THE PLAZA AT PPL CENTER ALLENTOWN, PA

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1 : MOMENT FRAME COMPARISON Introduction Design Intent IBC 2000 and ASCE 7-98 recognizes steel frames designed with a response modification factor, R, less than or equal to three as structural steel systems not specifically detailed for seismic resistance. If a structural steel building falls into low seismic design categories A, B, or C, as the Plaza for PPL Center does, the lateral system can be designed for an increased base shear using R=3, which exempts it from the requirements specified in the AISC Seismic Provisions. Economy In general, systems designed with a higher R-factor result in smaller member sizes, but are subject to more stringent connection provisions as dictated by AISC. Connections can account for around 5% of the total weight, but can be up to 60% of the total costs. Larger connections with stiffeners and doubler plates require a significant amount of shop labor, which drives up the cost of fabrication. Design The moment frames in the east-west direction of the Plaza at PPL Center were originally designed as Special Moment Frames. The lateral system was analyzed as four different frames as defined by AISC: 1. Special Moment Frames: R=8 2. Intermediate Moment Frames: R=6 3. Ordinary Moment Frames: R=4 4. Moment Frames with R=3 The four systems will be compared to determine which system is most economical by comparing material and labor costs as well as fabrication and erection times. 21

2 : MOMENT FRAME COMPARISON Lateral Loads under IBC 2000 and ASCE 7-98 Wind When wind controls over earthquake the building can be detailed using the procedure in the AISC Load Resistance and Factor Design Manual, provided that R is taken as 3 or less than 3 for seismic force calculations. Low-Seismic If the force due to earthquake controls and R is taken as 3 or less, the building falls in to the category of low-seismic applications and the building is designed to meet the provisions in the LRFD Manual using structural steel systems of normal ductility. High Seismic When R is taken as greater than 3, the building must be designed to meet the provisions in both the AISC LRFD Manual and the AISC Seismic Provisions for Structural Steel Buildings. Regardless of whether wind or earthquake controls in this case, the use of R greater than 3 in the calculation of the seismic base shear requires the use of a seismically detailed system that reflects the assumed ductility of the system based on the response modification factor that was chosen. 22

3 : MOMENT FRAME COMPARISON Evolution of Seismic Design for Steel Buildings in Pennsylvania 1996 BOCA National Building Code ref: ASCE 7-95, 1993 AISC LRFD Manual BOCA 96 does not recognize low seismic design or structural steel systems not specifically detailed for seismic resistance. The conventional assumption was that ordinary moment frames were ordinary and required no special detailing for seismic loads 1999 BOCA National Building Code ref: ASCE 7-98, 1998 AISC LRFD Manual, 1997 AISC Seismic Provisions BOCA 99 does not specifically address the design of structural steel frames, which are not specifically detailed for seismic resistance. However, the code does reference ASCE 7-98, which indicates that a lateral system which is not specially detailed must be assumed to have a response modification factor, R, less than or equal to 3. All other frames should be designed according to the AISC Seismic Provisions for Structural Steel Buildings. IBC 2000 ref: ASCE 7-98, 1998 AISC LRFD Manual, 1997 AISC Seismic Provisions and Supplement 1 IBC 2000 is the first building code to acknowledge steel buildings designed for low seismic. Table is a reproduction of ASCE 7-98 Table which assigns an R=3 for buildings not specifically detailed to resist seismic forces exempting these frames from the detail requirements in the AISC Seismic Provisions. 23

4 : MOMENT FRAME COMPARISON Seismic Provisions Amplified Load Effects In addition to the load combinations in IBC 2000, columns in frames are required to meet the AISC Seismic provisions and must be checked for two additional load combinations where Ω o =3 for steel moment frames and steel frames with a response modification factor equal to 3: 1.2D + 0.5L + 0.2S + Ω o E 0.9D - Ω o E The amplified load effects are to be applied without considering the effect of moment of the columns. Ductility For buildings with rigidly connected members, members can be designed to resist significantly lower seismic design forces providing that frames achieve a certain level of ductility. Under the AISC Seismic Provisions for Structural Steel Buildings, inelastic yielding must occur in beams before connecting elements if the frame is to be considered as detailed for seismic resistance. As ductility increases, the required strength or yield level decreases. Through the Response Modification Factor, R, building codes have tried to accommodate for this occurrence. A higher R-factor indicates that a system is more ductile and can therefore be designed for a smaller seismic force. 24

