Seismic Structural Separation and Expansion Joint Hardware

[sam4106]

I have a moment frame structure divided by expansion joints. I've sized the required structural separation and have provided this information to the architects for expansion joint cover selection. They've asked me to clarify the movement criteria needed to select an assembly.

For example, if the maximum inelastic displacement for both structures is 3.5", SRSS says I need a 5" joint. The architects chose an assembly accordingly but the minimum opening size listed on the products performance data is 1.00". So, 5" - 1" = 4"/2 = 2". This doesn't accommodate the two structures moving 2.5" towards each other and definitely not 3.5".

So, how are we to understand what the SRSS method is telling us regarding the direction and magnitude of movement for two adjacent structures? Additionally, does the expansion joint assembly need to be sized to accommodate that total movement? Since the assembly is non-structural and we expect inelastic behavior during an earthquake, is damage to that component acceptable?

Thanks in advance!

 

[milkshakelake]

This is an article discussing that. Another one, more recent.

Some approaches:
1. A conservative way is to calculate maximum displacement using deflection amplification factor for both buildings.
2. The way I've done it before is to consider maximum displacement without deflection amplification factor, since it's unlikely that both buildings will be out of phase at maximum deflection at the same time. I'm not saying this is the right way; it's just how I've done it. Because when considering deflection amplification, the separation becomes massive and starts affecting the building layout.
3. Building codes have a minimum amount of setback per amount of building height.

This is an excerpt from the Seismic Design Handbook (2001):
While building codes place modest limits on drift (for example, 0.005 time story height) based on static analysis, actual experience with drift and calculations of realistic figures provide some startling numbers. Freeman(6-13) calculated the actual drift on flexible buildings up to 20 stories under 0.4 g acceleration as being 0.020 - 0.055 times the story height. For a 12 -story building this translates into 40-110 in. for a 14-ft story height. A separation that could accommodate two such buildings vibrating out of phase would have to be 18 ft. 4 in. wide.

I think that's a bit far fetched, but it goes over some case studies where pounding caused huge damage. I think your best bet is to read the articles I posted at the beginning. I personally would check if the expansion joint becomes super stiff at 1" (so it's like part of the slab and causes damage through the structure), or it just deforms inelastically and only the expansion joint gets damaged.