Why do I sometimes need to add oversized mounting plates to restraints attached to concrete
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.