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"Assuming the diaphragm as rigid, there will be negligible in plane displacement of diaphragm with respect to its supports,which are vertical elements of lateral load resisting system, at particular story-level. Lateral displacement of diaphragms is with respect to story-levels. Hence, the support provided by slab to basement wall, at a particular story-level, can be considered as hinge as it will resist translational movement due to in-plane stiffness of rigid diaphragm." All written above regarding rigid diaphragm is correct,but in fact I am indicating the net displacement of diaphragm along with connecting LFRM rather than its in plane deformation relative to LFRM which is assumed as zero in case of rigid diaphragm. For eg if we suppose six basements of 12' each and assume that inter storey drift in each basement is 1" then net displacement of diaphragms w.r.t origin at 5th,4th,3rd,2nd & 1st level will be 1",2",3",4" & 5" respectively and as these basement slabs are acting as a horizontal support to basement walls therefore the support deflection @ wall's 1st,2nd,3rd,4th & 5th support will also be 1",2",3",4" & 5" i.e these slabs will not be acting as a rigid horizontal support to basement wall at storey levels but instead they will be undergoing lateral deflection along with wall.In this case the deflected shape and correspondingly flexural behaviour of wall will be quite different from the case of propped canti lever model that enforces assumption of basement slabs as rigid support with zero horizontal displacement.While propp canti lever assumption works adequately for the case of entirely confined basements from all sides but will not simulate the condition defined here due to absence of lateral confinement from other side. I have modeled both cases separately and have attached herewith deflected shape diagram extracted from both cases that indicates the difference in their behaviour and correspondingly in flexural forces. Why would someone be worried about out of plane displacement of basement wall, when it will be proportioned for out of plane forces( earth pressure). As it has been correctly been pointed out that drift limits in building codes are intended to control NON/STRUCTURAL damage, and to limit secondary forces due to P-delta effect. The point is: one should be worried about drift of frame (in/plane relative displacement), rather than the out of plane displacement of basement wall. It is correct that basement wall's out of plane bending is not a sensitive issue when it is proportioned for out of plane forces as you have indicated.Similarly all indicated above is about its correct proportioning w.r.t condition defined by Rana (significant lateral drift in below grade levels). I doubt if you can do that. The reason is that basement wall backfill would least likely to be compacted. Imagine doing that would increase load >> significantly which is not desired. Normally the lateral spring coefficients provided by geotech report are for undisturbed soil or soil well compacted. Your doubt is meaningful for shallow depth basements but I think in case of several below grade levels, soil bed could be reasonably compacted by overburden pressure from above lying soil itself as an example overburden pressure @ depth of 40' could be 4800 psf for a soil density of 120 pcf. This high overburden generally enables soil to create a massive passive resistance. For an eg passive resistance of soil for a basement wall of 200' length at same depth (in cohesion less soil) will be as follows, (Brinch Hansen's eqn), P=3.H.B.Kp.Density=3.(40)(200)(3)(120)=8640 Kipps that seems to be a considerable value. Dumped backfills would provide much softer springs that I would suggest be considered as void regions (Same thing as designing a pile for lateral). Above mentioned cases demand sound engineering judgement for consideration\unconsideration of lateral soil support but if loose characteristics of adjacent soil is certain,then all below grade levels (regardless of single\all side's confinement) should be included in seismic analysis and base location for seismic shear must be at foundation level that automatically includes all below grade levels in drift analysis without any side support. Thanks & Regards.

