Grade beams supporting concrete shear walls 2

[Okiryu]

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.

 

[Okiryu]

One more question, I think that grade beams are commonly used between pile caps, but does grade beams can be supported between footings? I guess it will depend on the restraint conditions at the end of the beams?

 

[wannabeSE]

Terminology differs around the world. In my area, grade beams typically require more reinforcing than continuous footings and the extra reinforcing is more expensive than any concrete savings. There are no rules that say you can't put grade beams on spread (isolated) footings. Grade beams are used with fixed base moment frames with or without piles.

 

[Okiryu]

wannabeSE, thanks for your reply. Do you consider the soil below the beam when you design it?

 

[jayrod12]

Not if you're spanning between piles or isolated footings, then you would typically provide some form of void below the grade beam.

 

[Okiryu]

Thanks. Reason why I am asking this is because the site may require some kind of soil improvements. If the shear walls can be supported in grade beams spanning between footings it will reduce a lot costs in terms of earthwork (soil improvement). So, soil improvement will be required only underneath the footings and not under the shear walls.

 

[BUGGAR]

Sort of similar: A Geotech encountered a soft spot below a conventional spread footing design during construction. He didn't change the footing design but widened the wall at the soft spot and put in a full cage of rebar, stirrups and all, to bridge the weak soil and transfer load to adjacent good soils.

 

[Okiryu]

Are there any benefits if large allowable bearing pressures are provided after soil improvements? My guess is that with large allowable bearing pressures, footings get smaller and less stable against eccentric and lateral loading.

 

[KootK]

A grade beam can support a shear wall between footings. Some do this intentionally even with continuous footings as a way to better distribute loads and control the vertical location of plastic hinging (if that happens). There are a couple of important things that should be considered by the EOR however:

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.

Are there any benefits if large allowable bearing pressures are provided after soil improvements?

Smaller, cheaper footings and perhaps less excavation.

My guess is that with large allowable bearing pressures, footings get smaller and less stable against eccentric and lateral loading.

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.

 

[dik]

you might consider the grade beam to be fixed at the edge of the wall... any dowels from the wall to the grade beam will likely be in tension.

Dik

 

[Okiryu]

Kootk, thanks for the reply.

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?

 

[Okiryu]

dik, I just saw your reply. That's a good idea to deal with comment #2 presented by Kootk...

 

[KootK]

Kootk, thanks for the reply.

You're very welcome.

Based on your comment #2 above, appears that a grade beam creates a hinge at the base of the wall, which complicates things.

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.

If you have a continuous footing supporting the wall, is that wall base considered fixed?

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.

That's a good idea to deal with comment #2 presented by Kootk...

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.

 

[Okiryu]

Kootk, thanks again. After re-reading from your previous post, it is more clear now.

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.

 

[Okiryu]

KootK, let me ask one more question: I understand that we will have anyways plastic hinges at the base of the walls, but even with a continuous footing supporting the shear walls, are we getting these monster shear loads? Also, are any other advantages/disadvantages between using grade beams or continuos footings for supporting shear walls? Thanks.

 

[hokie66]

There is a third option: no strip footing or grade beam, just the walls spanning between footings or pile caps. If the walls are cast in place, a blinding layer under the wall may be all that is required in certain cases.

 

[Okiryu]

hokie, thanks. I will pass that option to the structural. However the footings are spaced 9 m apart. Can this option still viable?

 

[hokie66]

For vertical loads, a reinforced concrete wall acting as a deep beam can easily span 9 metres. Using a grade beam in effect just means the wall is cast in 2 parts, the grade beam being the part with the bottom reinforcement. For lateral loading, an assessment is required as to whether the no footing method provides the strength required.

 

[Okiryu]

Thanks hokie, this info is very useful. I will talk with the structural next week and see what they say. I will pass all the information I learned from this thread. Thanks again.

 

[KootK]

I understand that we will have anyways plastic hinges at the base of the walls, but even with a continuous footing supporting the shear walls, are we getting these monster shear loads?

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.

Also, are any other advantages/disadvantages between using grade beams or continuos footings for supporting shear walls?

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.