# Stiffest structure in seismic design?

Hi @coachmarissayee what is the most stiff structure in seismic design? I thought it was braced but I found that it’s shear is that correct? Is there a difference between stiffness and strength?
Like comparing moment frame, braced frame, and shear frame?

Hi @coachadamcoers do you get this question and can you explain it to me?

@sergioclaure93 there’s several different questions within this question, so i will do my best to break it down.

First - with seismic design, the loading that is controlling here is going to be your lateral loads (the loads that apply to the structure in the X direction, such as wind or in this case seismic.) So if you have a building in a high seismic area you’re going to need a framing system that is able to resist moment (the bending that is caused by applying lateral loads to a structure).

The difference between stiffness and strength is going how these frames react based on different loads. For example, strength is usually taking into account gravity loads (downward forces) such as building dead loads, and vertical live loads (snow, people, furniture, snow drift, etc).
Stiffness however is how well the structural system reacts to lateral loads. Think about it this way, if you build an 8’ tall x 8’ wide wall, but you only use a top plate, a bottom plate, and (2) vertical studs - if you push on it from the side, the wall will move to the left or the right and ultimately fail; now say you build the same wall, but this time you put vertical studs at 16" o.c. and add a layer of 1/2" OSB on one side of the wall… if you were to push on it, it’s not going to move most likely as it’s a much stiffer, more rigid structure.

This last example with the studs at 16" o.c. and plywood on 1 side is a great example of what is known as a shear wall. The addition of the plywood is what is strengthening this system and making it rigid. - these are going to be most common in bearing wall construction with either steel or wood studs.

A braced frame on the other hand is an example in a structural steel system. In this case, the frame is braced using additional steel members (for example, an X-Braced or K-Braced Bay). This is a more cost effective option for lateral bracing as you are able to utilize a standard pin connection (connections that are either bolted or welded) and additional diagonal members to help transfer the load. The connections between the beam and columns do not transfer any moment to the foundation. These are some of the most common types of lateral bracing systems in construction today.

The next option is Moment Resisting frames. In these systems the connections are designed to take the moment generated from the lateral load and transfer it to the building foundation without the need for additional x bracing. These frames are a lot more rigid and allow for more open spaces due to not requiring the additional braced members. These connections are often times welded and bolted connections are much more expensive and labor intensive to install compared to a more traditional braced frame.

Hope this helps.

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Ok great this helps, thanks.