Okay, here is something important that I wanted to write but didn't had the time as I was travelling. I doubt if you can do that. The reason is that basement wall backfill would least likely to be compacted. Imagine doing that would increase load >> significantly which is not desired. Normally the lateral spring coefficients provided by geotech report are for undisturbed soil or soil well compacted. Dumped backfills would provide much softer springs that I would suggest be considered as void regions (Same thing as designing a pile for lateral). That is correct. However, If you have basement walls all around, then you get equal lateral pressure, thus net force on diaphragm due to lateral earth pressure is zero. However, I appreciate your comment as for the building being discussed basement walls are unsymmetrical. But, if you consider the lengths of basement wall and all the lateral force resisting system, the stiffness would be huge and I don't expect a significant response in unsymmetrical case, but, it depends on a lot of variables and number crushing in every situation is required before judging the situation. Good Catch though! Exactly! For Rana, I would suggest to keep modelling approach simple unless deemed necessary. For buildings with a number of basements, if you really want to complicate things, you should probably check your stiffness for basement below stories and basement above stories and use 2 different R values for both systems and apply recommendation in code for 2 different R systems. Thanks.

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It is good to see detailed discussion on a topic, and a very good way a modelling passive resistance of soil has been presented. However, I have reservations on some comments. In this case the support condition hinge (i.e zero horizontal displacement) cannot be assumed for basement walls at slab levels as diaphragms are undergoing considerable lateral displacement.Therefore, in order to evaluate flexure in basement walls it is advised to model a line element along the height of wall (meshed at wall area edges) to compute flexure in basement walls rather than using theoretical model of prop canti-lever assumption that seems to be invalid due to diaphragm's horizontal movement . Assuming the diaphragm as rigid, there will be negligible in plane displacement of diaphragm with respect to its supports,which are vertical elements of lateral load resisting system, at particular story-level. Lateral displacement of diaphragms is with respect to story-levels. Hence, the support provided by slab to basement wall, at a particular story-level, can be considered as hinge as it will resist translational movement due to in-plane stiffness of rigid diaphragm. Why would someone be worried about out of plane displacement of basement wall, when it will be proportioned for out of plane forces( earth pressure). As it has been correctly been pointed out that drift limits in building codes are intended to control NON/STRUCTURAL damage, and to limit secondary forces due to P-delta effect. The point is: one should be worried about drift of frame (in/plane relative displacement), rather than the out of plane displacement of basement wall.

Below grade seismic drift consideration. Considering building structure having several below grade levels and basement walls on a single (or two) sides, I like to share my views as follows, First of all drift limit states are intended to limit the damages in non structural elements in case of lateral movement and from structural point of view to limit the second order effects to a tolerable extent. Therefore, drift indexes defined in code are subjected to diaphragms rather than individual structural elements (like basement walls).It is advised to check serviceability considerations in accordance with diaphragm's drift limitation of code. As long as underground structural drift analysis with single side confined is concerned,two separate cases that seems to be required for consideration are as follows, 1, When structure moves towards the fill (passive condition). In this cases due to infinite stiffness of soil (assumption to be verified as defined later) adjacent to basement wall,displacement of soil will be negligible and correspondingly differential displacement of basement wall from bottom to top will also be negligible,that creates almost zero drift in below grade floors (diaphragms) if they are rigidly connected to basement wall (i.e the case of monolithic slab-wall joint with adequate reinforcement anchorage for tension directed in plane of slab). Modelling Strategy: This case can be simulated in model by assigning relevant basement wall throughout its height with compression only spring where spring represents lateral soil support.(Usually lateral soil support is modeled using P-Y curve).Then an application of seismic force in the direction towards soil will yield a realistic value of lateral deflection of soil (could violate infinite stiffness assumption) incorporating the effect of lateral soil support. In this case considerations must be given to the design of basement wall as it will be experiencing different earth pressure profile due to seismic excitation.Therefore it must be designed for this (seismic) pressure condition as well. 2, When structure moves away from the fill (active condition). In this case as there is no wall to confine structure on the other end,therefore the net drift will be Seismic drift + lateral earth pressure's displacement.Both of these forces will be unidirectional in this case and will be effective in diaphragm drift. Combined drift can be evaluated using loads combination from UBC97 section 1612.3.7 with exception 1612.3.3 (note that there "H" indicates lateral earth pressure). As seismic drifts are required to be amplified to represent inelastic effects then there amplified values must be used in above mentioned serviceability load combinations (could be done through scale factors) whereas drift due to lateral earth pressure doesn't need to be amplified. "2) I still did not get why there would not be any problem in basement for drift. I have multi-basements. Do you mean there would not be excessive drift between basement stories? I am getting it and it is more than allowable seismic drifts....guidance needed pls!" If this is the case i.e you have considerable inter-storey drift then an important consideration here must be the thickness of basement wall.In this case the support condition hinge (i.e zero horizontal displacement) cannot be assumed for basement walls at slab levels as diaphragms are undergoing considerable lateral displacement.Therefore, in order to evaluate flexure in basement walls it is advised to model a line element along the height of wall (meshed at wall area edges) to compute flexure in basement walls rather than using theoretical model of prop canti-lever assumption that seems to be invalid due to diaphragm's horizontal movement . It is recommended to design basement walls first (before analysis for drift) based on above methodology and model its accurate thickness, as the in this case basement wall is serving as a tie member between underground diaphragms and its thickness will effect differential diaphragm movement between different levels. "Sir but the point is that slab will distribute the lateral load to all supports. Like seismic or wind. Lateral load is a lateral load and that would keep the earth pressure drift to minimal." I think that lateral earth pressure for several below grade levels could be significant enough to be considered. For eg if we consider 6 below grade levels 6@10' with length of basement wall=200' and Ca=0.33 + soil density=120 pcf then net horizontal diaphragm force due to lateral earth pressure in bottom most (5th diaphragm) above foundation will be as, p(intensity@mid height b/w 5th& 6th level)=Ca.We.h=(0.33)(120)(55)=2178 lbs/ft2 p(intensity@mid height b/w 4th& 5th level)=Ca.We.h=(0.33)(120)(45)=1782 lbs/ft2 Force (tributary to 5th diaphragm per unit length of basement wall)=(1782x10)+0.5x(2178-1782)(10)=19,800 lbs/ft Net force tributary to diaphragm=200x19,800=3960 Kipps It seems to be a significant figure but needs to be checked as in contrast with magnitude of seismic force at same level. Thanks

