Leaderboard

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