Brent Ellmann Structural Option 200 Minuteman Park, Andover, MA Structural Consultant: Dr. Hanagan

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1 Brief Building Overview: 200 Minuteman Park stands as a 200,000 square foot Class A office building in Andover, Massachusetts, worth roughly $15,000,000. Although the building has a large square footage, it stands only three stories above ground plus a roof that contains two mechanical penthouses (See Building Section). In plan this office building appears to be a large semi-circle, with the flat side of the building facing to the south and the curved façade pointing towards the north (See Building Plan). The southern wall faces a small courtyard with a pond and park, while the northern façade faces a road. The southern façade is comprised of metal cladding and low-e reflective glazing, designed to reduce energy costs and improve thermal efficiency. The curved northern façade also has the same low-e reflective glazing, but instead of the metal cladding there is a limestone brick veneer in between the glazing. In the center of the structure a large atrium, with a clerestory at the top, has been placed to allow natural light to penetrate into the interior of the building and to separate the structure into two halves. Early on in the design of this building the owner, Brickstone Properties, wanted to allow for future use by multiple tenants; therefore the building has two separate mechanical penthouses, each supplying half of the structure. The systems found in 200 Minuteman Park are typical of an office building. The building is fully sprinklered and has an addressible fire alarm system. On the roof, 6 air handling units sit, three each in the two mechanical penthouses, to provide ventilation and cooling. On the ground floor there are 2 natural gas-fired cast iron boilers that Page 1 of 11

2 provide the heating for the structure. The power supplied to this building comes from utility transformers at a voltage of 480/277 volts. Sticking with the idea of allowing for multiple tenants, there is a dual main-tie-main switchboard, with each switchboard having a 2,500 amp rating. Applicable Building Code(s): The building code used in Massachusetts is called the 780 CMR: The Massachusetts State Building Code, 6 th edition. Although the state of Massachusetts has its own building code, the code established by the 780 CMR is actually based off of the BOCA National Building Code (1996). In fact, one of the other accepted building codes for the state of Massachusetts is the 1996 BOCA code. According to the 780 CMR building code this structure is considered a 2B Protected system. The B stands for business and the building is called protected, because it has fireproofed structural steel and is fully sprinklered. Other accepted codes used are the NFPA 101-Life Safety Code (1997) and the NFPA 80-Standard for Fire Doors and Windows All of the masonry work done in this structure is in conformance with ACI 530 and ACI The concrete used has the requirement to be in compliance with ACI 318 and ACI 301. Finally, the steel in the structure is in accordance with the revised 1989 AISC specifications for steel structures. Description of Structure: 200 Minuteman Park has a structural system composed primarily of wide flange steel beams, girders, and columns, with composite concrete on metal deck floor systems. The columns and large girders in 200 Minuteman Park are ASTM A572 Grade 50 steel, Page 2 of 11

3 steel tubes or pipes are ASTM 500, Grade B steel, and the rest of the steel in the structure is ASTM A36. Concrete footings and piers used in the foundation have a minimum required strength of 3,000 psi after 28 days. The concrete used in the rest of the building, i.e. interior slabs, exterior slabs, and walls, has a required minimum strength of 4,000 psi after 28 days. Concrete strip footings, ranging in size from 1 x 2 to 1 x 2-8, were used around the perimeter of the building, while the steel columns are supported on concrete footings and piers of various dimensions (See Foundation Plan). The piers are all 24 x 24 square and extend up to the base of the concrete footings located below the columns. 200 Minuteman Park has no basement or sub-terraineous levels; therefore the first floor is a four inch thick concrete slab on grade reinforced with 6x6 W1.4 x W1.4 welded wire mesh. The basic framing plan consists of 30 x 30 interior bays and an exterior arc that is broken down into 30 sections. A typical bay consists of two W24 x 76 girders supporting three W18 x 35 beams (See Typical Bay Layout). Two more W18 x 35 beams connect directly into the columns, completing a full bay. The arc comprises of a series of W24 x 55 and W24 x 131 girders that link the arc s steel tube columns. For this bay configuration, a four inch thick concrete slab on top of two inch deep 20 gauge composite steel deck, with 6 x 6 W2.0 x W2.0 welded wire mesh reinforcement, creates the floor system. The roof framing consists primarily of 22K6 open web joists spanning between W24 x 55 girders. In a typical bay there are 4 joists between the girders and two joists that frame directly into the columns. Sitting on top of the joists is one and a half inch Page 3 of 11