5 : MOMENT FRAME COMPARISON Moment Frames Under IBC 2000, there are four classifications of moment frames, which vary depending on the ductility of the system. These descriptions according to the AISC Seismic Provisions for Structural Steel Buildings are: Special Moment Frame(SMF) SMF s are expected to resist significant inelastic deformations when subjected to seismic design forces. Intermediate Moment Frame(IMF) IMF s are expected to resist moderate inelastic deformations when subjected to seismic design forces and shall be designed so that the inelastic deformations are accommodated by the yielding of members of the frame when fully restrained connections are used and by the connection element when partially restrained connections are used. Ordinary Moment Frame(OMF) OMF s are expected to withstand limited inelastic deformations in their members and connections when subjected to the forces resulting from seismic design forces. Structural Steel Systems not specifically detailed for seismic resistance The AISC Seismic Provisions have also evolved since the 1997 publication. With the issue of the second supplement, the required rotation for special moment frames was increased and the rotation requirement for ordinary moment frames was eliminated, but the connections for OMF s are required to resist and increased design capacity over frames designed with R 3. 25

6 : MOMENT FRAME COMPARISON The AISC changes are summarized in the tables below: Moment Strength Capacity Class AISC 1997 Suppl.1 AISC 1997 Suppl.2 AISC 2000 SMF M 0.03rad or M 0.04rad or 0.8M 0.04rad 0.8M p for buckling or RBS 0.8M p for buckling or RBS IMF M 0.02rad or M 0.02rad or 0.8M 0.02rad 0.8M p for buckling or RBS 0.8M p for buckling or RBS OMF 1.1RyMp 1.1RyMp 1.1RyMp M 0.01rad Shear Strength Capacity Class AISC 1997 Suppl.1 AISC 1997 Suppl.2 AISC 2000 SMF 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + shear from 2(1.1R y M p ) shear from 2(1.1R y M p ) shear from 2(1.1R y M p ) IMF 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + shear from 2(1.1R y M p ) shear from 2(1.1R y M p ) shear from 2(1.1R y M p ) OMF 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + 1.2D + 0.5L + 0.2S + shear from 1.1R y M p shear from 1.1R y M p shear from 2(1.1R y M p ) In 2000, the Federal Emergency Management Agency (FEMA) issued a series of documents to direct the seismic design of steel moment frames. The two documents of most relevance to this study are: FEMA 350: Recommended Seismic Design Criteria for New Steel Moment- Frame Buildings FEMA 353: Recommended Specifications and Quality Assurance Guidelines for Steel Moment-Frame Construction for Seismic Applications The class of moment frames as describe by AISC and IBC 2000 are not consistent with the classes in the FEMA 350 series documents. FEMA refers to two categories of moment frames: 1. Special Moment Frames 2. Ordinary Moment Frames 26

7 : MOMENT FRAME COMPARISON Though, the two categories in the FEMA documents use the same nomenclature as AISC, the frames are required to meet different rotation requirements: Class Interstory Drift Class AISC 1997 Suppl.1 AISC 1997 Suppl.2 AISC 2000 FEMA 350 SMF IMF N/A OMF 0.01 N/A N/A 0.02 Currently, these documents regarding connection behavior and recommended design procedures include the best available information on seismic detailing. However, the adequacy of test data and accuracy of some of the design procedures that have been published are currently being questioned. The Structural Engineers Association of California published Commentary and Recommendations on FEMA 350, which included additional information for engineers with a focus on seismic design in California. The FEMA 350 series documents are undergoing review by national building code committees as well as AISC in an effort to develop a set of provisions, which are appropriate for adoption by building codes. 27

8 : MOMENT FRAME COMPARISON Design Requirements For the comparison, the design of the frames is based 1997 AISC Seismic Provisions and Supplement 1 as referenced by IBC 2000 which includes the following requirements: 1. Special Moment Frames a. 23% reduction in allowable width to thickness ratios b. Strong column-weak beam: the nominal flexural strength of the columns has to be greater than the nominal strength of the beams unless the connection meets certain criteria and even then special weak column-strong beam connections are required by FEMA 350 c. Connection Design Capacity An inelastic rotation of at least 0.3 radians Flexure: the nominal plastic moment capacity, M p Shear: 1.2D+0.5L+0.2S plus the resulting shear from 1.1R y F y Z applied in opposite directions at each end, where R y =1.1 for A992 Steel 2. Intermediate Moment Frames a. Must the meet the criteria above except, except connections must demonstrate an inelastic rotation of at least 0.2 radians 3. Ordinary Moment Frames a. Connection Design Capacity An inelastic rotation of at least 0.1 radians Flexure: the nominal plastic moment capacity, 1.1R y M p Shear: 1.2D+0.5L+0.2S plus the resulting shear from 1.1R y M p for fully restrained connections or 1.2D+0.5L+0.2S plus the resulting shear due to the moment capacity of a partially restrained connections 4. AISC Frame (R=3): No special requirements 28

