Gif file not mine. Credit goes to whoever owns this.webp.

Continuing our exploration of the different lateral analysis models, we will now discuss the wind drift/lateral displacement model.

Regarding wind load, one must ensure that the applied wind load is in service level when checking displacements. So our equation should be gross/uncracked modifiers = service wind loads.

This entails that we either use the stiffness of gross geometric properties of members or 1.40 times that of cracked properties of clause 6.6.3.1.1 of ACI 318-14. That is if our structure is made up of concrete.

In steel structures it’s easier: you don’t mess at all with the modifiers.

Let me reiterate: service wind load should correspond to “uncracked” member properties.

Aside from property modifiers, it is important to know that wind drift is the super structure’s response, ergo if you have springs and the basement level from the base model you have to remove it when analyzing wind displacements.

What is the significance of this model?

You might’ve gotten too amped after modelling and loading your structure that you proceeded right away to check the design, and told yourself after which that that’s all to it. Of course, the individual members should pass the design of combined gravity and lateral loads (both seismic and wind) BUT there’s more to that.

There are structures that are more susceptible to wind loads than seismic like flimsy, lightweight structures such as billboards or towers. And it is in these types of structures that serviceability requirements need to be compliant to set limits. These are the induced lateral displacements that need to be kept within the threshold.

And why is this important? First, excessive vibrations would make the occupants go bananas thinking collapse is imminent. Second is to ensure that brittle attachments such as façade and glazing won’t “explode” when the supporting system is swaying like crazy.

So keep in mind that members that pass strength design do not always guarantee that the structure will perform well under serviceability requirements. In other words, you must check both the lateral displacements under wind load and strength requirements. Because when you fail in one, you fail both.