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I'd call it a triangular building but the answer you are looking for is pyramid but that is NOT what a triangular building is called.
Generally, the horizontal members in a building are called beams. Beams can be girts, which hold up joists, headers, which are found above windows, plates, that hold up rafters, etc. Beams are usually structural members.
· beams · boards · bricks
They are triangular structures
steel beams
If the cytoskeleton is the framework of a building, microfiliments are the wood beams.
Steel beams and metal
connecting between footings
Steel
A square pyramid has a square base and four triangular sides.
square based is much stronger
An A-Frame