Question about an x-brace detail
Question about an x-brace detail
(OP)
Hi,
I was wondering if I could get a quick opinion on the detail shown in the attached jpg. It's the connection to a concrete footing for the tension member of an x-brace. The steel rod is bent approximately 11° and welded to a steel plate that is attached to a concrete footing with wedge anchors.
I can show the rod and the weld good for the tensile load, but are there any further checks I should do because of the bend in the rod?
Thanks,
Cap
I was wondering if I could get a quick opinion on the detail shown in the attached jpg. It's the connection to a concrete footing for the tension member of an x-brace. The steel rod is bent approximately 11° and welded to a steel plate that is attached to a concrete footing with wedge anchors.
I can show the rod and the weld good for the tensile load, but are there any further checks I should do because of the bend in the rod?
Thanks,
Cap






RE: Question about an x-brace detail
divide this by 2d^2/6 to get the perpendicular shear stress in the welds then check the weld size for the resultant of the 2.
You will also get some tensile load in the anchors.
RE: Question about an x-brace detail
RE: Question about an x-brace detail
csd72 - I see what you are saying - but doesn't your 2d^2/6 concept assume that the tension rod is stiff in bending?...which it is not.
The rod isn't a rigid body and I would think that you'd get a very high stress concentration at the end of the weld - much higher than assuming the weld lines receive shear and orthogonal stress from a reaction eccentricity.
I see the weld simply unzipping starting at the left end and progressing down -
RE: Question about an x-brace detail
If the force in the rod is big you might "unzip" the weld before the rod can distriute the force along the length of the weld.
RE: Question about an x-brace detail
RE: Question about an x-brace detail
RE: Question about an x-brace detail
Thanks for your help, I appreciate it.
RE: Question about an x-brace detail
I would "guess" that all the force would initially all go into the first 1 inch of weld.
RE: Question about an x-brace detail
RE: Question about an x-brace detail
Assume the length of the weld is 1 inch (it's actually 2+ inches). Resolve the tensile force (T) in the rod into horizontal and vertical components:
H = T cos (theta)
V = T sin (theta)
where theta = angle of rod with the horizontal (please see the jpg file attached to my initial post)
Shear stress in the weld, fv = H/Aweld
Bending stress in the weld, fb = M/S = Ve/S
where e = the distance from the edge of the weld to the center of the weld
Resultant stress, f = (fv² + fb²)^0.5
I would then compare the resultant stress to the allowable stress, Fw.
RE: Question about an x-brace detail
You are on the right track. Here are few suggestions:
- Yes, resolve the force in horizontal and vertical components. These components will act on respective shear planes of the weld. The size of weld on these planes is a/sqrt(2), where 'a' is the effective throat thickness (ie) weld size.
- Calculate the shear stress on two planes:
fv=V/(a/sqrt(2)*Lw)
fh=H/(a/sqrt(2)*Lw)
where Lw is the length of weld
- Combine them to find resultant stress using huber-henky criteria.
f = sqrt(3*(fv^2+fh^2)) to be less than allowable stress.
In my opinion bending is not a problem once you have considered stress in two orthogonal planes and combined correctly. Although there will be vertical shear stress concentration, however vertical component of force is quite small.
RE: Question about an x-brace detail
RE: Question about an x-brace detail
I disagree with hanif08, the vertical component is applied eccentrically and this needs to be taken into account.
RE: Question about an x-brace detail
It depends on the rod diameter as well.
In all, to me it is not a very good detail and one I wouldn't waste my time on. I'd use a vertical gusset-type plate which would have more vertical rigidity to establish load distribution over the full length of weld. It would still have the eccentricity that csd72 states.
RE: Question about an x-brace detail
Considering that this weld is a two sided flare groove, its strength could be somewhat questionable especially if it was not backgouged properly. I'm not sure what kind of weld inspection was done, but lot of inclusions can result if it was not done properly.
If the first inch of weld cracks as a result of poor welding practice, maybe the unzipping of weld theory is possible.
One other question. Is this a wind brace subject to stress reversal? If so, perhaps its fatigue stress range would come into play?
RE: Question about an x-brace detail