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 PRO-CON STRUCTURAL STUDY OF ALTERNATE FLOOR SYSTEMS Executive Summary The Plaza at PPL Center is an eight-story mixed-use building, with first floor rentable retail space, office space on the second through sixth floors and securities trading on the two upper floors. Core areas border the east and west sides of a central atrium. For the given site, steel framing is best. Steel systems are light and that is desirable for the Plaza at PPL Center since the soil conditions are poor. Steel also allows for large open plans which the architect has designed for the Plaza at PPL Center. A precast concrete system may be worth investigating for the building on an alternate site because it has many advantages, but does require a review of the lateral system. Existing Framing System Members The existing structure for the Plaza at PPL Center is a composite steel system. The floor framing is typically W-shape, composite steel beams spaced at 10 on center with a maximum span of 42. A 5-1/4 a light-weight concrete slab on composite steel deck spans 10 from beam to beam in no less than a three-span condition. See Figure 1 for typical member sizes. Strengths A992 Steel: F y = 50ksi Steel Deck: F y = 40ksi Lightweight concrete slab: f c = 4ksi Figure 1: Existing Framing 1

2 System Characteristics With an extended slab and spray-on fireproofing, this system offers a two-hour fire rating. The Plaza at PPL Center site had poor soil bearing capacity, which was supplemented by compaction grouting, however, the building is still at high risk for differential settlement. A steel system is typically lighter which is beneficial given the soil conditions. Lightweight systems are more subject to vibration and this will have to be considered. In the typical office area, the height of the floor structure is nearly two feet. The HVAC design had to be coordinated with the structural design to allow ducts to penetrate some of the beams. The long spans require the steel beams to be cambered typically 1-1/2 to counteract the effects of deflection. The Plaza at PPL Center has a strict time constraint, since the unregulated subsidiaries of PPL Corporation must be relocated prior to April 30, Steel construction requires early planning to account for fabrication time but goes quite quickly from there. Variations on the Existing Steel Systems Removal of Moment Connections Moment connections are costly, in time and money. As designed, the Plaza at PPL Center direction resists lateral loads with braced frames in the north-south direction and moment frames in the east west-direction to allow for flexibility within the plan. However, the primary tenant, PPL Energy Plus, has signed a thirty-year contract with the owner, Liberty Property Trust. The tenant has designed large conference rooms and storage areas along the central core of the building. Unlike office cubicles these spaces will have full-height walls. Provided there is coordination with the tenant s architect, it would be possible to eliminate the moment connections in the east-west direction and use braced frames to resist lateral loads in this direction as well. 2

3 Diagonal Girders The architect for the Plaza at PPL Center located columns so they would align with the window mullions. It is desirable to have girders, which carry a large portion of the floor load to frame from column to column. For the exterior columns to be aligned with the window mullions they were not aligned with the rest of the columns in the building. Therefore, the girders framing from column to column ran along a diagonal line (see Figure 2), forcing the beams framing into the column to be of staggered lengths and sizes. The floor plan is close to symmetrical. It would be economical to recenter the core to align with the exterior Figure 2: Diagonal Beams column lines. The windows are in 5 panes that would be a convenient module to work with. As the building is laid out the spaces appear to be symmetrical but in actually it is not. Realigning the columns would permit the smaller sizes as they are now, but it would also allow for more redundancy among the beams. It would be possible to run the girders into the perpendicular beams; however, this results in heavier beams, which would be less economical than the current layout with staggered lengths. Steel Bar Joists Members Steel bar joists could be used instead of steel beams as the primary framing. The 24 joists would be spaced at 2 on center and span the 42 to W-shape girders. With the closer spacing, a 4 slab can be used instead of the original 5-1/4 slab or a shallower slab can be used but this may cause vibration issues. Steel Deck is 22 ga. B-lok deck with no studs. See Figure 3. Figure 3: K-series Joists and Metal Deck 3

4 Strengths Steel Deck: F y = 33ksi Normal Weight Concrete Slab: f c = 3ksi Steel Joist Chords: F y = 50ksi Steel Joist Webs: F y = 36ksi Characteristics It is more difficult to obtain a 2-hour fire rating as required for office buildings with a bar joist system. However, it is possible to do so when used in conjunction with a fire-rated ceiling assembly. Steel bar joists are commonly used in the Lehigh Valley and are often an economical system. The downfall comes in the installation and labor. The original design has beams at 10 on center and the steel joists spaced at 2 on center will call for five times as many members to be placed. Mechanical ducts up to 10 square, 13 round and 8 x13 can run through the 24 K- Series joist. The joist manufacture requires the size and location of mechanical ducts to be noted on structural drawings if they are to pass through a joist. This would be possible for the Plaza at PPL Center. While most of the sub-contractors are contracted at a maximum price, the mechanical contractor is working as design-build. It would be convenient for coordination between the mechanical and structural designs. A bar joist system is lightweight which would be advantageous for the site. The lightweight system may turn out to be problematic in that it will be more prone to vibration. 4

