SDC D special concrete shear/retaining wall

[OPengineer]

I have a few questions regarding special concrete shear walls that are also acting as a retaining wall. It will be restrained at the roof and at the bottom of the foundation (we won't be doweling into the slab since it is only 4" thick). These are squatty walls with H/L<=2

Our geotech told us to use the Woods approach for a dynamic lateral force:
ftp://

http://www.nuquake.eu/AnalyzaSeizmi...ical Earthquake Engineering (Kramer 1996).pdf

Page 502/672

I've been told that this is an additional dynamic EQ load, however I do not know whether it is already service level (0.7E) or if it is ultimate (1.0E). I believe it is 0.7E (mainly because it was derived from the 70s and LRFD wasn't around).

Two, how should I go about designing the retaining/shear wall?
I was thinking:
-Design retaining wall (for non seismic cases)
-Check in plane what the compression is at the outer boundaries and get a 1 foot chunk. Design it as a column with moment from the retaining wall and shear from both directions (worst case will be at the base). Then pray I won't get anything that triggers boundary elements.
-Check in plane what the tension is at the outer boundaries and get another 1 foot chunk. Design it as a column with moment from the retaining wall and shear from both directions (worst case again will be at the base). Adjust steel as required.

I'm assuming I won't have to use the extra dynamic force (Woods) during the in plane analysis, only in plane + at rest pressures. Is this a correct assumption?

Does this sound like a reasonable approach? I'm only an EI in the Midwest so hearing some different advice from around the country is always good.

 

[KootK]

Tricky problem. So you've got a wall spanning from the basement to the roof without a main floor in between? How does that come about? Large stairwell? Subterranean atrium?

Your approach strikes me as pretty darn rational overall but I still have some concerns.

1) It seems to me that you'd need to look at a 70/70 non-orthogonal EQ case where seismic earth pressure loads acted on the wall in conjunction with in plane.

2) For your R factor to be valid, you need to shear yield your squat wall which means yielding all of your vertical bars. I have a hard time wrapping my head around whether or not you'll still have capacity available for resisting earth pressures in that state.

3) I worry about your roof deck if it's just bare steel. Seems as though you'd be dumping a lot of simultaneous in and out of plane load into it under seismic.

4) In general, I worry about the behavioural predictability about such an unconventional system.

Economy aside, I'd prefer to laterally restrain the wall at main and prevent any yielding from occurring below that level. A counterfort wall below grade with a pair of split, slender shear walls on top would feel much better. I imagine that would be a tough sell however.

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.

 

[OPengineer]

Well KootK, I'll elaborate a little more. We have two conditions. It's a 20ft masonry wall on top of a 8ft retaining wall. So the concrete portion is a squatty wall.

I was just thinking that if it were a masonry wall on top of a concrete retaining wall, there would essentially be a hinge at that interface. The cantilever retaining wall is so rigid enough that the masonry will behave as a pin from floor to roof.

In this area, the wall at the top is load bearing and the joists and seats will be designed for X amount of load.


Yeah, I believe the counterfort wall would be hard to sell.

 

[KootK]

Is this the story that we're telling? How long are the concrete and block walls? What's driving the decision to go wit a special shear wall?

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.

 

[OPengineer]

Very similar to that but it's a dock well instead of a basement, so soil is on the other side. The only reason why it's a special reinforced concrete shear wall is because in SDC D it is the only type of concrete wall that is allowed. The masonry above is a special masonry wall. Both have same R=5 factors.

I'm limiting it to the length of the masonry walls which are typically a minimum of 16ft in length, maximum 24 ft.

 

[KootK]

I'm limiting it to the length of the masonry walls which are typically a minimum of 16ft in length, maximum 24 ft.

Is the concrete wall longer than the masonry walls in reality? That could be helpful.

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.

 

[OPengineer]

The building length overall is about 400x250. About 300 feet of that 400 feet length is concrete wall on masonry.
We have contraction joints I believe at 2H of the wall, max joint spacing 24 ft. Construction joints are a lot longer than that.

 

[mike20793]

What value is controlling the seismic design category? Can you use the "stiff structure" exception to limit the seismic design category to being based on Sds only?

 

[OPengineer]

What value is controlling the seismic design category? Can you use the "stiff structure" exception to limit the seismic design category to being based on Sds only?

I've never heard of that exception before. Would you mind referencing that portion of the code?

Since it's a flexible diaphragm, am I allowed to even use that provision?

 

[mike20793]

It's listed in ASCE 7-10 Section 11.6. There's some criteria based on the fundamental period and the diaphragms have to be rigid but they can be flexible if the diaphragm is spanning less than 40 feet. I've used it on projects that have a lot of walls to get out of SDC D and into C since in my area, it's usually SD1 that pushes me into SDC D.