Capacity of through bolts in concrete
Capacity of through bolts in concrete
(OP)
For bolts through concrete, loaded in double shear (equal both sides), and far from any edge, what design procedure would you follow? Assuming the bolt is cast in, or otherwise completely encased within the concrete matrix, would bending of the bolt have to be considered? Concrete bearing would obviously be important, but over what length of the bolt? Would edge failure of the concrete control, even if steel side plates restrain the concrete? Sorry to be so general, but just looking for advice on how others would approach the design.






RE: Capacity of through bolts in concrete
Mike McCann
MMC Engineering
RE: Capacity of through bolts in concrete
I would consider edge failure as a reasonable potential, even with confinement, since the edge stress is going to create tension in the concrete for a failure wedge.
RE: Capacity of through bolts in concrete
For a 3/4" dia. A307 bolt, (thread included in shear plane) in an 8" thick slab of 4000 psi concrete:
1) Factored shear 39.6*2 = 79 kN or 17,800#
2) Bearing area = 0.75*8 = 6.0 in^2. Shear stress = 2970 psi. CSA A23 would permit this bearing stress if clearance is adequate all around.
3) Mf = 17,800 * 8/8 = 17,800"#.
Z = (0.75)^3/6 = 0.0703 in^3 (plastic modulus)
Mr = 0.9 * Z * 36,000 = 2,278"# = 0.128 Mf (No good)
If bending is a valid consideration, the factored load would have to be reduced to 2,278# total, or 1,139# each side.
This seems ultra conservative, especially when compared to the shear-friction provisions of the CSA code. As long as the anchorage is adequate to fully develop the ultimate strength of the bar or bolt, I don't believe it is necessary to check bending.
BA
RE: Capacity of through bolts in concrete
RE: Capacity of through bolts in concrete
In case the concrete is old or weak I would also check the bolts as if embedded in masonry, just in case.
RE: Capacity of through bolts in concrete
When holes for thru bolts are drilled in the field they won't perfectly match the bolt diameter. Also when trying to match holes in steel members on both sides of a wall the contractor will likely drill from both sides and meet in the middle so there won't even be bearing full width. I have always wished someone would do some tests for this condition.
RE: Capacity of through bolts in concrete
If we were talking about drilled holes, I would be inclined to use a higher safety factor, but I thought we were talking about bolts cast in the concrete.
BA
RE: Capacity of through bolts in concrete
RE: Capacity of through bolts in concrete
To make things a bit more specific, I am talking about high strength bolts, say A325 or grade 8.8, 1 1/4" or 30 mm, in a wall 10" or 250 mm thick, 50 MPa or 7000 psi concrete. Cored holes a bit bigger than the bolts and epoxy injected.
I know, the use of higher strength (which may not help) and larger bolts aggravates the problem, and I think means the concrete will always control rather than the bolt strength, but it differs from a one-sided connection, so I don't know of any actual design guide which fits.
RE: Capacity of through bolts in concrete
I have a similar condition to check...
1 1/2" diameter grade 8 high strength bolt to be installed in a cast 1 5/8" diameter hole placed in a 14" thick wall subject to shear on one-side only.
I agree that bolt shear and bending, and concrete bearing are to be checked. The bearing length on concrete to resist bolt shear is likely going to be the within the first couple of inches of the wall.
I am not aware of a specific design guide for this condition of a thru-bolt subject to shear placed in cast or drilled hole
RE: Capacity of through bolts in concrete
So, start with an ideal case: the bolt is in perfectly "hard/rigid concrete" and cannot "wiggle" in the hole (it is cast in place or fastened with epoxy filler in the hole or inserted into a very tight hole, the load is applied in pure shear (in parallel to the surface of the concrete), and that load is applied very close to the surface of the concrete.
What can bend? How can it bend? It's simply not possible.
--
Real world: The hole is oversized or the concrete around the cast-in-place bolt deforms, the epoxy yields a little. The baseplate is pulled sideways and "up" away from the concrete (which yields a little bit) and thus there is a bending action on the baseplate. This pries up one half of the bolts on the "high side", and bends the bolts closest to the bend in the baseplate on the "low side".
Real world, the loads are applied up above the surface of the concrete, and those loads have a vertical component: usually much smaller than the shear component, but it is there. The baseplate holes are larger than the bolts = which allows the bolt to yield sideways a little bit, and so the bolt will be able to bend at the surface of the concrete: it can't be kept perfectly vertical.
But it is the prying action of a "flexible" baseplate in soft (or cracking) concrete that will cause the most problems.
With a sufficiently rigid baseplate, the yielding (failure) is usually in pulling the bolts up out of a hole out of their anchor (that, in this case, is resisted because the bolts are through the concrete slab), or in stripping the threads and pulling through the bolts on the high side.
Then the sign or post fails by fallin down, thus bending the bolts on the low (downwind) side. Yes, they bent, but after the initial failure.
RE: Capacity of through bolts in concrete
My design issue has not yet been resolved. Your problem with one-sided shear is more like a post installed anchor, and you can probably use Hilti's design methods.
racookpe,
The concrete failure in my case will control, but I would still like to have a design guide rather than depend on my own assumptions. The load is not a wind load, and the bolts are in pure shear. The small moment is taken in another way.
RE: Capacity of through bolts in concrete
The resulting moment is P/2*a/2 or Pa/4 which is the factored moment required to be resisted by the bolt.
That is consistent with ultimate strength theory.
BA
RE: Capacity of through bolts in concrete
RE: Capacity of through bolts in concrete
Put another wallaby on the barbie!
BA
RE: Capacity of through bolts in concrete
RE: Capacity of through bolts in concrete
BA
RE: Capacity of through bolts in concrete
BA
RE: Capacity of through bolts in concrete
Mike McCann
MMC Engineering
RE: Capacity of through bolts in concrete
I disagree that bending can be completely ignored as therecould be spalling at the edges. I usually allow for an inch of spalling. All these things are included in the chemical anchor capacities.
RE: Capacity of through bolts in concrete
When in doubt, just take the next small step.
RE: Capacity of through bolts in concrete