Grade beams supporting concrete shear walls
Grade beams supporting concrete shear walls
(OP)
Hi, a question from a geotechnical guy: I am just looking at some structural plans, and noted some 300 mm thick shear walls with loads ranging between 40 kN/m to 50 kN/m. These walls are supported in continuous footings. I was thinking that since loads are not too high, it may be possible to support these walls in grade beams? or shear walls should be supported always by footings? Any special reasons on doing one or the other? Thanks in advance for your input.






RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
1) The grade beam may introduce additional flexibility and impact building drift and period.
2) When shear walls hinge at the base, you tend to get a monster compression point load at one end and a monster tension load at the other. These may represent a rather high shear demand on the grade beam. For this reason, having footings or piles centered under the ends of the shear wall is a nice feature. You know, if you can swing it.
3) Depending on your seismic environment, the grade beam may well be an element that needs to be capacity designed for the forces delivered to it by the shear wall. Take those monster forces from #2 and turn them into T-Rex level forces.
4) If the grade beam ends at the end of the shear wall, you may have a detailing problem as the shear delivered to the grade beam by the hinging wall compression zone will tend to tear a wedge of concrete off of the end of the grade beam. Use small-ish bars there with hooks or other kick-ass anchorage. Better yet, if possible, run the grade beam a couple of feet past the wall ends.
Smaller, cheaper footings and perhaps less excavation.
This is possible but certainly not a given. In many instances, the bulk of the overturning and shear resistance will be coming in from the superstructure. The EOR should be able to advise you on this.
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.
RE: Grade beams supporting concrete shear walls
Dik
RE: Grade beams supporting concrete shear walls
Based on your comment #2 above, appears that a grade beam creates a hinge at the base of the wall, which complicates things. If you have a continuous footing supporting the wall, is that wall base considered fixed? It will be interesting if I can see a shear diagram for both options (shear walls on grade beams and shear walls on continuous footings).
Our footings are centered under the ends of the walls, so it may help. However we don't have piles (BTW, that is a nice idea). If there is not eccentricity, perhaps large allowable bearing pressures can help to deal with those monster compression and tension loads. What do you think?
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
You're very welcome.
There would always have been a hings at the base of the wall. The grade beam just forces the hinge to move from the top of footing elevation to the top of grade beam elevation.
Both arrangements are considered fixed. It's the big tension and compression forces that do the fixing. Since most shear walls are vertical cantilevers they are, by definition, fixed at the base.
It may be a good idea for other reasons but it does nothing to alleviate #2.
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.
RE: Grade beams supporting concrete shear walls
Also, BUGGAR's post above is interesting, how do you know how much additional reinforcement do you need to transfer the load in case you find soft soil spots? Does this depend on how much deflection (settlement) you can expect? Thanks again.
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
Often shear walls are required to hinge at the foundation but not always. Check with the EOR on that. If the walls are squat or the foundation is below a basement level, for example, this may not necessarily be a requirement.
There are a number of different ways that designers use to model seismic shear walls and the choice of modelling dictates the presence of the monster shear force. Some walls rely on their foundation system to creat a fixed connection to the earth. That's often the case with pile support and support on large raft footings. Other walls are allowed to rock and break contact with the ground. That's more common with narrow strip footings.
When there is no grade beam, the monster shear that would have presented as one way shear in the grade beam winds up manifesting itself as monster punching shear in the footings. Basically, if a plastic hing must form at the foundation level, then there's a large localized compression being delivered to the system somehow.
The disadvantages are cost, which we discussed above. The advantages are performance. Some of those include:
1) The grade beam can be made longer than the wall which can be beneficial in improving overturning resistance.
2) The grade beam helps to deliver the shear wall loads more uniformly and predictably to the foundations. That can help with issues like the punching shear issue that I mentioned above previously.
3) Where high levels of shear are being delivered at the base of the wall, that sometimes feels much better when you're delivering it to a stocky beam with stirrups etc. This is particularly true with piles.
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.
RE: Grade beams supporting concrete shear walls
Dik
RE: Grade beams supporting concrete shear walls
@dik, that is interesting, so you just assume a large point load acting at the middle of the span of the grade beam?
RE: Grade beams supporting concrete shear walls
Truly, I don't know enough to speculate meaningfully. It could be a rocking foundation, a squat wall system, a low seismic foundation, or a foundation beneath a deep basement. I wouldn't invest any time in this until you get clarification from the EOR regarding their intent.
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.
RE: Grade beams supporting concrete shear walls
RE: Grade beams supporting concrete shear walls
Not in the middle of the span, but at the ends of the shear wall... As I noted, any dowels from the grade beam to the shear wall are in tension due to the relative stiffness of the wall to the beam. The beam is designed, however, for any construction loading prior to the concrete setting... generally not significant.
Dik
RE: Grade beams supporting concrete shear walls