how to make sure concrete wall doesn't take shear in a frame

[Rainbowtrout]

This has confused me for a while. I have a concrete frame with beams and columns (2'-3' SQ). I am detailing concrete walls (9") flush with the outside face of columns. All wall reinforcement will have a developed hook in columns/beams.

My question is: If I am only designing the concrete walls for out-of-plane loads, does it mean I am being un-conservative because I am ignoring the in-plane shear force potentially in the walls from the beams and columns? How do you typically design for this?

Thank you.

 

[jayrod12]

provide expansion joints where it would be contacting the concrete frames.

Design the rebar to span across the expansion joints for the out-of-plane loads.

Depending on the wall height and out-of-plane loading it may be easier to design the walls as cantilever off of the floor structure and not fasten them to the frame at the sides and top at all.

 

[hokie66]

Rather than "un-conservative", I think you are being overly conservative in ignoring the walls. Why not use their capacity?

 

[KootK]

I'd either use the wall as a shear wall and design it for the anticipated loads or detail it as you might a masonry infill wall. Structures are notoriously bad listeners when asked not to participate.

Whether or not ignoring the wall's contribution is conservative is a murky issue. On the one hand, the wall's presence may draw load from other hard working frame elements. On the flip side, the wall may present a detrimental torsional irregularity if not detailed to be isolated from the intended lateral system.

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.

 

[Rainbowtrout]

Thanks for your thoughts! One thing that has been troubling me is the offset between the wall centerline and the column/beam centerlines. (Since columns are 2' SQ and walls are 9" thick, and they are flush outside.) How would this offset affect shear-wall behaviors? I couldn't find anything online so would appreciate any guidance.

For people that is familiar with STAAD, would you use "surface" elements with "offset" to model the walls? (Is it possible to set "offset" with "surface" elements"?)

Thanks!

 

[KootK]

One thing that has been troubling me is the offset between the wall center line and the column/beam centerlines. (Since columns are 2' SQ and walls are 9" thick, and they are flush outside.) How would this offset affect shear-wall behaviors? I couldn't find anything online so would appreciate any guidance.

I'll assume that you're still planning to cast the walls and columns monolithically as you described initially. In that case, some consequences off the offset are:

1) Shear in the wall will be delivered as axial force to the columns off of the column center lines, resulting in some additional moments to be considered in the columns.

2) Technically, the shear center of the column/wall assembly will outboard of the exterior side of the wall, similar to how it works with steel channel shapes. I wouldn't sweat the impact that this will have on your overall distributions of load throughout the structure however. The inaccuracies that we accept in performing elastic analysis more than outweighs any inaccuracy in the determination of the wall assembly shear center.

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.

 

[Rainbowtrout]

Thanks kootk. Your #2 point is what has been hanging on my mind. What made you say so?

 

[KootK]

What made you say so?

Sorry Rainbow, I'm not clear what you're asking hear. Can you elaborate?

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.

 

[Rainbowtrout]

@ KootK, I am interested in why you think the difference in shear center is not significant. Thanks!

 

[JLNJ]

I did a low-rise all concrete building with many concrete walls (low seismic). The walls were to be solid since it was a food facility. I figured if it worked as a shear wall and it worked as a concrete frame, then it would work if they were combined. Overly conservative? Maybe.

If it were a really serious structure I would have fought to make the walls independent, but the Owner didn't want any joints or gaps due to hygiene reasons.

 

[hokie66]

"if it worked as a shear wall and it worked as a concrete frame", then it works as a shear wall structure, not as a concrete frame. The loads always follow the stiffness.

 

[KootK]

@ KootK, I am interested in why you think the difference in shear center is not significant. Thanks!

1) As I mentioned above, inaccuracy in the location of the wall shear centre gets lost in the noise of more significant approximations that we make. One such approximation commonly used in modelling concrete is linear elasticity. Park and Paulay point this out in the shear wall section of their classic concrete book.

2) Presumably, you've got lateral resisting elements spread about your building that are separated by distances that are very large relative to the shift in shear center of your wall assembly. Therefore, an error in the location of the wall assembly shear center will not materially affect how your loads distribute to the various lateral load resisting elements present.

3) In your wall assembly, part of the stiffness is associated with flexural deformation and part is associated with shear deformation. The portion associated with shear deformation will remain close to the centreline of the wall no matter what interconnectivity is created with the adjacent columns. And, the squatter the wall, the more important a role shear deformation plays.

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.