4 thick 20 gauge galvanized wide rib deck that is continuous over three spans. In the center of the structure two mechanical penthouses sit, housing the six total air handling units and four smoke exhaust fans. In the areas where these mechanical units sit, the framing changes from K series joists to non-composite W18 x 35 beams to accommodate for the increased dead load (See Roofing Plan). The lateral force resisting system used in this structure is a series of braced frames placed in the east-west and north-south directions of the building. As can be seen from the provided building plan of 200 Minuteman Park, there are 10 braced frames placed throughout the structure. Typical frames are composed of two W10 columns and a variety of W24, W10, and TS compression and tension members (See Typical Lateral Bracing Elevations). The loads which these compression and tension members must carry are listed on the drawing. The geometry of the frames varies by their position in the structure, because at the location of some of the braced frames openings are needed for a loading dock and other passage ways. Design Theory: 200 Minuteman Park was built and finished close to five years ago now. At that time, and even now, Atlantic Engineering Services used Allowable Stress Design (ASD) to design all their steel buildings. For the purpose of this thesis design project, however, the use of Load and Resistance Factor Design (LRFD) will be implemented. Both methods of design are practical and safe, and the choice to use LRFD instead of ASD is by no means a statement that the original design is flawed. The choice to use LRFD instead of ASD is one done for two reasons. First, although ASD is still used in practice, Page 4 of 11

5 LRFD has been the design method demonstrated to new and upcoming engineers. The second reason involves the hope of comparing results from the two methods. Since the member sizes found in ASD are typically slightly larger than those of LRFD, using LRFD to spot check and re-design this structure should show slightly varying results. As noted earlier, this structure is steel system using composite beams. The choice of using steel to build this structure was actually not that of the structural engineering firm. The owner, Brickstone Properties, is a very experienced developer in the Massachusetts area. In the past Brickstone Properties has built many structures from steel and have adopted that as their company s preferred method of construction, because of steel s quick construction and capability for future renovation. For this reason no other structural systems or alternate floor systems were ever even considered in the design of this structure. This reason also explains why braced frames were used as the lateral force resisting system for 200 Minuteman Park. Again the owner specifically asked for a certain type of steel structure. Thus, other methods of lateral force resistance, such as shear walls, were never even considered in the design development of this structure. Design Loads: Dead Loads: Superimposed: 10 psf Mechanical 5 psf Flooring Total: 15 psf Page 5 of 11

6 ***Partitions not included in superimposed dead loads for this office building. They have been included in the floor live load to account for future renovations of the office layouts. Roofing: 3 psf Roof Deck 15 psf Ballasted EPDM Roof Membrane Mechanical (Penthouse Area): 150 psf Steel Members Self Weights: Roughly 2 psf for 22K6 joists and 5 psf for all others Composite Decking: 45 psf 2-20 Ga. decking with 4 thick concrete slab Metal Cladding: 15 psf Metal cladding and low-e reflective glazing Live Loads: Roof: 30 psf Base ground snow load Floor: 100 psf 80 psf + 20 psf for partitions Corridors: 80 psf Stairs: 100 psf Wind Loads: The following wind load diagrams were developed using ASCE The basic design wind speed for the location of 200 Minuteman Park is 90 mph, according to Massachusetts State Building Code. The surrounding site of 200 Minuteman Park allows the building to be classified as an Exposure B building, and the occupancy and use of this structure gives it an Importance Factor of 1.0. In order to carry out the calculations, the Page 6 of 11