Waqar sahib! Can you give some practical example, how a "conservative" design snow load value can be selected for a particular city? What should be the design snow loads for Quetta and Murree? IMHO, as structural engineers, we are responsible to design safe and economical structures. Using unlikely heavy loads, will result in larger structural members or more reinforcement, thereby making the structure uneconomical. As such, selected loads should be very realistic. In this purview, I am interested to know how the other structural engineers are selecting snow load values for various hilly areas of Pakistan? I think, seniors should share their experience here to tell us about what snow load values they have used for their designs?, and what criteria they adopted to determine the selected snow load value? This sharing of knowledge would certainly help the younger engineers to decide at least about selection of reasonable design snow loads, in absence of standard or code-recommended values.

snow fall data u may get from the MET department or use any conservative value based on the snow pattern of the area .

waqar sahib! Chapter 5 Division II of Building Code of Pakistan (Seismic Provisions-2007) provides the following information only: 1. Which load combinations are to be used for snow load. 2. How the snow load reduction factor can be calculated. It however DOES NOT tell us recommended values of snow load (in psf or kN/m^2 etc) to be used for the design of building roofs in various areas of Pakistan. Snow does not fall in all areas of Pakistan. Moreover, snowfall intensity is not the same in different hilly area. For example, Murree receives much more snow, as compared to Quetta. As such, design snow load for these two cities will certainly be different. I am interested to know where can I find design snow load values, applicable for structural design of building roofs located in Murree, Quetta and other hilly areas of Pakistan.

there is a newly established council for technologist or the persons who has Tech degrees PTC (Pakistan Technology Council) headed by VC UET Lahore,it would be working like PEC,technology degrees are good for those who have good execution experience in the industry,in Pakistan B-Tech is not valued much but abroad B-Tech get good jobs according to there experience. UET,Lahore offers B-Tech programes as Govt. universities are concerned up to my knowledge ,there might be other universities as well.many private sector universities offer B-tech Programes .