on the days when a deck is being poured. Fortunately, the building has an alleys on both the north and east sides where a pump truck could be staged. Obviously there are other construction issues besides these mentioned, but an experienced construction management team should be able to handle any problems which may arise. 38

41 Conclusions The conclusion of this report is that by changing the structural system of the building to a composite steel design several benefits can be realized. First, in the redesigned system a plenum space is provided in which the mechanical ductwork and sprinkler piping can be located which removes these eyesores from view. Second, the impact of the exposed columns can be minimized in the steel system since they will be small and encased in drywall providing a finished look which is lacking in the existing building. Finally, these improvements can be made without sacrificing any of the acoustical performance of the existing two-way concrete system and without adding any significant cost to the building or seriously affecting the schedule. The overall recommendation of this report is to use the composite structural steel system if a finished ceiling is desired or if the decision was made to build 9 stories of residential units instead of 7. This recommendation is made for two reasons; the first is if the extra money is spent to install the ceiling it would make sense to also receive the benefits of the smaller columns the steel system provides. Since the steel system really doesn t cost any more it would not make sense to install a finished ceiling in the concrete building since the columns would still be exposed. The second reason for using the steel system is: if the concrete system gets taller, it will become even more massive and would most likely require larger columns which would scar the building interiors even worse than they already are. After investigating the design of the existing two-way concrete slab and redesigning the building using composite steel a realizations was made. Looking back, it would be worth while to investigate a redesign of the two-way slab while putting an emphasis on moving the interior columns off of a typical grid and making a better effort to conceal them in the walls. This is typically one of the reasons two-way slabs are used in condos, and could solve the problem of the unsightly columns which protrude into some rooms. 39

42 Acknowledgments I would like to thank the following people for helping me in completing my thesis or for helping me simply get through it. David Heselbarth of Walsh Construction for providing me with a full set of drawings The entire AE Department faculty for answering my infinite number of questions All of my friends in the AE major for showing me how to use RAM and explaining everything I couldn t figure on my own. 40

43 Bibliography ACI Building Code Requirement for Structural Concrete (318-02) Farmington Hills, Michigan: American Concrete Institute. AISC Manual of Steel Construction, Load and Resistance Factor Design, 3 rd Edition Chicago, Illinois: American Institute of Steel Construction. Egan, David M Architectural Acoustics. New York, New York: McGraw-Hill. Murray, Thomas M Floor Vibrations Due to Human Activity. Chicago, Illinois: American Institute of Steel Construction. Nilson, Arthur H Design of Concrete Structures. New York, New York: WCB McGraw-Hill. Underwriters Laboratories, Inc. 41

44 APPENDIX A 42

45 Fig. 12 Potential moment frame layout

46 Fig. 11 Wind Pressures in E-W direction per IBC 2000 (windward + leeward pressures shown)

47 Fig. 10 Floor map showing beams designs

48 Fig. 9 Typical floor framing model used in RAM Steel

49 Fig. 8 Wind loading per IBC 2000

50 Fig. 7 Transverse building section

51 Fig. 6 Longitudinal building section

52 CENTER OF RIGIDITY BUILDING CENTER Fig. 5 Typical residential floor plan

53 Fig. 4 Parking garage floor plan

54 Fig. 2 - West side of 111 Morgan looking north Fig. 3 Typical condominium interior