How to stop ramps acting as braces on ETABS 2

[Respawn]

How can I stop my ramp elements in ETABS from acting as a brace? The main ramp in question is the ground level ramp. It hinders the story drift ratio at that level.

 

[Colostruct]

I don’t know ETABS. But, could you model the base of the stair restraint as a “roller” rather than a pin? In other words no restraint in the “x” direction? I wonder if it should be detailed this way too for
Construction?

 

[jimstructures]

Respawn,

It has been 30+ years since I have run any ETABS models but in theory if you put in releases for forces and moments at any connections between your ramps and structures then they should not restrain or brace any elements in the model. I realize it may not be as simple to actually do this in the model. Let the ramp be a lump of mass on the floor and do not allow it to brace any part of the structure.


Jim

 

[jimstructures]

Looks like Colostruct beat me to the reply.

Jim

 

[human909]

Surely you have the question backwards. Surely the question should be

"How can I stop my ramp elements in ETABS from acting as a brace?"

 

[rscassar]

probably best to put in etabs forum. Etabs shells do have edge releases, need to find the edge you want and release the direction you are after. Or you can mesh a thin strip of slabs and then assign very soft stiffness modifiers in the F11 and/or F22 direction.

 

[271828]

Do you have a detail at the base that will prevent the ramp from taking axial load?

If the garage is multi-story, then same question for the ramps above.

 

[JoshPlumSE]

I have never worked directly on a parking garage project. But, I've always wondered the same thing. What do you do with your concrete detailing to prevent the ramp from being the main lateral force resisting brace? Anyone want to share any details.

I have gotten involved in the analysis side a number of times when engineers would ask me why their shear wall forces were so low.... or such. With the answers being what JimStruct and ColoStruct said.

 

[Colostruct]

I've not done a large parking structure. But, I did need to isolate a structure from an entry bridge to avoid temperature and unintentional "bracing" reactions on a retaining wall. I don't have access now to that detail. But, I used to design bridges in a previous life and I used the same approach on a building. Basically, I attached the bridge to the structure and used a bearing "haunch" on the retaining wall with teflon sliding plates. The tricky part is isolating the joint from debris.

 

[JoshPlumSE]

Thanks Colo.... Next time I'm in a parking garage, I'm going to take a nice stroll along the ramps and such just to see if I can spot these joints.

Note: This is the type of thing that I like about Eng-Tips.... Reminders to look at structures that I haven't thought about much in my career and expand my knowledge and understanding about how they get put together.

 

[Colostruct]

It is always good to get another input when designing. I detail and design by deflection when I layout a building or structure. The goal is to either reduce the deflection or accommodate the movement. I think your concern about story drift and unintentional bracing is a great example and reminder to all of use to keep this in mind. Another example of this is the unintentional bearing that might be provided by a partition wall in a building subject to even moderate deflection. It’s surprising how much strength a wall has...until it doesn’t.