9 : MOMENT FRAME COMPARISON Calculated Lateral Loads The base shear was calculated for each system using IBC 2000 and is noted below: 1. Special Moment Frames V=350K 2. Intermediate Moment Frames V=350K 3. Ordinary Moment Frames V=506K 4. Moment Frames w/ R=3 V=675K The complete calculations can be found in the spreadsheet in Appendix 1, which distributes the force to each story based on the story weight and elevation. The plan for the Plaza at PPL Center is fairly regular, but a small eccentricity exists in the east-west direction. IBC 2000 also requires an accidental torsion to be considered when considering the effects of earthquake loading. The torsion is calculated in a spreadsheet in Appendix 1 and the load distribution to each frame is summarized in the table to the right. E-W Moment Frames, N-S Braced Frames Frame At Mid-Height At Roof A B C D ~~~~ Typically, the response modification factor directly affects the calculated load since the seismic response coefficient, C s, is divided by R in the equations: 1. C S = S DS /(R/I) 2. C S = S DS /[(R/I)T] The governing C s value for both special and intermediate frames was 0.044S DS I, therefore the base shear was equal for both systems and independent of the R- factor since V=C s W. The resulting member sizes were the same for both systems, but the connection requirements do vary. 29

10 : MOMENT FRAME COMPARISON Governing Load Combinations When designed with an R>3, the controlling load combination varied depending on the member. Seismic load controlled for columns, but for beams the governing load combinations were dependent on the location of the beams within the frames. However, since the seismic load was calculated with R>3, the connections had to be designed for the increased capacity regardless of the controlling load. With R=3, the greater seismic load was the controlling load for most of the members with the exception of some beams near the top of the frame. Drift No building code prescribes limitations for total building drift, but ASCE7-98 does make recommendations in Commentary Appendix B. ASCE7-98 recognizes that the common practice limits drift to somewhere between 1/600 and 1/400 of the building height and a 3/8 interstory drift to minimize damage to non-structural elements such as cladding and partition walls. ASCE7-98 also suggests a load combination using D + 0.5L + 0.7W to use when considering the effect of drift. While wind load is the primary concern for building drift since it affects the occupants on a regular basis, the interstory drift is limited by IBC 2000 in Table for seismic loading. For the Plaza at PPL Center in Seismic Use Group I, the interstory may not exceed 1/50 of the story height, which would equal 4.8 for the tallest story. The building is important to PPL Corporation housing personnel for its PPL Power Generation and EnergyPlus divisions. In the event that the lateral design loads are achieved, it would be important for damage even non-structural damage to be minimized to reduce the time the building would be out of service. 30

11 : MOMENT FRAME COMPARISON Besides preventing damage to the building, limiting the allowable drift also limits the Pδ effects on the columns reducing the required moment capacity. The total drift for the Plaza at PPL Center was limited to 4 (1/400 of the building height), but for a reduced seismic load equal to D + 0.5L + 0.7E or the allowable interstory drifts dictated by IBC 2000, whichever is smaller. Frame Analysis The frames were analyzed with STAAD for the following load combinations: D + 1.6L D+0.5L+0.2S D + 1.0E + 0.5L + 0.2S D + E D+1.6W+0.5(L+S) D + 0.5L + 0.2S + Ω o E D - Ω o E The STAAD input files can be viewed in Appendix 2. The output was checked against a portal analysis calculation which is also included in Appendix 2. STAAD code checks indicate that beams fail due to the interaction of bending and axial loads. However, the lateral system is such that it is the slab which resists axial loads due to lateral forces and these forces in turn cause bending in the beams. The beams can therefore be checked for flexural strength and it is not necessary to include the axial load. The columns were checked for combined axial and bending and the beams were checked for flexural strength. The members were checked in a spreadsheet and a representative spreadsheet is included in Appendix2. Lower columns and beams were governed by strength, while stronger members were required for higher levels to control drift. The member sizes for the four frames designed under different provisions are included in Appendix 2. 31

12 : MOMENT FRAME COMPARISON Connection Design Frames designed according to the AISC Seismic Provisions, are expected to have connections that are governed by yielding in the beam. Connections for intermediate and special moment frames require a minimum rotation to ensure the frames have enough ductility. A flange-bolted, web-bolted moment connection, as seen to the right, was chosen for the analysis. The flange-bolted, web bolted connection was chosen because it: 1. Has been tested by FEMA and is included in FEMA Provides sufficient ductility to be used for both AISC SMF and IMF (FEMA SMF and OMF) 3. Does not require welding the beam flange to the column, which is being reviewed after the failures which occurred during the Northridge Earthquake 4. Does not require field welding 5. Allows for fabrication tolerances, unlike the moment end-plate connections 6. Is recommended by AISC for seismic design, unlike the moment end-plate connection. Flange-bolted, web-bolted moment connection 32

