GRADE BEAM DESIGN

[tbone73]

Our office has had much debate about this one.

The scenario is you have a grade beam spanning from drilled pier to drilled pier, supporting more rigid load bearing concrete walls (several levels) above. Do you design the grade beam taking into account the stiffer wall elements above will deflect less than the beam and try to transfer the loads directly to the piers? Or do you design the grade beam as it if is transfering the full load to the piers? The two approaches can yield significantly different grade beam design results, not to mention pier reactions.

Your thoughts & input are greatly appreciated.

 

[frv]

We don't even design grade beams underneath the walls.

The walls are more than capable of spanning between piers.

 

[PUEngineer]

I would agree with frv. If anything they should be able to carry the weight of the concrete pour and formwork. Although the soil is most likely able to support that amount of weight.

 

[DST148]

In general, agree with above two responses. But sometimes the grade beams need to be formed due to underlying services and utilities. The contractor typically strips the grade beam bottoms, adds re-shoring, forms the walls and the slabs, pour walls and slabs in one shot, strips slab after two days, reshores the slab..... a common scenario. Moreover, many times the contractors assigned for below grade, below cellar work, are different from the ones who take up superstructure work.
It is difficult to predict field conditions and means and methods the sub-contractor may employ during construction. We always design grade beams to support load from at least one or two storys without counting on the underlying soil.

 

[BAretired]

In my area, due to potential frost heave, grade beams are always underlain by a void form, usually four or six inches thick. The void form is capable of carrying the weight of the grade beam without too much settlement but not much more so the grade beam must able to carry the wet concrete above it.

If the first story wall is continuous, it may be relied upon to support the upper stories. If there are door or window openings, engineering judgment is required as to what portion of load is carried by the grade beam.

BA

 

[msquared48]

One reason for the presence of the grade beam over a drilled pier, similar to a pile cap, is to allow for variance from the plan of the actual drilled pier location. The grade beam is able to be placed over the pier, not necessarily centered over it, but directly under the wall, serving as a lateral location adjustment for the foundation system. It also serves as an expanded area for the anchorage of the wall to the foundation system if needed.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[haynewp]

I have used grade beams with void forms below (may not be your case) and I would design the beams to carry an amount of the wall weight as it is being poured (I would note this on the drawings), subsequent to the walls reaching some cured strength level they should be able to carry the additional walls poured above and the floor weights if the walls are detailed for continuity between piers.

 

[slickdeals]

I agree that the grade beam may not be needed and the wall should be able to span.

We once had a building plans reviewer, rightly so, do a strut-and-tie analysis for a planar shear wall that was supported by pile caps at each end. A finite element stress analysis indicated that this wall acted as a purely compression member until the first floor (about 10 feet) high and then as a deep beam, looking at the compression struts. The wall was 30 feet long. We ended up providing some hooked #8 bars at the base.

 

[tbone73]

Thank you all for your comments. As always it's appreciated. Personally I think the design falls somewhere in between the two approaches. The grade beam will take some load, as much as its stiffness allows, and the stiffer walls will transfer more of the load to the piers. That is not to say that the walls above could not span between the piers without help from the grade beam.