Two very simple yet often overlooked tools for the structural engineer when it comes to lateral load analysis are the plots of the story shear and story drift.
But before we get to that, what are these story shears and story drifts anyway? Story shear is the graph showing how much lateral (read: horizontal) load, be it wind or seismic, is acting per story. The lower you go, the greater the shear becomes (see figure under story shear below).
Story drift on the other hand is the plot of the resulting drifts per floor. Drift is a unit less number resulting from dividing the relative lateral displacement to the story height, and it shows how much a story is displaced by a given load. The usual limit for wind drift is H/500 while for seismic drift it is 0.01H. A graphic definition of a story drift is given below.
That settled? Ok, let’s begin our detailed discussion.
In story shear, you can visualize the possible governing lateral load on a certain floor at a given direction. And this will aid to understanding or investigating why wind governs over seismic on certain direction or vice versa. Aside from which, you can check whether something is “fishy” – or something just doesn’t seem right according to the guts of us structural engineers.
Cases in point include:
- Seismic base shear. Base shear has an upper and lower limit be it ASCE or UBC. Based on the above graph, I can now visualize and confirm that the base shear on both X and Y directions should be the same since the resulting Cs (seismic response coefficient) is less than the lower limit of 0.01 (ASCE 7-05 Equation 12.8-5)
- Why is the wind base shear along Y greater than that along X? Normally, the larger the area that the building catches, the larger or the more wind force it can attract, or simply, it’s a function of the tributary area. The yellow plot which is the wind force dominating the podium area (5th step down). You will notice that the seismic shears on its left is relatively small compared to the wind force and thus, load combinations containing wind load in the Y direction governs the design of walls at the podium levels.
Viewing the story drift can tell us many things about the structure we are designing. Sensibility checks that can be derived from the said graph are the following:
- The larger the drift, the less stiff the structure is. If the drift is greater on the X-direction than that of the Y-direction, the Y direction may be stiffer. And as such, you can start to trace whether this should really be the case by looking at the structural plans. If it says otherwise, then you will need to do a detailed check.
- You can view which specific floors require “strengthening” or which floor requires beefing up the stiffness. Does the roof sway like crazy? It’s completely possible if the roof area is less than the story below it. Does it sway excessively on the first level? If you have a multi-story structure then you might want to add shear walls or lateral bracings to address it. On the graph above, the wind drift on the 13th floor is excessive as this is the framing of the helipad walkway and it is made up of a steel framing.
- Serviceability is a basic requirement that aims to limit the story drift. Is the resulting drift within the prescribed limits? If not, then you might want to add a few stiffener walls if the architects permit or maybe suggest another framing that will work.
As I mentioned earlier, the story shear and story drift plots only aid the understanding of a building’s behavior when subjected to lateral loads. These are no end-all and be-all solutions that will solve or explain all related issues of the building by just plotting and looking at it. Detailed and other rigorous checks might be required.
I just discussed the importance of the said graphs and this does not include how to actually extract the forces and deformations from a structural analysis program. But I bet that since you are reading this, you already know how to. But just in case you don’t and you want to know, or you want to know anything structural engineering in nature, you can use the comment section below and I’ll try to answer what I can answer.