Length of Wall Effective Either Side of Opening

[Engrman]

In a cast-in-place conc wall containing openings, the wall above and below the opening has to span horizontally under lateral load to the full height portions either side. How much of the wall either side of an opening is considered effective? 2'-0"? Some multiple of wall thickness?

 

[AlohaBob]

I think the clear span is the starting point. The full height walls act like stiff columns rigidly connected to the wall header over the opening. Consider the stiffness of the header by clear span and its measured depth for elastic section properties.

 

[JedClampett]

This is one of those questions where we could argue for all time. I think you're on the right track in relating the effective thickness to the wall opening size. One to two times the thickness seems reasonable.
The typical detail where I work adds the interrupted reinforcing in an area one-half the opening size on each side. Every companies' typicals I've looked at has a similar detail. Somehow this works, even though the analytical basis is a bit vague.

 

[Engrman]

Howdy AlohaBob,

Either you didn't understand my question or I don't understand your answer. I'm not asking about design of the wall above the opening. A strip of wall either side of the opening resists its own loads and those contributed by the wall above and below the opening. What is the length of that strip?

 

[Engrman]

Howdy Jed,

I've seen that typ detail as well. It's the same principle as with wood framed walls. Maybe I'm wrong, but to me that would work only for small openings. When you get to something like large overhead doors I think you need something a little more scientific to avoid congestion at the jambs. Normally I would put pilasters there and not worry about it. But, the contractor is using a pre-fabricated remain-in-place extruded plastic forming system and since the system does not include column forms he wants to avoid them if at all possible. I can't find anywhere in the code where this is specifically addressed. Maybe I can use the tbeam effective flange criteria as a starting point. My thinking is that you can use whatever strip length you need, within reason, as long as the horizontal rebar is properly developed and engages all of the verticals. Any thoughts or comments?

 

[apetr26542]

If this is a uniform wall, a finite element model might be more accurate in determining the distribution of this opening and how it is delievered to the wall portion, or you can be conservative and try 2x the wall thickness and provide additional reinforcing if needed. It sounds like this is a plate element with a hole, perfect for a finite element model.

 

[JedClampett]

Our detail is limited to 48 inch openings unless special analysis is done. A garage door would require a special detail.
I'm not familiar with your forming system. However, your problem reminds me of designing tilt-up. In that case, don't be afraid to put in two layers of reinforcing locally. The "Tilt-Up Design and Construction Manual" by Hugh Brooks has a procedure for designing openings. If you can't get two layers of bar in, the contractor might have to break his forms and pour in a column.

 

[AlohaBob]

For lateral load, stiffness of the elements is going to make end moments in the header. That's what I thought you were asking. For those forces, inducing axial load in a shearwall, a boundary element may need to be designed. See ACI 21.6.6. Let this control effective width of your column.

For effective width of jamb in a wall for vertical load, There's a bit of interactive design goes with this. Try a square in wall column first. When I get to a point 3x thickness perhaps and can't make the column design work, I start increasing the width with a square column jamb as required. I use the slender wall analysis.