Concrete Tilt-up Addition

[ivanga7]

We are doing an addition to an existing 18 feet high concrete tilt-up building with a flexible roof diaphragm (See attached for layout). There is an existing re-entrant corner with its respective drag lines. Our addition consists of squaring off the building by adding to new walls.

My question is:

Is there a way to neglect the lateral resistance of the walls that make up the re-entrant corner and assume that the outer walls of the newly created box will resist the seismic loading?

 

[JAE]

I don't think you can neglect it if it indeed is connected to the diaphragm and has stiffness. The load will always follow the stiffness.

By adding the corner, you are increasing the lateral demand on the drag lines and associated shear walls so you'd have to simply check those with the added load.



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[msquared48]

By the position of the interior walls respecting the new walls, I do not think the existing drag lines would be affected, but the interior panels will. I am assuming the drag line links are at the interior corner at the existing walls.

Also, the diaphragm chord forces may or may not be impacted. Check those too.

Mike McCann, PE, SE (WA)

 

[ivanga7]

That's what I thought, I was just trying to see if I could simplify my work.

Thank you for your input.

 

[KootK]

Is the goal to avoid having to evaluate the existing drag lines or is it to shelter them from new load that would cause overstress? If it's the former, then the only strategy that I can think of would be to install bracing against the re-entrant corner and set it up as two separate buildings. If it's the latter, there are a couple of options:

1) If you can prove that the re-entrant walls would fail in a ductile fashion, you might be able to redistribute load out to your new walls. I'd think the odds of this working out to be pretty low.

2) If the individual wall segments of the re-entrant corner are stitched together to act compositely for lateral, you could remove that stitching and thereby reduce the stiffness of those walls. Folks on site may think that you've lost it.

The proportions of the building being what they are, I wonder if you might be able to justify rigid diaphragm behavior, even with an untopped steel deck. I'd think that a rigid analysis would shift a good deal of load away from the re-entrant corner walls.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[ivanga7]

KootK,

In a way I was trying to avoid both, but given the existing conditions I won't be able to take any shortcuts and I will just have to analyze the re-entrant corner and drags for the additional loads.