Why do I sometimes need to add oversized mounting plates to restraints attached to concrete
ANSWER:
The seismic restraint manufacturer has probably designed the steel restraint so that it is stronger than the material it is anchored to. A good example is when there is a requirement to anchor to a thin layer of lightweight concrete. The concrete may not be strong enough on its own to hold the anchors. The addition of a larger mounting plate or embedded steel plate will distribute the forces over a larger area.