5 Concrete Pan Joists and Girders Members Concrete pan joists spanning in the 42 direction are 29 deep and 8 wide. The joists would be spaced at 4 o.c. since shear reinforcement will be required due to the long span and this module is achieved with 40 pans and 8 joists. Turning the framing, pan joists running the 30 direction required a minimum depth of 23, but the concrete girders would be spanning 42 instead of the 30 direction which is more practical for the heavier members. A five Figure 4: Concrete Pan Joists inch slab is required for the two hour fire-rating. Girder depth should be equal to the joist depth, so interior T-beam girders will be 29 deep and 24 wide. See Figure 4. for typical framing plan. Strengths f c = 4ksi F y = 60ksi Characteristics This system will be a heavy system given the weight of concrete and the large member sizes, so it is not the most appropriate system give the poor soil conditions. Cast-in place concrete is not common in the Lehigh Valley and the labor force is not readily available. This system is the deeper than any of the other systems and does not allow for mechanical penetrations. Concrete is a good system in that it does not have the lead time that steel has, but it generally takes more time to place in the field. Concrete is inherently fireproof and not as susceptible to vibrations which are advantages over the steel systems. 5

6 Hollow Core Plank and Prestressed Beams Members For the Plaza at PPL Center 12 hollow-core planks of lightweight concrete could be used to span the 42 direction with a 2 normal weight concrete topping to provide a flat surface for carpeting. The planks span 42 to pre-cast, pre-stressed girders that are 24 deep. L-Beams used around the perimeter and T-beams used in center spans provide a flush floor condition without dropping beams See Figure 5 for typical framing layout. Strengths Concrete: f c = 5ksi Steel Strands: F pu = 270 ksi Figure 5: Precast Concrete System System Characteristics A concrete system will be heavier than steel, which may be problematic considering the soil conditions at the Plaza for PPL Center site. The site on which the Plaza at PPL Center is being constructed was not always available. A hockey arena was initially planned for the site, until PPL Corporation declared interest in the lot. If PPL Corporation could not develop the site next to their current headquarters, the company stated that it would look to sites outside the city limits. Had the corporation been denied the site, the soils conditions may not have been such a governing factor. A different structure could be proposed for a building on an alternate site. Precast concrete systems allow for quick on-site construction. The precast members can be fabricated as the foundations are started. The floor and framing would be supplied by one contractor, which can also speed up construction. The existing building uses precast panels for the façade. It would be convenient to incorporate a precast structure as well. 6

7 Concrete systems offer good resistance to fire. Additional fireproofing is not needed as with steel construction. With the conversion from a steel system to concrete the lateral system has to be reevaluated as well. It is possible for precast systems to incorporate rigid frames however; it is not typically economical to do so. It is most efficient to use precast elements in simple span conditions because: 1. It is more difficult and expensive to achieve negative moment resisting capacity in precast members 2. Moment resisting connections are more expensive 3. Rigidity of connection may cause cracking since it cannot respond to temperature movement. It is most desirable to incorporate shear walls into the structure to act with floor and roof diaphragms to resist lateral loads. The Plaza at PPL Center has adequate room for shear walls in the north-south direction. As stated previously the layout could be planned so that shear walls could be incorporated in the east-west direction as well. Comparison System Advantages Disadvantages Potential? 1 Existing Lightweight Susceptible to Vibration YES Fast On-site Erection Long Lead Time Spray-on Fireproofing Crane Required Common to the Area Camber required Can incorporate Mech. Eq. 1a Braced Frames Econmomical Need Arch. Approval of Floor Layout YES 1b Move Columns Provides redundancy Need Arch. Approval of Floor Layout YES 2 K-Series Joists Lightweight Susceptible to Vibration YES Fast Erection More pieces to erect Common to the Area Fire-rated Ceiling Assembly Can Incorporate Mech. Eq. Crane required Deep System 3 Conc. Pan Joists Short Lead Time Heavy NO Fireproof Large Members Less Susecpitible to Vibration Deepest System Less deflection Mech. Eq. Must be Dropped Not Common to the Area 4 Precast Conc. Quick Heavy YES Already using Precast Panels Crane Required Shallowest System Economical Fireproof 7

8 References PCI Design Handbook, 5 th Edition AISC Load Resistance and Factor Design, 3 rd Edition United Steel Deck Design Manual The New Columbia Joist Company Steel Joists and Joists Girders 8