THE PLAZA AT PPL CENTER HAMILTON BOULEVARD AT 9 TH STREET - ALLENTOWN, PA

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1 THE PLAZA AT PPL CENTER HAMILTON BOULEVARD AT 9 TH STREET - ALLENTOWN, PA Amy Graver, Structural Option - Dr. Hanagan, Advisor THESIS PROPOSAL Executive Summary Architecture The Plaza at PPL Center is an eight-story mixed-use building, with a partial basement, first floor rentable retail space, office space on the second through sixth floors and security trading on the two upper floors. Core areas border the east and west sides of a central atrium which can be seen in Figure 4. Structural System The gravity system is a composite steel system with both composite beams and composite deck. Typical beams are W18 s which span 42 in the north-south direction and are spaced at 10 on center. A representative floor plan is pictured in Figure 1 below. Figure 1: Typical Floor Framing 1

2 Proposal The proposed thesis will include three variations on the existing structural steel system for the Plaza at PPL Center. The purpose of this thesis is to examine the economy of different system configurations. This thesis will compare high and low seismic design for lateral systems, the economy of braced frames to moment frames and will consider the cost effects of full and partial composite action for floor beams compared to deeper members. The results will be presented in a series to also determine the effects of combining the three variations. Altering the structural system affects other aspects of the building, one of which is the mechanical system. The analysis of the composite floor system will consider the economy of deeper beams which will intrude upon the floor plenum space. Designing shallower ducts to run beneath the floor beams would help retain the original floor depth. The primary objective of this thesis is to compare the costs associated with variations on a steel structure. Money can be saved in material, but it can also be saved in time and constructability. Therefore, a schedule and budget will be prepared to use in the comparison of the various designs. 2

3 Background Building Function and Primary Use The Plaza at PPL Center is a multi-use building with the ground level designated as retail space. Since government deregulation of energy companies, it became important for the unregulated subsidiaries of PPL Corporations: PPL Energy Plus and PPL Generation to be located within a separate building from the regulated divisions of the company. The second through the six floors of the Plaza at PPL Center are office areas which will be rented by these subdivisions. The primary function of Energy Plus is to buy and sell buys gas, oil and electric and the top two floors are reserved for traders. Location and Site The site for the Plaza at PPL Center is in the center of downtown Allentown, PA. It is located on the northeast corner of 9 th and Hamilton streets adjacent to the PPL tower, which stands on the northwest corner. The site was formally Hess s department store and the building was demolished in 2000 after being vacated by the Bon-Ton department store. The Corporate Plaza was built in 1986 and was the last commercial building to be built in downtown Allentown prior to the Plaza at PPL Center. However, the Corporate Plaza was demolished in 1994 after a sinkhole opened underneath the building causing irreversible damage. The geotechnical report indicated that there were not active sinkholes at the Plaza at the PPL Center site but there is a high possibility of long-term instability. Compaction grouting was required to increase the bearing capacity. Architecture One of the design considerations for the eight-story building, designed by Robert A. M. Stern Architects of New York, was the fact that downtown Allentown is densely developed with little open space. To help with this, the architect included a plaza in his design, which sets the building back 85 from Hamilton Street. This is uncharacteristic of most of the buildings along this main street. The architect wanted to bring nature into the building and he provided an atrium that allows light into all the floors from above. Winter gardens on the third floor and fifth floors can be viewed from the floors above with the exception of the two upper trading floors. A roof-top garden provides additional green space for the Plaza at PPL Center. 3

4 Building Envelope The building façade consists of a curtain wall with aluminum framed windows between ribbons of pre-cast concrete panels. The roof of the Plaza at PPL Center consists of two levels. The lower roof is constructed similarly to a typical floor with composite steel beams spanned with a lightweight concrete slab on composite metal deck. The larger portion is curved. This profile is obtained with curved steel beams spanned by metal deck. Project Delivery System Method The primary tenant, PPL Corporation, sought out Liberty Property Trust to develop the former Hess s lot and lease the building to PPL. The owner then proceeded to hire the architect, Robert A.M. Stern, under a time plus fee contract and the construction manager, L.F. Driscoll, was hired after submitting the lowest bid in the guaranteed maximum price (GMP) job. The contracts held by the design team are traditional time + fee agreements and L.F. Driscoll is acting as an At-Risk contractor holding cost + fee contracts with its subcontractors. However, the MEP contractor, H.T. Lyons, is also owned by PPL Corporation. L.F. Driscoll holds a design-build contract with H.T. Lyons and hired Lehigh Valley Engineering as a consultant. Robert A. M. Stern still holds the contract with PPL Energy Services and the MEP Engineer for the base building work. Composite Construction Steel framing was chosen for the Plaza at PPL Center because of its favorable performance in long span conditions and light weight. The Plaza at PPL Center has clear spans up to 42 feet. Without pre-stressing, concrete beams cannot reasonably span this distance. The soil bearing capacity after compaction grouting was only 5000psf. A concrete system would have increased the overall dead load of the system and the column reactions. The light weight of steel allows for smaller footing sizes. W-shape beams and columns are the primary framing members. The steel beams act compositely with the concrete slab on composite metal deck. Columns lining the perimeter also support the exterior façade. 4

