Precast Moment Frame

by Micah Jacobson

I wanted to talk about a cool piece of structural engineering technology. It is called the precast moment frame. As we know from good old Norm, precast is a great building material. It has almost limitless appearances and material covering, it is very strong, goes up fats and required no on site forms (less labor cost). A disadvantage it has compared with in-situ (cast in place) concrete is the connections carry no moment, they just sit there. So you have very large, heavy and rigid members setting on one another. This is a almost worst case scenario for seismic design. In an in-situ concrete construction the rebar is continuous throughout the pores and creates a monolithic structure. All of the joint (almost) are moment connections and the whole building is one rigid structure. This works very well for seismic design.

The precast hybrid moment frame presents a solution to this problem. It uses a combination of standard reinforcing steel and post-tensioned steel cable (can be made of steel that has a Fy ,yield strength, of almost 300 ksi, that’s strong!). These steel strands are run through pvc tubes that are places in the concrete, they are un bonded (is some post-tension system the tube is filled with grout to create a continuous bond throughout the length of the member). They are tensions to be with in there elastic range, so when the building sways do to wind or earth movement they can stretch (strain in engineering terms, remember δ=∆ L/L). This acts like elastic bands to bring the building back into its original position in these events.

So you can have a precast structure, that has the benefits of moment connection, Norm Lock would be proud! And it’s all because of steel, John Dobbins would be proud too!

Picture Link:
http://www.gostructural.com/magazine-article--11-2011-how_the_precast_hybrid_moment_frame_works-8596.html