7 fundamental period of the building was found to be seconds, which means that the frequency of the building is 2.79 Hertz. A frequency of 2.79 Hertz classifies the building as being a rigid structure, which makes sense because of the large building footprint compared to the low building height. The results of the wind loading analysis are shown below in the pressure loading diagrams and shear diagrams. From the wind shear diagrams it can be seen that the total base shear in the North-South direction is 346 kips and 126 kips in the East-West direction. Page 7 of 11

8 Seismic Loads: The following seismic loading diagram has been calculated using the procedure outlined in ASCE It has been assumed that the site soil classification for this site can be conservatively chosen as Site Classification D: Stiff Soil. 200 Minuteman Park is classified as a Group I Seismic Hazard Building with an Importance Factor of 1.0. The total base shear due to seismic forces is 1,435 kips. As can be seen from the floor shears shown in the Shear Due to Seismic Loading and the two Shear Due to Wind figures, the shear developed by a seismic event can be almost five times as large as the shear due to wind loading. Since 200 Minuteman Park stands only 45 feet above ground, the wind pressure never develops into the stronger wind pressure regions found at higher elevations. The shear due to seismic, on the other hand, is larger than normal because 200 Minuteman Park is located in a region where Page 8 of 11

9 seismic events have the potential to be catastrophic. Also, the building weight for this structure is rather large because of its large square footage, and building weight is a determining factor in seismic shear. For these two reasons it makes sense that the shear due to seismic loading is the controlling shear loading. Member Spot Checks: Floor Decking and Roof Decking: The basic spot check for both the floor decking and the roof decking was carried out by using the United Steel Deck Design Manual and Catalog. Although this may not be the exact manufacturer used in the original construction, the values used in the table suffice for the spot checks. In checking the roof and floor decking it was found that both decks were used in the building were actually over-designed. For the roof deck a 22 gage deck was strong enough and for the floor decking a 22 gage decking could have worked. However, it should be noted that the tables used in this manual only went down to concrete slabs with a thickness of four and a half inches instead of the four inches used in the building. This difference could explain why the obtained results were lighter deck gages than the original design. Roof Joists, Beams, and Girders: The roof joists were checked using a steel joist and joist girder manual published by The New Columbia Joist Company. Again, although this manual is for a specific manufacturer, it is reasonable to use the tables in this manual for this building. From this manual it was found that 22K6 joists are an exact match for the loading of the roof. Checking the other beam type by hand calculation showed that the W18 x 35 checked Page 9 of 11

10 out. The girders, on the other hand, resulted in different member sizes than those used. Where the plans showed a W24 x 55 and a W24 x 117 the analysis resulted in a W21 x 44 and a W24x 103, respectively. Floor Beams and Girders: The beams used in the floor system are composite beams and were designed taking that into account. After analyzing a typical beam in the layout it was found that a smaller beam than the W18 x 35 would work for the given loading. However, when the deflection was checked, it was found that the beam deflected more than the allowed L/360. By using the Lower Bound Moment of Inertia Tables in the steel manual, it was found that in order to reduce the deflection down to the required level a W18 x 35 beam was needed. The check of the girder, unlike that of the beam, showed that beam used in the original plans was adequate and the most economical choice. Columns: A typical column in the middle of structure was checked to see if it held the required gravity loading. To find the moment in the column, due to loading on the girders framing into it, a case where two adjacent spans were loaded, one with both dead and live loads and the other with only dead load, was used. The analysis resulted in a W10 x 88 column being the best size for the loading, just as the column schedule in the plans indicated was used. Lateral Bracing: A preliminary check on the bracing elements in the lateral frames showed that the members used were slightly over-designed. But the difference between the analysis Page 10 of 11

11 members and the original design members was very minimal. For all the members both a compression and tension check was needed to determine the capacity of the bracing. As was expected the controlling check was the compression check, since lateral torsional buckling is a factor that must be considered. Note: In some of the spot checks different member sizes, when compared to the existing member sizes, were found. This was expected since the building was originally designed using ASD, but this analysis was carried out using LRFD. It is also important to note that in some of the cases the deciding factor in the member sizes was actually the deflection of the member caused by only the live loads. Most notably this occurred with the W18 x 35 beams used in a typical floor bay. Page 11 of 11