13 : MOMENT FRAME COMPARISON The SMF and IMF connections were designed according to the recommended procedure in FEMA 350. The calculations were written in a spreadsheet which is included in Appendix 2. The FEMA 350 design procedure for web-bolted, flange bolted moment connections is as follows 1. Confirm column is W12 or W14 for SMF (not limit for OMF) 2. Calculate the moment at the column face, M f, and the moment at the column centerline, M c 3. Calculate the moment at the onset of flange yielding, M yf 4. Check panel zone thickness 5. Determine the required dimensions of the flange plate 6. Note: the best connection has simultaneous yielding of the beam flange, flange plate and panel zone 7. Check 1.2M yf <M fail for: a. Bolts in flange plates b. Bolts in shear tab c. Fracture of flange plate d. Fracture of beam flange e. Bolt bearing and tearout 8. Check block shear according to the LRFD manual 9. Design shear tab according to the LRFD Manual 10. Design continuity plates Connections for ordinary moment frames and systems designed with R 3 can be designed using the procedure prescribed by the third edition LRFD manual. The connection summary for moment frame A is included in Appendix 2. The calculations for LRFD connection design were written in a spreadsheet, which is also included in Appendix 2 checks the following limits states: 33

14 : MOMENT FRAME COMPARISON 1. Beam Limit States a. Reduced Flexural Strength b. Flange Block Shear c. Bolt Bearing and Tearout 2. Tension Plate Limit States a. Block Shear b. Tension Yield and Rupture c. Bolt Bearing and Tearout d. Bolt Shear 3. Compression Plate Limit States a. Bolt Bearing and Tearout b. Plate Buckling c. Bolt Shear 4. Shear Tab Limit States a. Bolt Shear b. Shear Yielding c. Shear Rupture d. Block Shear e. Bolt Bearing and Tearout f. Plate Buckling Overturning The moment frames are along column lines B and C are the narrowest and are therefore the most critical frames for overturning. Earthquake forces produce 72,600 ft-k of overturning moment. Moment frame B takes 21% of the seismic load. The resulting overturning for this frame is 15,250ft-k. The 166 x13 x6 concrete footing provides 150,000ft-k of resisting moment. Therefore, even without the added resisting moment from the dead load of the building, the overturning is resisted. 34

15 : MOMENT FRAME COMPARISON Results Since the base shear for the SMF s and IMF s were equal the members were also identical. The required rotation varies for the frames, but the moment and shear capacity requirements of the connection are the same for both frames. The flange-bolted, web-bolted connection is capable of producing an inelastic rotation which is acceptable for SMF s and therefore exceeds the required rotation for and IMF. Ultimately, the lateral system for the Plaza at PPL Center was the same whether the moment frames were designed as SMF s or IMF s. The AISC Seismic Provisions require strong column-weak beam connections, for frames detailed for seismic resistance. This requires shallower, heavier beams, which increases the tension and compression forces in the flange plates due to the smaller moment arm. To resist the larger forces in the flange plates, the connections have thicker flange plates, more bolts and column flange stiffeners. With an R=4 for OMF s, the moment frames resulted in larger members. AISC has determined that the connections for OMF s can be designed according to LRFD procedures providing that the connections withstand a moment equal to 1.1R y M p. Using R=4, the OMF requires larger member sizes than required for the SMF and IMF. The larger column sizes help reduce the number of stiffeners, which reduces the fabrication costs. On the other hand, the simplified connection design required the connection to be design for an amplified moment which adds cost by increasing the number of bolts and plates thicknesses. For the Plaza at PPL Center, the frames designed with an R=3 were the most economical. Using R=3, the frame resulted in heavier members, but took less time to fabricate. For most of the connections, the heavier members eliminated the need for stiffeners and doubler plates. Low seismic frames can have deeper and lighter beams because there is no strong column-weak beam requirement. 35

16 : MOMENT FRAME COMPARISON The lighter beams were a problem for some connections where the thinner flange failed in block shear resulting in an increased number of bolts. This was only the case for a few members and the cost of additional bolts was negated by the reduced number of column stiffeners and lighter beams. The design saved money in detailing and labor costs, and this will be discussed further in the following section. 36