5 Lateral System As seen in Figure 2., the existing design of the Plaza at PPL Center uses four moment frames(a-d) in the east-west direction and two braced frames(3,8) in the north-south direction. Moment frames do not obstruct openings in the façade or plan and were useful in the long direction. Braced frames were appropriate for the short direction because there was space along the core where the diagonals will not block openings. Seismic force is the controlling load for the Plaza at PPL Center. Member sizes for the lateral resisting frames are governed less by strength and more by stiffness to help reduce building drift. Figure 2: Lateral System Layout 5

6 Moment Frame A Moment Frame B/C Moment Frame D Braced Frames 3/8 Figure 3: Frame Elevations 6

7 Design Considerations Economy Cost dictates design and it is imperative to consider the economy of the structural system. Connections constitute a considerable portion of the cost of a steel structure. AISC states that connections consist of only about 5% of the material weight but can be nearly 60% of the cost. The Plaza at PPL Center will be analyzed with a series of modifications to the current steel structure and the cost implications compared. Lateral System Moment connections are more costly in both erection time and fabrication cost. It is much more economical to use braced frames than moment frames as the lateral system in a steel building providing that the braces do no impose upon the circulation paths and views. It is also worthwhile to consider low seismic design and normal ductility for the lateral system. If the response modification factor(r) is taken as 3 or less the system does not have to be designed using the American Institute of Steel Construction Seismic Provisions. Using an R equal to 3 or less results in a higher base shear and ultimately larger members, but allows for less severe detailing and more cost-effective connections. Composite Construction If there is no height restriction on a building, partial composite beams and girders may be more cost effective. The cost of one shear stud is equivalent to ten pounds of steel and it may be more economical to use a deeper beam than a higher number of shear studs. The deeper beam will also be more effective against deflection. Mechanical Modifications Changes to the structural system affect other building systems as well. The ducts for the mechanical system run below the main floor space at the Plaza at PPL Center. When considering the size of these beams for the composite construction analysis, the mechanical system has to be reviewed as well. The existing structure provides duct openings in the girders. These openings will also have to be considered if the sizes of the ducts change. 7

8 Construction Management The modifications to the structural system are all an attempt to optimize the constructability of the building. When considered, constructability has a huge effect on both the cost and time for construction. Time is also a critical factor for the Plaza at PPL Center since it is important for the unregulated resources of PPL Corporation to be separated from the regulated divisions in the Tower building as it creates an unwanted association between services. April 30, 2003 is the deadline for the relocation of these companies and any modifications are not feasible if it would prevent the building from being completed on time. Proposed Investigations Response Modification Factor The controlling lateral load for the Plaza at PPL Center was seismic force. The building was designed for high seismic applications using R=5 for braced frames and R=8 for moment frames. This higher factor decreases the seismic base shear, but this choice requires that the frames and more specifically, the connections, be designed to meet the more stringent requirements of the AISC Seismic Provisions. It has been proposed that designing for low-seismic applications by using an R=3 could be more economical in that connections are not subject to the stricter detailing requirements. Within the lowseismic applications, the higher seismic base shear will require larger member seizes but this may be more cost-effective when compared to the heavier connections that would otherwise be required. The lateral system for the Plaza at PPL Center will be will be designed for both provisions and a cost comparison will be made. Frame Design Four moment frames resist lateral forces in the east-west direction. Moment frames offer unobstructed bays, however the connections are expensive. The Plaza at PPL Center has a central core area with two elevator shafts and a restroom area that is consistent on every floor as seen in Figure 4. At these locations it would be possible to incorporate braced frames into the lateral system. It will have to be determined whether two braced frames along the elevator shafts are adequate to resist the lateral force. If not, additional frames can be incorporated. The combination could include frames along the restroom area; however this area is not included on the first floor. Since the first floor is designated as retail space it is important for this floor to retain its open plan. 8

