U.S. Pharmacopeia Headquarters Consolidation. Rockville, MD

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1 Consolidation Jeff Rothermel Structural Option Advisor: Dr. Ali Memari Technical Report 2 Submitted: Oct. 24 th, 2008

2 Table of Contents Executive Summary.1 Floor Systems Existing Two Way Slab with Drop Panels..2 Composite Steel Alternative..4 Two Way PT Slab Alternative One Way Slab with PT Beams Alternative Conclusions 11 Appendix A 12

3 Executive Summary This technical report is installment 2 of the 3 part series. It lays out a comparison between the existing floor system and 3 alternative systems. The other systems designed are two way PT slab, one way slab with PT beams, and a composite steel system. The overall pros and cons of the various systems are discussed in terms of the actual designs. The first section of this report is devoted to the structural systems and their designs. Each system is broken down into a brief description along with the final design. Each corresponding section lists the pros and cons of each particular system as well. Ending the report is a brief comparison and an attempt at determination of the best system. This is followed by an appendix containing the calculations for the one way slab with PT beams and the two way PT slab system. Overall the different systems designed appear to be reasonable with the exception of the two way PT slab. The two way PT slab has much more reinforcing than the existing mild two way slab though theoretically it should have less. This design needs looked at more closely because it is possible that a refined design may prove to be most efficient. One way slab with PT beams has good potential for huge span increases though they will require big ceiling height losses. And steel seems to be the best system in terms of a combination of ease or erection, structural slimness, and span/weight efficiency. The determination of floor system is a preliminary decision and the various systems will need to be studied more in depth in order to gain a more accurate design. This will make the comparison more accurate and give a better idea of which system truly is the most efficient. 1 P a g e

4 Floor Systems Existing Two Way Flat Slab The first system analyzed was the existing two way flat slab with drop panels around some columns. Analysis was performed on the Second floor level because it is representative of typical floor layouts. The analysis was done using the PCA Slab program which performs strip design for elevated slabs. Using the existing dimensioning on floor slab, columns and drop panels, column lines A, B, C, 11 and 12 were run in Design mode in order to compare the design to the existing design. The existing design was performed on ADOSS, another slab strip program. Figures 1 and 2 show the existing design and the PCA comparison design respectively. The PCA slab design used ACI respectively. The existing slab design has a bottom mat of #4 bars at 12 o.c. plus the additional bottom and top reinforcement. After the PCA analysis was performed the bottom mat area of steel was subtracted to obtain the additional bottom steel numbers for design. The loading for analysis was taken as 100 psf unreducible live load plus 10 psf dead load, though actual loading varies from 100 to 250 psf depending on area function. Normal weight concrete was used for the design (145 pcf, f c = 5000 psi). By looking at these figures it can be seen that the existing slab has more reinforcement than the PCA minimum design throughout. Existing values are often several square inches larger than the required design; also existing design uses larger bar sizes than used in the comparison design. These extra values may have been added through engineering judgment or simply because the ADOSS program may have designed slightly different reinforcement than the PCA Slab program. Figure 1 (Existing Design) Figure 2 (Comparative Design) 2 P a g e

5 Pros and Cons Two way flat slab systems have a lot of advantages not seen in other floor systems. One of the biggest advantages to a two way system is the great reduction in floor heights. Floor thickness can be reduced from 2-3 feet for other systems down to <1 even with drop panels in place. Reduction in floor thickness leads to many other pluses, such as lower floor to floor heights, greater ceiling heights, lower façade costs and faster construction time. Two way flat slab is also efficient in its use as a thin rigid diaphragm in order to use concrete columns as moment frames. Fireproofing is also a big advantage of the two way slab system, since it requires no additional fireproofing materials. In terms of disadvantages span length is the main one. Spans are generally small using the two way slab system. Another disadvantage is the sensitivity of two way systems to openings. Two way flat slabs are extremely susceptible to cracking in opening corners and must be specially detailed, though this is still not a foolproof solution. 3 P a g e

