If you add a section onto an existing building, does that section need to resist lateral load? 1

There are other requirements. The building must function as a complete system and meet all the respective code requirements. If you are adding to a structure you need to analyse the interactions between the new elements and the existing.

When it comes to lateral load you need to consider the loads, the location of the loads and their relative stiffness's. In brief if your new structure is less stiff laterally then it will likely impose lateral loads on the existing structure. If it is more stiff then it will likely attract lateral loads which could be very significant. You can somewhat avoid the transfer of lateral loads if you provide separation joints between the two structures. But that could have its own challenges if differential deflections are high.

The above is an overview of a thorough analysis. The easy way is to attach your structure and rely on the lateral system of the existing larger building. Of course you have to be sure there is sufficient capacity in the existing lateral system and the load paths to it.

@human909 thank you that makes sense. When you say "attach to the structure" could you point me in the right direction for how to do so or explain a bit further? How is a new section typically attached?

reverbz said:

@human909 thank you that makes sense. When you say "attach to the structure" could you point me in the right direction for how to do so or explain a bit further? How is a new section typically attached?

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That really depends on the structural system being used. I'll talk about steel as that is most of the context I deal with. If you have a 25mx25m building in plan on a 5m grid pattern and are extending things in one direction by 5m then it might be as simple as adding 6 new columns spaced 5m off the side of the building and then attaching each level together. By attaching I mean connecting the new beams to the old columns eg through welded cleat plates of clip angles. You might need a floor diaphragm or floor bracing to ensure lateral loads are transferred.

If you had a concrete floor system then "attaching" could be doweling things in assuming that can calculate out.