Grade beam design 3

[jenofstructures]

I'm having a problem on a certain project site because it is swampy

do you have any good reference for Grade beam or tie beam design?

thank you

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[msquared48]

I assume you are placing grade beams over pile. Just desighn them at 2 or 3:1, depth to breadth, and try to place the pile in such a spacing so you can get away wiyth one grade beam section - except for brick support, etc.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jenofstructures]

how about if there is no need of piles, but we really need to have just a grade beam

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[rscassar]

What is the magnitude of load you need to resist and reactivity of the soil.

 

[Lion06]

I always thought the point of a grade beam is that you're not counting on the soil for support. If you're counting on swampy soil for support then it's just a strip footing, not a grade beam.

How are you using shallow spread footings for those (swampy) soil conditions?

 

[jenofstructures]

the senior engineer told me that we need to use a grade beam for walls because the soil is not that good....

from what I understand, grade beam can be design just like the ordinary beam.... But I still want to find a reference so that I can justify this

Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[msquared48]

There is a basic difference between a grade beam and a strip footing.

Grade beams are used between piles to support structure above.

Strip footings are used to support walls bearing on soil, unless that wall is also being supported on pile, then the wall sits on a grade beam.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[TXStructural]

Grade beams are designed like any normal beam. Since there is only one, not a bunch of repetitive members. Design the longest/most heavily loaded, and make them all the same. Do not change each span, it is unnecessary. Your section should be proportioned something like 24"w x 36"d or 18"w x 24"d, and designed as a continuous beam across your piers/piles.
Reinforce the same in all spans, unless there is significant span or load difference. I usually design with the same reinforcing top and bottom. As usual, choose the (largest, fewest) bars for maximum permitted spacing. (Like 4#6 T&B).

Usually, you end up with minimum shear stirrups (uniform spacing @ d/2) since your section will probably be large enough that Vc is sufficient. Use 2-piece closed stirrups, rather than 1-piece which are difficult to place.

 

[Lion06]

If you're using grade beams then you have to be spanning to something (most likely piles). A grade beam on soil that doesn't span to anything is a strip ftg (meaning it's not a grade beam).

As noted by others, it's no different than any other concrete beam.

 

[southard2]

You need to ask the senior engineer for a soils report. If he doesn't have one then make sure you are not being asked to use your seal on the project. I agree with everyone above on what the difference is between a grade beam and a strip footing. Try to have the geotechnical engineer give you a prescription for the pile designs such as diameter, reinforcement, and spacing. The depth of the muck will change your braced lengths, etc.... Its been a very long long time but pile design is a specialty and has its own design equations, etc....

Over expansive soils I have used grade beams without piles. In this case the difference between the grade beam and the regular spread footing is that you have top and bottom steel and stirrups if required. That way if you have a soft spot somewhere along your foundation it will bridge over it.

John Southard, M.S., P.E.

http://www.pdhlibrary.com

 

[csd72]

The only exception to the above definition is when you tie two eccentrically loaded footing together to take out the effect of eccentricity. The beam between these is often also called a grade beam.

 

[Splitrings]

I thought a beam between two footings for the purpose of redistributing the moment was a strap beam?

 

[Once20036]

I typically assume that the grade beams will be cast directly against the earth (even if we specify otherwise) and design with 3" clear cover, as opposed to 1-1/2" for beams. Otherwise, I agree with the above comments that it's just another beam.

 

[jenofstructures]

thanks for the reply

the concrete cover changes on the grade beam because it is exposed to earth

and yes we are actually asked for the soil report, that is why we are in need using grade beam for this project



Poems are made by fools like me, but only God can make a tree. engineers creates wonderful buildings, but only God can creates wonderful minds

 

[hokie66]

If you need grade beams, you need piles. The soil report has little to do with grade beam design, as the piles will be the founding elements.

 

[csd72]

a few things to note for grade beams.

As the spans are often short then the end sections where shear does not need to be designed for become significant.

If they are very short compared to depth then you may find that you need to analyse it by strut and tie method.

The width is often dictated by the size of the piles/piers/caissons under.

 

[TXStructural]

Jen, from rereading all of your posts, it seems that there may be a misunderstanding of terms.
A grade beam is used to span between deep foundation elements (piers, piles); it is only cast against soil where the soils are not expansive. Grade beams act to carry loads to the deep foundations and are not designed to bear on the soil, even where cast against soil. With expansive soils, a void must be maintained below the grade beam to prevent the expanding soil from lifting the grade beam and compromising the deep foundation (and the structure.)

Strip footings are used to bear loads directly on soil at or near grade, with each increment of footing transmitting the load applied to it to the soil in the same increment. A strip footing may also have a sort of grade beam on top of it acting as a stiff element to distribute loads relatively equally along the footing.

 

[rscassar]

A few things that you will need to consider:

1. It will be more expensive to bore piles than to provide a deeper grade beam so it may be worthwhile to use 24" piles opposed to 18" and have the grade beam span further.

2. If the soil is reactive, consider the construction sequencing. What I have done in the past is use collapsible cardboard void formers under the slab as sacrificial formwork. For a 28"x12" grade beam, the first 20" of the grade beam is placed first with all shear ties crossing the horizontal construction joint. Between the grade beams is then backfilled with a free draining material before the 7" slab is pour on cardboard void formers with the 20"x12" grade beam supporting the slab pour.

If you don't get the construction sequencing correct and the cardboard void formers take on moisture too early than the pour could fail epically.

It is would be advisable to include test certificates from the cardboard void former manufacturer and an installation procedure. Also specify the horizontal cold joint to be rough (but not too rough).

 

[BAretired]

kikflip,

I don't think the grade beam mentioned by jen is supporting a structural slab, although maybe it should be if the site is swampy.

I typically use 24" deep grade beams with thickness equal to the wall thickness but not less than 8". Under the grade beam, a compressible void former or "cushion" is placed to prevent heaving of the beam. Piles are used to carry the grade beam reactions.

Design of grade beams is no different than design of any concrete beam except that top and bottom reinforcement is normally continuous with laps of bottom steel at piles and top steel at midspan.

BA