6 Alternative 1: Composite Steel Frame The first new floor system design is a composite steel system. It is constructed of a concrete slab on top of metal decking with shear studs welded to the beams to provide shear transfer between the two materials thereby adding strength and making the slab structural. Composite was chosen due to its great increase in strength over noncomposite design. The design was performed using RAM Structural System utilizing the existing column layout of 22 square bays with 25 on the North bay and 21 on the East bay. The loading was taken as self weight of the system plus 100 psf live load plus 10 psf superimposed dead load. Vulcraft 2VLI19 metal decking was chosen based on the maximum unshored span length so that each bay can have only one intermediate beam. This decking with a 3 ¼ lightweight concrete slab (115 pcf, f c = 3000 psi) achieves adequate thickness (5 ¼ slab) for a 2 hour firerating. Figure 3 shows the various beam designs for the typical floor spans. The diagram shows that W14x22 beams with (12) shear studs exterior and (14) shear studs internal are used for the typical 22 span W16x26 beams work well as girders with varying shear stud quantities based on the spans they are supporting. RAM Structural System used LRFD 3 rd Edition for design. Figure 3 4 P a g e

7 Pros and Cons Steel has a variety of pros and cons. One major advantage is erection time. Steel construction has a much faster pace than concrete construction. Faster construction time leads to shorter construction periods and cheaper costs. Another advantage to steel is its ductility as a material, also the relatively low weight/strength ratio compared to concrete. This leads to a lighter system and therefore lower lateral loads and a much more responsive lateral system. Steel is also a recyclable material making it a relatively good choice for clients concerned with green building aspects. Steel floor systems also allow for greater spans if the client is willing to sacrifice ceiling height. There are several disadvantages to a steel system however. One is the need for additional fireproofing of the steel. Also one of the most important disadvantages is the need for much thicker floors up to 2 and 3 depending on the spans and required beam sizes. This disadvantage does have one positive in that though beams may be very deep often times holes can be strategically cut so as to allow MEP penetrations while maintaining structural integrity. Another very subtle disadvantage is that the current structure uses moment frames as the main lateral system. Moving to a steel system would require much more expensive moment frames to maintain the current structural load path or moving to a much more intrusive braced frame system. 5 P a g e

8 Alternative 2: Two Way Post Tensioned Flat Slab Alternative 2 is a two way post tensioned slab utilizing the existing column layout. Two way PT slab utilizes PT cables to add strength and reduce reinforcing amounts. The concrete used in design is 5000 psi normal weight concrete. The design was hand calculated and can be seen in Appendix A. It was calculated that 9 tendons be used in the banded direction and to supply the appropriate force in the uniform direction strands were placed at 2 o.c. Reinforcement needed is rather high with 21#5 bars at supports and #7 6 midspan. Figure 4 shows the typical bay reinforcing and Figure 5 shows a span in section. Figure 4 Figure 5 6 P a g e

9 Pros and Cons Two way slabs with PT are generally more efficient than regular mildly reinforced slabs. One big advantage is the requirement for less reinforcing steel than a regular mild slab. A second plus is the reduction of deflections and also minimization of cracking. PT tendons essentially offset deflections due to loading which makes them less prone to cracking. Also PT slabs have greater span capabilities than mildly reinforced slabs. A major disadvantage of two way PT slabs is the cost of construction. PT can be over 10-20% more costly than regular mild slabs. PT cables, having such high strength, are more prone to failure from fire requiring larger clear cover distances. Also core drilling that may lead to damage of one cable may lead to complete system failure. PT cables are susceptible to corrosion as well, particularly if there is cracking on the upper surface of the slab where water may filter into the cracks and stagnate. 7 P a g e

