Shear flow into fastners of built up sections
Shear flow into fastners of built up sections
(OP)
Hi,
I am doing a project where I am strengthening an existing structure that is insufficient. The current structure utilises a channel for a beam and as a method of strengthening it I am fastening an addition channel to it back-to-back (webs vertical).
I am struggling to visualize how to calculate the spacing of the fasteners, I have reviewed the methods of fastening together flanges to webs etc using Tau = VQ/I.
The main part I’m struggling with is to calculate Q as the sections as the sections are side by side and there is no eccentricity of the sections in y direction. Is my method of approach correct or should I calculate the shear stress in the section at the bolt location using sigma = M/Z and calculate a force on the bolt from there?
Also is there any pros or cons to fastening at both the top and bottom of the web as opposed to just fastening in the centre of the web?
I have attached a rough drawing to illustrate
Thanks in advance
I am doing a project where I am strengthening an existing structure that is insufficient. The current structure utilises a channel for a beam and as a method of strengthening it I am fastening an addition channel to it back-to-back (webs vertical).
I am struggling to visualize how to calculate the spacing of the fasteners, I have reviewed the methods of fastening together flanges to webs etc using Tau = VQ/I.
The main part I’m struggling with is to calculate Q as the sections as the sections are side by side and there is no eccentricity of the sections in y direction. Is my method of approach correct or should I calculate the shear stress in the section at the bolt location using sigma = M/Z and calculate a force on the bolt from there?
Also is there any pros or cons to fastening at both the top and bottom of the web as opposed to just fastening in the centre of the web?
I have attached a rough drawing to illustrate
Thanks in advance






RE: Shear flow into fastners of built up sections
RE: Shear flow into fastners of built up sections
Furthermore, if the line load is applied from both directions evenly (and again, if the channels are approximately the same stiffness) you could technically get away without any fastening at all. Just think of it as 2 joists closely spaced. Of course, you'd still want to connect them nominally.
RE: Shear flow into fastners of built up sections
I agree with Atrizzy’s approach to the problem, with a couple additional comments. The load comes in on the existing beam member now, so you must bolt to distribute something a little more than half the load to the new member, to make them act together. This will cause some amount of torsional loading on those light cold formed steel channels, not a particularly good thing. You might try to rearrange the load application a bit so it comes into the new beam more centered and/or applied to both members directly, then the torsion issue goes away and the bolting becomes even less critical, except to hold the two channels together. I’d bolt only on the centerline, except out at the bearings you should apply some bolts t&b. The biggest issue is that you should unload the existing beam, maybe even camber it a little bit, by jacking it up a little more than just straight (no deflection, rather a little camber). Then apply, drill and bolt the new member in place, to that same cambered shape. This allows both members to take all the loading when it is reapplied. If you just bolt the new member in place with the existing member, as is, the new member will only pick up new loads, additional loads. The existing member will pick up some of those new loads too, along with all the existing loads, and may then be overloaded.
RE: Shear flow into fastners of built up sections
I'd be inclined to stagger the bolts about the center-line as I expect that you'll be depending on the existing joist to provide lateral torsional bracing to the new joist and staggered bolts will do a better job off that. Besides, what does it hurt? Same number of bolts.
One note of caution. If your new member doesn't make it out to the bearings independently, then you'll need to transfer 100% of the end shear out of the new joists and into the existing joists locally. Hopefully you're getting out to the bearings.
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.
RE: Shear flow into fastners of built up sections