Syed Umair Haider

Dear All, Kindly share your experience regarding type of support assignment (hinge or fixed) in super structure models for computation of pile loads in case of, a, Columns supported in isolated pile caps. b, Columns supported on piled mats. Emphasizing on following parameters, 1, Difference in Pile loads extracted between both cases. 2, Difference in super structure column design between both cases. 3, Difference in lateral stability of super structure between both cases.

Dear Waseem, Your first two points from NEHRP are logical as they are directed towards the implementation of an upper limit on time period to be used in seismic forces evaluation to eliminate the possibility of excessively flexible structure.These provisions are present in all building codes and seem to be logical and valid. For usage of Ta when Tactual < Ta, The RS curve you have shown indicates Ta & Tc both on constant acceleration zone , so whether you use Ta or Tc you will get the maximum ground acceleration or in ELF terms whether you use any of them equation of max base shear will govern.So both are equal mathematically. However, ASCE endorses what you are saying. But as long as usage of Ta (approximate period) when Tactual < Ta is concerned it still seems illogical to me and will be associated with potential inconsistencies, some of them could be as follows, Drifts evaluated from Ta will be lesser than actual, therefore building separation widths (most importantly) will be under estimated. Code defined equation of max base shear will be of no value for a certain height of structures, while following this. As the actual seismic forces will be greater than that used in design, therefore structure will require high energy dissipation demand than anticipated, as members will be yielded and extended to inelastic range of stresses earlier (due to under estimation of seismic forces) than anticipated stage (associated with R used and detailing performed). Many other issues will also arise due to under estimation of seismic forces that you can also consider. Therefore, although this document is published from a credible source but they didn't justify the adequacy of this provision and didn't address the probable consequences.So it seems a non-engineered dictation to me. Practically this situation is very rare but I will go for UBC97 in such situation rather than following this.

Dear Waseem Your concern is valid but the lower limit given in FEMA example doesn't make sense to me as in a general the usage of time period in seismic analysis is not more than the evaluation of ground acceleration that will be imparted in structure in accordance with specific structural characteristics. Similarly if the specific structural characteristics of a building reveals a time period lesser than generalized approximate time period , then it doesn't make sense to skip the accurate time period and to use approximate one when code itself defines approximate time period as a basis to start analysis for actual T. More interesting is to note that indicated FEMA example is based on 2009 NEHRP seismic provisions , whereas in a separate document issued by NEHRP afterwards (named expanded seismic commentary to ACE-10) it is recommended to use "Tcomp" if "Tcomp < Ta". Moreover, i didn't find any lower bound on time period in UBC97 and even in ASCE 7 which is based on same ELF procedure as given in FEMA example. Therefore, it seems that indicated provisions in FEMA example are overlooked that they have fixed later and usage of Tcomputed if "Tcomputed < Tapproximate" seems valid.

I think both ETABS and UBC 97 provisions are correct. As ETABS completely follows UBC97 therefore only UBC's provisions are elaborated below. UBC seismic design philosophy limits the minimum time period (in the form of maximum base shear ) to Ts (UBC response spectrum) which is equivalent to Cv/2.5Ca. For elaboration , V=CV.I.W/R.T Substitutue T=Ts=Cv/2.5Ca in above eqn therefore V=(CV.I.W/R)X(2.5Ca/Cv) i.e V=2.5Ca.I/w (the maximum base shear that ETABS and UBC97 uses) Therefore time period regardless of its shorter value below Ta cannot be lesser than Ts which is in contrast with UBC97 provisions that are inherent in ETABS. However, analysis indicating time period lesser than Ta indicates a over stiff/over design structure, therefore it is recommended to maintain a natural period closer to 0.1x (no of floors).

W salam, In monolithic construction , T or rec beam is not a matter of choice of designer but its indicated by analysis that if depth of compression block lies below flange (flange = slab thk in beam-slab system) then the compressive force in concrete is balanced by a certain width of flange + total width of web, otherwise only web balance compressive force i.e example of rec section carrying slab load in monolithic construction. Example of rec beam sections also exists in some specific cases of construction for eg RC beams supporting metal deck system,hollow core slab panels and precast construction system. ETABS checks the eqn "a < or > ds" for maximum analyzed Mu-bott and design positive R/F accordingly.

Dear waqas, Calculation of Ie for live load only doesn't make sense as live load will always act on the section after the application of dead loads and will act simultaneously with dead loads. Therefore , Ie must be calculated for possible combinations that could be 1.0D,1.0D+0.5L & 1.0D+1.0L as per ACI-435. Detailed calculations of deflection for 1-way NP flexural members as per ACI 435 is shown in attachment.

In a multi storied building with post tensioned floors, What could be the effective way to model or include secondary (hyper static) forces on vertical supporting elements i.e columns & walls due to post tensioning, that seems to be typically includes inplane diaphragm force (P) & moment due to eccentricity of tendons to cross sectional centroid of slab (P.e). Kindly share your views and practical approach towards it.

