THE PLAZA AT PPL CENTER ALLENTOWN, PA

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1 Introduction Moment Frame Analysis The moment frames in the Plaza for PPL Center were analyzed as four different systems defined by AISC: 1. Special Moment Frames 2. Intermediate Moment Frames 3. Ordinary Moment Frames 4. Moment Frames not Specifically Detailed for Seismic Resistance The frames are listed in order of decreasing ductility. The idea behind seismic design depends on the response of the lateral force resisting system to the ground motion. Under IBC 2000 and ASCE 7-98, steel frames can be designed for a lower lateral force provided that the frame achieves certain ductility. The AISC Seismic Provisions dictates the detailing criteria for the various categories of frames and assigns more stringent requirements to frames which are assumed to be more flexible. The result is a trade-off between heavier members with smaller connections and lighter members with larger, often stiffened connections. Construction Management Comparison The economy of each system was compared in both cost and time savings. With the increasing wages and unchanging material prices, most of the cost for a steel system lies in the detailing. As predicted, the most economical moment frame system resulted when a response modification factor of three was used for design which made the less stringent detailing requirements applicable. This design required the least fabrication and erection time and resulted in the largest cost savings. 38

2 Cost Comparison Assumed Costs Cost data associated with steel construction was acquired from two fabricators: Stewart-Amos Steel of Harrisburg, PA and Grossi Steel of Bensalem, PA. The costs used for the lateral system comparison are summarized in the table below. Stiffeners and Doubler Plates Total Material Fabrication 4 Stiffeners 600 lbs. steel $125 $20 $105 2 hrs. 2 Doubler Plates 700 lbs. steel $150 $15 $ hrs. Erection Labor Fabrication Labor 1 Ironworker $15/hr Shop Labor $15/hr. 3 min./bolt Overhead $35/hr Rolled Steel Plate Steel A992 $0.21/lb. 50ksi $0.32/lb. 36ksi $0.25/lb. The estimated costs of the connections are included in the summary tables from the previous section. The connections for moment frame A are included as a representative spreadsheet in Appendix 2. The total estimated cost of the frames is included with the frame diagrams. The frame diagrams for moment frame A have been added in Appendix 2 as a representative summary of the design costs. A summary of the total system cost is show in the table on the following page. 39

3 Fabrication and Erection Time The estimated time comparisons are listed in the table on the previous pages. The times were derived from the equivalent weights of steel, which are suggested by AISC, along with wage information from RS Means and Stewart- Amos Steel. The fabrication data only indicates the additional time that would be required to fabricate column stiffeners. The difference in fabrication time due to a greater number of bolts or thicker plates is negligible and is therefore assumed to be equal. The final costs are listed in the table below: Special Moment Frames A $50,145 $4,294 $22,480 $2, B $38,662 $3,310 $17,332 $1, C $38,662 $3,310 $17,332 $1, D $46,986 $4,023 $21,064 $1, Sum $274,617 1,676 hrs. Intermediate Moment Frames A $50,145 $4,294 $22,480 $2, B $38,662 $3,310 $17,332 $1, C $38,662 $3,310 $17,332 $1, D $46,986 $4,023 $21,064 $1, Sum $274,617 1,676 hrs. Ordinary Moment Frames A $60,753 $4,367 $11,180 $3, B $44,048 $3,167 $8,106 $2, C $44,048 $3,167 $8,106 $2, D $51,362 $3,692 $9,452 $2, Sum $262,953 1,334 hrs. R=3 A $70,906 $1,607 $2,340 $2, B $51,058 $1,157 $1,685 $1, C $51,058 $1,157 $1,685 $1, D $58,839 $1,334 $1,942 $1, Sum $252, hrs. 40

4 Comparison Results Total System Results It was proven in the structural comparison of the moment frames that a lower R-factor resulted in heavier members, but smaller connections. As shown in the table on the previous page, this combination results in a more economical frame. As the R-factor is increased, the efficiency of the frames is decreased. The chart above illustrates the trend of decreasing cost and time with a higher R-factor. Connections Connections drive the cost of steel construction. The heavy members and less stringent detailing requirements associated with low seismic design eliminated the need for stiffeners and doubler plates, which are pictured in the right. Stiffeners and doubler plates are easily designed with today s technology, but drive the fabrication costs up significantly because of the extensive labor required for the welding of the plates to the columns. Column Flange Stiffeners Stiffened Moment Connection Panel Zone Doubler Plate 41

5 Schedule Comparison Project Requirements PPL Corporation will occupy the Plaza at PPL Center on April 30, PPL Corp. wants to relocate the unregulated subsidiaries as soon as possible to minimize the negative implications of having the regulated and unregulated divisions of the corporation in one headquarters. If the building is not turned over to Liberty Property Trust by April 30, 2003 a string of penalties occurs. PPL will receive compensation from Liberty Property Trust, who will in turn collect compensation from L.F.Driscoll, who will fault the sub-contractor responsible for the delay. As a result, it is very important for the project to stay on track. Schedule Modifications All items associated with the structural steel design, fabrication and erection fall on the critical path. Therefore, any time savings in the steel sequence would be beneficial. The existing east-west lateral system for the Plaza at PPL Center is special moment frames. The previous comparison indicated that is was more economical to design the frames using R=3 because it offers both cost and time savings. As shown in the table to the right, the low seismic design offers a time Connection Labor Special Moment Frames Frames not Detailed for Seismic Fabrication Erection Fabrication Erection ,676 hrs. 602 hrs. savings of 1074 man-hours, which is due to the time savings in the fabrication. For the special moment frames, the fabrication time was substantially increased by the addition of stiffener and doubler plates. The erection time is slightly higher for the low-seismic since more bolts were required to prevent block shear in the beam flanges. 42

6 Schedule Comparisons Designing with a response modification factor equal to three, 1090 hours are saved during the fabrication time, but sixteen hours are added during erection. Assuming an erection crew of six ironworkers, the sixteen hours has almost no effect on the erection schedule. However, the change in fabrication time will have a considerable effect on the overall project schedule. The difference in fabrication time can be contributed to the difference in the number of required stiffeners. Assuming a welding crew of 4-5 men, working eight hours each day for five days each week, the 1090 man-hours equates to about six weeks. Since steel fabrication in on the critical path for the project, the time savings takes the activity off the critical path allowing for six weeks of float time. With the existing schedule the decrease in fabrication time does not change the project completion date because the steel erection is dependent on the completion of sitework and foundations as seen in the schedules in Appendix 3, but it does allows for flexibility in the schedule and opens up other possibilities. The steel fabrication was scheduled to begin immediately after schematic design. There are inherent problems associated with this schedule. More often than not, major changes occur in the design after schematic drawings are issued. Fabricators will often charge the owner a larger contingency when fabrication is started earlier to prepare for future change orders. The time savings could allow for the steel fabrication to start at a later date. This would allow for a more complete set of design documents to be issued resulting in a lower contingency for the owner and less change orders. 43

7 A second option would be to retain the float within the schedule to allow for any problems that occur later in the construction. This is a sizeable float time and would probably be better utilized for delaying the fabrication for at least part of the time. Changing the foundation and sitework, could be a third possibility. The start of steel erection is dependent on the foundation. If possible, it would be beneficial to fast-track the foundation to allow for earlier erection of the steel. This change would allow for an earlier project completion date since all the steel related activities would be put back on the critical path. Conclusions In summary, designing the east-west lateral for the Plaza at PPL Center as moment frames not specifically detailed for seismic resistance instead of special moment frames results in a cost savings of $23,000 and saves six weeks of fabrication time. The savings can be attributed to the more economical connections that can be designed under low seismic. 44