THE PLAZA AT PPL CENTER ALLENTOWN, PA

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1 Building Function and Primary Use The owner of the new Plaza at PPL Center is Liberty Property Trust, but the primary tenant is PPL, which will use the building as the headquarters for its new subsidiaries EnergyPlus, an energy marketing and trading organization and PPL Generation. The Plaza at PPL Center is a 256,000 square feet multi-use building, with the ground level designated as retail space. PPL will lease around 200,000 square feet, including the second through the six floors for office areas, which will be rented by the unregulated subsidiaries of PPL Corporation, PPL Energy Plus and PPL Generation. The primarily function of Energy Plus is to buy and sell gas, oil and electric. The top two floors are reserved for traders and equipped with screens to allow them to monitor energy prices. Location and Site Site The site for the Plaza at PPL Center is in the center of downtown Allentown, PA. As picture below, 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 6

2 formally Hess s department store and the building was demolished in 2000 after being vacated by the Bon-Ton department store. The Plaza at PPL Center is an important project for the downtown area since, like many cities, businesses continue to close and new development has been rare. PPL Corporation is a primary corporation in downtown Allentown and the current headquarters occupies the tower on the northwest corner of Ninth and Hamilton streets. Soil Properties 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 the soil is of poor quality only capable of supporting 3000 psf without sitework and that is with possible long-term instability. Most of the soil on the site is fill from remains of previous structures. 7

3 Architecture Architect s Vision 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 try to bring nature into the building and he provided an atrium that allows light into all the floors from above. A winter garden on the third floor can be viewed from the floors above with the exception of the two upper trading floors and a roof-top garden provides additional green space for the Plaza at PPL Center. Building Envelope The exterior of the building consists of pre-cast concrete panels with 7-6 ribbons of windows on an aluminum-clad frame. The roof of the Plaza at PPL Center consists of two stories. The lower roof is constructed like 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. 8

4 Overall Structural System 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. Steel has the advantage again in that its light weight adds less load and therefore allows for smaller footing sizes. W-shape beams and columns are the primary framing members. The steel beams act as composite members with the concrete slab on metal deck. Columns line the perimeter of the building to support the glass curtain wall exterior. Lateral System Four moment frames resist lateral loads in the east-west direction and two braced frames resist lateral loads in the north-south direction. During schematic design, it was decided that moment frames were therefore needed in the long direction to minimize obstructions in the façade or plan. The eccentric braced frames were appropriate for the short direction because there was space along the core where the diagonals will not block openings and the links allow openings for doorways. 9

5 Plan View Elevations Moment Frame A Moment Frame B/C Moment Frame D Braced Frames 3/8 10

6 Foundation System The Plaza at PPL Center has a partial 16-6 high basement level. Columns of the superstructure sit on the cast-in-place basement walls, which are supported by continuous spread footings. Interior columns which are part of the moment frames in the E-W direction, also sit on a continuous spread footing. The continuous spread footing has advantages over single square column footings. It is easier to excavate and pour because it remains at a constant level and the added mass helps to resist the uplift from the moment frames when lateral loads are applied. The site for the Plaza at PPL Center has poor soil bearing capacity. With compaction grouting, the soil capacity was increased to 5,000psf, but differential settlement is highly probable. The continuous center footing allows for a more consistent settlement across the building. 11

7 Mechanical System Air System The Plaza at PPL Center is supplied by a variable air volume system. The outdoor air intake is located at the first floor and the air is sent to the basement where it combines with the return air to enter the air-handling unit. This custom air-handling unit is actually built into the walls of the basement. A 2000 MBH heat recovery unit is located on the rooftop. Hydronic System The building has two 240 ton, chillers in the basement. A 1500 gallon per minute cooling tower is located in the adjacent North Building, which is another building occupied by PPL Corporation. A 2000 MBH boiler is required for the building. An interesting feature of the mechanical system is that there are three ice tanks in the basement. The storage tanks have a 761 ton-hr latent capacity. The tanks accumulate ice throughout the night and utilize the ice during the peak hours of the day to provide chilled water to the building. 12

8 Construction Information Project Delivery System Method The primary tenant PPL needed space in which to expand its developing subsidiary, Energy Plus. The tenant sought out Liberty Property Trust to develop the former Hess s lot and lease the building to PPL. The owner then proceeded to hire an architect under a time plus fee contract and a construction manager. The construction manager, L.F. Dricsoll was hired after submitting the lowest bid in the guaranteed maximum price (GMP) job. Dates of Construction Construction began in January of 2002 and the steel was topped out ahead of schedule on July 24, The building is expected to be complete by April 30, The top two floors are energy securities trading floors and will be occupied prior to the six floors remaining floors. Therefore, fitout of the seventh and eighth floors as well as the first floor lobby and other public spaces will take place first and completion will proceed to the sixth floor and down from there. Cost Information The new PPL Center projects are estimated to cost $60 million. This figure includes a new parking deck, which falls within a different scope of work. The estimated base building cost is $29 million and the cost of tenant fitouts is $19 million. 13