Auto meshing doesn't ensure adequate connectivity between member to member and is therefore recommended for horizontal area elements enclosed by line elements only (floors), where there is no structural connection between floor and any element in between the panel. In case of vertical elements, connectivity between vertical and horizontal elements is of due importance and is better to be achieved through manual meshing. In case of auto meshing as you indicated, change in size of auto mesh could solve the problem as its possible that connections inadequate (nodes not coinciding) on 1m element size can be adequate for 1.2m size (nodes start coinciding) and so on. For p-delta,a possibility exists that due to any meshing error some connection is modelled with inadequate lateral stiffness i.e when program try to impose lateral deflection due to seismic loads,modes start yielding frequency below shift. If you are interested in studying the problem,then easy approach is to check each mode shape and investigate the member that is going in unrealistically large displacement. Solving this member's connectivity inadequacy will solve your problem.

This message is indicative of some modelling error in your model as Shift is the centre of cyclic frequency range that is used to limit the modal frequency range i.e to enforce program to neglect modes with frequencies below shift,By default shift is a very small value something like .0001,any mode with eigen value lesser than that of shift means a mode with a very low frequency or a very high modal period that indicates some modelling most probably member connectivity error in your model.

Yes you seem to be correct.I am not familiar with ldh restriction in zone 4, I will check it.

Regarding point # 3, suppose at any support strength requirement shows 3 #6 bars,here to counteract ldh you provide greater area i.e greater bars for ex if you provide 6 # 6 bars then ldh should be multiplied with As-3#6/As-6#6 i.e 0.5. Concept behind this provision is that if at any support say half ldh is available for stressed bars then these bars will develop half of their strength through bonding in concrete and will be able to take stresses till 0.5xFy,if you add up same no of bars with same ldh then they will also bear stresses upto 0.5xFy and as a whole you will get the required resistance Fy.

Waqas, First note that development length in indicated case is only the straight length as shown in figure and doesn't include bent portion and secondly this requirement can be fulfilled as follows, 1,you can use greater no of lesser dia bars complying ldh requirement and if beam width is insufficient to accommodate greater no of bars then you can distribute them in effective flange width (ACI 318-11 section 10.6.6). 2. you can multiply ldh by 0.8 if you tie hooked end reinforcing bar with ties parallel or perpendicular to the bar, spaced not greater than 2db (db = hooked bar diametre).(ACI 318-11 figure R12.5.3). 3, You can provide excessive reinforcement and multiply ldh by As req/As prov. 4, You can multiply ldh by 0.7 by maintaining minimum side cover to the longitudinal bar to 2.5".

Uzair, See chapter "foundation structures" topic foundation structures for frames.

Asad, It is not necessarily required to extend the seismic analysis's storey range till basement levels as for buildings with several below grade levels supported by basement walls, two stage static analysis procedure is used (ASCE 7-10 Section 12 & UBC97 section 1630.4.2) that consists in distribution of building in flexible upper portion (above basement levels) and rigid lower portion (basement levels), provided the lower portion have a stiffness minimum 10 times greater than upper and time period of whole structure should not exceed 1.1 times of flexible upper portion's period while it is considered as a separate structure. You can simply check these limitations as, 1, by computing stiffness ratio (EI/L ratios of basement walls + LFRS in rigid lower portion) to the (EI/L ratios of LFRS in flexible upper portion) 2, computing time period of whole structure (Eigen vector) and computing time period of upper portion alone modeled without basement levels. Having satisfied these, seismic analysis is required to be performed till base of upper portion only & rigid lower portion is required to design only for seismic forces transmitted at the base of flexible upper portion modified by the factor Rupper/Rlower. In ETABS you have to define "ground level" as bottom storey in analysis storey range and seismic shear imparted on ground level will be automatically transmitted to the levels below through diaphragm action.It will be just required to compute "R" value for lower portion considering it separate and to modify seismic load case's scale factor by Ru/Rl for the design of below grade structure. In this way the maximum seismic shear will be acting at the ground level not at B4, that will reduce the magnitude of force and could be beneficial in mentioned below grade serviceability issues particularly drift will be considerably reduced (also compute drift using user defined time period obtained from eigen vector analysis see UBC Section 1630.10.3). As long as below grade torsion is concerned, it is just required to satisfy that Ax (UBC97 Eqn 30-16) should not exceed 3 and required to be noted that amplification of diaphragm eccentricity is of no meaning there since seismic forces are imposed from upper portion and are not calculated & applied separately. Secondly, load combinations should be inclusive of minimum seismic vertical effects and dynamic load combinations.

It is applicable to SMRF i.e beams & columns and structural walls with their coupling beams.In general all members effective in resisting lateral force.