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Waqas! You should remember that we have to satisfy all applicable DESIGN CODE requirements, while designing a structure. IMO, in case of special moment frames subjected to bending and axial load (required in seismic zones 3 & 4 or SDC D, E & F), ACI 318-08 Section 21.6.4.4 is certainly applicable, and HAS TO BE SATISFIED. Chapter 29 of PCA Notes on ACI 318-08 contain very elaborate examples on how to make design calculation for earthquake resistant structures. You might like to have a look at Example 29.3 section 5, describing in detail the method of calculating transverse reinforcement (hoops, ties etc.) Regards.

Assalam U Alaikum, Muhammad Tayyab Asif B.Sc. Civil Engineering, UET Lahore Passing Year 2002 Working as 1- Deputy Director(BPS-18),Construction Deptt, University of Agriculture,FSD 2- Managing Partner, Vertical Engineering Services, FSD

Good article; I will throw my two cents. Seismic drift values are much larger than wind values. UBC uses maximum inelastic response displacements rather than the design level displacements to verify the performance of the building. As stated above, the seismic drift limits are 2% & 2.5% of the story height for long and short -period buildings. So, for a floor to floor height of 12 feet the max. allowable inelastic drift value would be 2% of 12 feet= 0.02*12*12inches=2.88 inches. For wind for a 12 story height, drift would be L/400=12*12/400 =0.36 inches, A comparison of both wind and seismic drift limits shows that earthquake inelastic displacements are quiet large compared to wind displacements. That is why proper detailing is emphasied in seismic design. Moreover, when calculating ΔS for seismic, make sure: you have included accidental torsion in your analysis. use strength design load combinations: 1.2D + 1.0E + 0.5L & 0.9D + 1.0E You are using cracked section properties for reinforced concrete buildings. Typical values are Icr walls= 0.5EcIg, Beams = 0.5EcI g & for Columns 0.5 - 0.7 EcIg.

For stiffness modifiers to slabs, If your framing is such that you don't have Stiff Beams (beams normally > 3*Depth of slab), you will see a lot of difference when you apply and don't apply stiffness modifiers. Consider flat plates, where you have thick slabs on periphery beam. Now when you don't apply stiffness modifiers to such a case, you get less moment in you beam as more load is transferred by the slab to the column, and slab is considerably thick and has a reasonable stiffness relative to the beam which is supporting it. For the same case when you apply a modifier, the slab stiffness is reduced and more load in transferred to your beams which are now stiffer than your slab because of the modifier. Why should we use a stiffness modifier? for the above example, a flat plate would have some sort of cracking when subject to seismic loading and because of that the stiffness would be reduced, as a result the beams would be carrying higher load than they were actually supposed to take(considering if the original design was based on no stiffness modification for slab). Thus by using modifiers for this case, a structural engineer is able to consider the effect of heavy loaded beams and designs beams to a greater load. Therefore, using stiffness modifiers where there is a reason can help you design robust structures. What I am trying to say is that for slabs like 4", 5" the difference in moments is very small for the cases with or without modifiers if the beams are deep. Don't worry too much about using modifiers unless you have a condition that demands one. Just, plug your numbers in and do your analysis as for most of the general cases, the difference due to modifiers would be close to nothing as explained above. Every thread I go, people are confused about them. Dont worry that much, just use the default numbers and be cautious of situations like the one explained above where you really need them. @Rana, why did you say that using stiffness modifiers for slab would result in higher sway? Isnt the lateral distribution (for this case) on rigid diaphragm assumption would result into same story force no matter how thick or thin the slab is, which will determine the story sway(considering frame stiffness as constant)..