10 Alternative 3: One Way Slab with PT Beams The third proposed system is a one way slab using post tensioned beams as supports. This system was selected in an attempt to eliminate columns. For this system the column layout was reduced and bay sizes became typically 44 x22, double the original 22 x22 size. Figures 6, 7 and 8 show typical beam layout and rebar detailing for both slab and beams. Compressive strength for the entire system is a robust 8000 psi. The one way slab is 5 thick with #4 bars at 12 o.c. for both positive and negative reinforcement. This slab sits atop 24 x36 (w x d) post tensioned beams at 11 centers. The beams that lie in line with columns frame directly in them and since the bays are 22 wide there is a center beam in each bay that frames into mildly reinforced girders then to the columns. The PT beams use (7)#8 bars to deal with tension in the initial prestressed, no load condition. Each beam uses 17 PT strands with a drape of 14 from the section centroid at midspan. Four inches of cover were provided to help achieve adequate firerating. There is no shear reinforcement required in the PT beams, their strength is sufficient based on the concrete strength. The midspan beams frame into girders of the same dimensions running in the 22 perpendicular length. These girders use only (5)#9 bars as their negative and positive reinforcement with #3 stirrups with 4 legs at 16 o.c. spanning the entire length. Figure 6 8 P a g e

11 Figure 7 Figure 8 9 P a g e

12 Pros and Cons One way slabs have many of the advantages of concrete and disadvantages of steel. The biggest advantage of the system chosen, using PT beams, allows for much larger spans. The PT beams allow for double the 22 bay size. The one way slab has less reinforcing than the two way slab and is therefore easier and faster to construct. Aside from the advantage of spans the PT beams make the floor slab stiffer than regular two way slab as well. The most important disadvantage of the one way slab system with PT beams is the overall structural depth. Though the slab itself is a slim 5, at 36 total this is the thickest of the slab systems and has major implications on floor-to-floor height. Since the building houses lab spaces the large ducts in addition to structural heights will lower floor-to-ceiling heights from the current ½ to 9-10 depending on duct size. Another major disadvantage is that with the one way system individual spans are much shorter than two way slabs and require girders to carry intermediate beams. At the current beam spacing construction may be very expensive due to the much higher amounts of concrete used, though actual beam sizing and spacing could be played with. 10 P a g e

13 Conclusions There are many criteria to use when choosing a floor system. These will be discussed in the following set of conclusions and comparisons. Figure 9 represents a basic comparison between the different systems. Floor System Cost/ft 2 Total Structural Depth Weight Bay Size Firerating Additional Fireproofing Two Way Flat Slab w/ Drops /2" 105 psf 22'x22' 2 hr None Two Way PT Slab w/o Drops /2" 85 psf 22'x22' 2 hr None Composite Steel /2" 68 psf 22'x22' 2 hr Spray on Fireproofing One Way w/ PT Beams " 129 psf 22'x44' 2 hr None The 4 systems studied are very different and make good comparisons with each other. It at first seems that the chosen system is the most efficient, and this is a logical conclusion since one would assume that the designers chose the most efficient system. The two way PT system calculations seem to be erroneous because one would expect a PT slab to have less mild reinforcing than a regular two way slab due to the initial forces from the cables. A more refined calculation of the PT slab system may lead to it being considered the most efficient system. As of yet however, it seems to be much less efficient than the two way mild slab. The one way system with PT beams seems very good if minimizing columns is the most important though the much reduced ceiling heights make it a less desirable system than that currently in place. If it can be further refined and reduced it may become more comparable but upon initial design it is one of the least efficient. Composite steel seems to be the system with the most potential. Though it has thicker structure this can be worked around by penetrations through steel; and if some penetrations are unavoidable it is much slimmer than the one way system. The steel system could also be refined and may enable larger spans with minimal structural thickness increases. Even at the maximum of 21-½ thick it is much smaller than the 36 PT beams. The composite system appears to be the best alternative though it will need more evaluation and determining how the connections and lateral system would make it more or less efficient. 11 P a g e

14 Appendix A: Hand Calculations One way slab calculations 12 P a g e

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23 2 Way PT Slab Calculations 21 P a g e

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