concrete wall effective flange 1

[haynewp]

I have a concrete room that is acting as the core of the lateral system of the building. Kind of like you would treat an elevator core but much larger. The dimensions are 24 ft x 60 ft rectangular x 14 ft tall. The walls are all 8".

In the shorter dimension direction, I have a sizable amount of shear and moment. I don't want to use a mat footing under the entire area for cost reasons, but rather use a 4 ft (or so) continuous footing under all the walls. But I am not sure what percentage of the 64 ft walls to include as the flange for the shorter 24 ft webs. (Kind of like the perpendicular to force walls are flanges of channels).

ACI 530 suggests 6 times the wall thickness for an effective flange in a similar type situation. ACI 318 has 6 times the flange thickness for a tee beam, which I am not sure if that would really apply here. And using a 45 degree angle from the top of the wall would give 14 ft of channel flange on each side, which seems like too much. Any thoughts? 6 times the wall thickness would only be 48 inches.

 

[WillisV]

haynewp, I would suggest looking to the seismic portion of ACI for guidance on this one - see ACI 318-09 Section 21.9.5.2

"21.9.5.2 — Unless a more detailed analysis is performed, effective flange widths of flanged sections
shall extend from the face of the web a distance equal to the smaller of one-half the distance to an
adjacent wall web and 25 percent of the total wall height."

So for your case, assuming your building is one story tall, your effective width would be around 25%(14')= 3.5'.

 

[haynewp]

Thanks Willis,

This is mainly for the design of the wall footings where I am trying to estimate how much the load will spread out without overstressing the soil as it has lower capacity at this site. It is hard for me to imagine this large wall system only spreading the load out 3.5 ft but I guess if that is what the code says...

 

[hokie66]

haynewp,

I don't see why you cannot use the 60' long x 14' high wall as a deep beam to spread your load onto the footing. You would need to consider how openings in the wall affect the stiffness, but I don't know any reason you would have to restrict the effective footing width to an arbitrary flange dimension. Maybe for seismic in the wall itself, but not the footing.

 

[haynewp]

Actually thinking about this again, I don't have ACI 318-09 but I am sure that ACI section must be addressing more of the boundary type elements for the wall design itself.

So maybe I was approaching the whole problem in the wrong way. For instance, I am thinking about how a standard 2 column combined footing is assumed to spread a column load out over a long length of footing. So why couldn't the same apply for my case? (the end of wall reaction from the moment spreads out over a long width of footing under the perpendicular wall). But how does that fit in with the limitation of effective wall length that you quote?

 

[haynewp]

hokie66, you posted at the same time I was writing.