9 Therefore, the braced frames will have to be reconfigured within the first floor to include a moment frame or a braced frame at a different location. Figure 4: Location of Core Area Composite Construction It is possible to design composite beams as fully or partially composite. Several factors go into making this decision: floor depth restrictions, deflection and the practical spacing of shear studs. Since, no height restriction is imposed on the Plaza at PPL Center it may be possible that a deeper beam with less or no composite action will be more cost effective than a lighter member with more shear studs. A comparison will be made to determine the most economical system. Mechanical Modifications Deeper floor beams, will take up space in the floor plenum. This may be counteracted by redesigning the ductwork. The required mechanical load will not change from the existing design, but the size of the ducts can be altered to retain the same floor height. Shallower and wider ducts will be considered as a solution. Construction Management The proposed modifications to the structural system will be compared to the existing design with a material, fabrication and labor cost analysis and a schedule will be prepared to contrast lead time and the erection sequence. 9

10 Research Method Response Modification Factor and Frame Design Initial sizes for the framing members will be calculated for gravity loads using IBC 2002 and the 3 rd Edition of the AISC LRFD Manual. The design of the frames will be conducted using the lateral forces determined using both the response modification factors for ordinary frames and for low-seismic provisions as defined in the LRFD Manual. The frames will be modeled in STAAD which utilizes a stiffness matrix to analyze the members. For the braced frames, the individual members will have to be checked for axial loading and for the moment frames the members will have to be checked for flexure. Connections for both applications will be designed using LRFD procedures and cost information will be obtained from a steel fabricator. Composite Construction Using design procedures published by the American Institute of Steel Construction and loads determined by IBC 2000, composite members will be checked. The program RAM steel can be used, but the members will also be checked using the AISC procedure because the output given in RAM is not always the most economical from a construction perspective. The beams will be considered with partial composite action to determine whether more shear studs or larger beams are more practical using the comparison of one shear stud equivalent to 10 pounds of steel. This comparison will be supplemented by a cost estimate from AISC SolutionsCenter. Mechanical Modifications The load for the ducts will not change and this can be obtained from the existing design. A shallower duct size will be chosen using a Ductulator. Construction Management Cost information will have to be gathered from a variety of sources: Fabrication of Connections: Steel fabricator Labor: R.S. Means, Construction Manager, Contractor Structural Steel: American Institute of Steel Construction A schedule will be developed using data from the construction manager and contractors. Both time and cost will be considered in the comparison of the different variations for the structural system and for combinations of the variations. 10

11 Task and Tools Response Modification Factor 1. Determine gravity and lateral loads from IBC 2000 and ASCE Calculate trial sizes using gravity loads 3. Input trial sizes and LRFD load combinations into STAAD a. Finalize sizes b. Obtain member forces from STAAD output 4. Design typical connections for moment and braced frames 5. Check costs with steel fabricator 6. Compare costs of two designs Frame Design 1. Determine gravity and lateral loads from IBC 2000 and ASCE Calculate trial sizes using gravity loads for braced frames at elevator shafts 3. Input trial sizes and LRFD load combinations into STAAD a. Determine if the two frames provide adequate resistance, if not continue with additional frames b. Finalize sizes c. Obtain member forces from STAAD output 4. Determine costs for the frames using information from steel fabricator and AISC 5. Compare costs of braced frames to moment frames Composite Construction 1. Determine gravity loads from IBC 2000 and ASCE 7-98 including live load reductions 2. Calculate trail sizes with RAM 3. Compare sizes from RAM with deeper beams and beams with less composite action 4. Check deflection 5. Check vibration 6. Compare costs of variations with AISC SolutionsCenter estimate and 10 pounds/shear stud ratio Mechanical Modifications 1. Determine design loads for existing mechanical system 2. Redesign for shallower ducts with Ductulator 11

12 Construction Management 1. Collect cost figures from construction managers and fabricators 2. Compile cost figures for material, fabrication and erection cost 3. Make cost comparisons between the systems 4. Obtain schedule information from construction managers or contractors 5. Create a schedule for the various structural designs 6. Compare schedule to existing schedule to ensure the building will be completed on time Timetable Week Tasks Jan. 6 Research Seismic Provisions and Calculate Loads 13 Analyze Frames with Different R Factors Design Typical Connections Contact Milton Steel for Cost Comparison of Different Connections 20 Calculate Loads for Lateral System with Two E-W Braced Frames at Elevator Shaft Analyze Lateral System with Braced Frames Determine if Additional E-W Frames are Required 27 Calculate Loads for System with Additional Frames if Required Feb. 3 Analyze Lateral System with Additional Frames if Required Compare Cost of a Braced Frame System to a Moment Frame System 10 Calculate Floor Loads for Composite Beams Design Composite Floors Beams 17 Compare Economy of Larger Beams to Smaller Beams with More Shear Studs 24 Compile Comparisons Mar. 3 Compile Comparisons 10 Spring Break 17 Mech: Resize 24 CM: Schedules and Budgets 31 Prepare Presentation Apr. 7 Prepare Presentation 14 Presentations 12