Failure of HSS rectangular tubing
Failure of HSS rectangular tubing
(OP)
Locally, engineers strengthen HSS columns near the top of the column, near the haunch, by adding rebar. A hole is drilled through the long side of the rectangle (the "height" dimension) on both sides. Then, rebar is passed through the holes and welded into place.
My understanding is: forces at the haunch will cause failure in HSS rectangular tubing via an inward collapse of the long side. Basically, in cross-section, the rectangle "dents" inward to form an hourglass shape. The bar forms a compressive reinforcement to resist this deformation.
Please comment! What do y'all think of this? Has anyone seen a formula to calculate the placement of the bar or the force in the long side of the HSS tube causing the concavitation?
Has anyone seen this type of failure in the field?
All comments appreciated, DD
My understanding is: forces at the haunch will cause failure in HSS rectangular tubing via an inward collapse of the long side. Basically, in cross-section, the rectangle "dents" inward to form an hourglass shape. The bar forms a compressive reinforcement to resist this deformation.
Please comment! What do y'all think of this? Has anyone seen a formula to calculate the placement of the bar or the force in the long side of the HSS tube causing the concavitation?
Has anyone seen this type of failure in the field?
All comments appreciated, DD
RE: Failure of HSS rectangular tubing
Surely a better knee brace would be another piece of HSS, of suitable geometry. The axial force in the HSS brace would then be transferred to shear force in the sides on the HSS column and rafter, instead of putting the HSS "face" into bending.
RE: Failure of HSS rectangular tubing
These other engineers are bracing a standard HSS rectangular tube, HSS 18x6x3/16ths, for example. The round bar (#6, I think) goes through the tube, completely. Two holes are drilled, one in each side to accomplish this. The end result is a Frankenstein-esque pair of rod ends projecting symmetrically from the middle of the the long sides of the rectangle of the HSS tube. Instead of a column, imagine Frankenstein himself standing on the footing with the beam resting on his head. Frank's bolts are the reinforcing rod.
I could explain better with pen and paper!
Thanks, DD
RE: Failure of HSS rectangular tubing
RE: Failure of HSS rectangular tubing
You are right on.
I used "rebar" where I should have used "round bar". And, judging by the savvy of the two engineers who do this, the qualities of the bar have likely been considered.
Yes, there is a connection here. About 1 to 3 feet above where the rod-stiffener is placed, is the column-beam connection, which is typically 1/2" to 5/8ths plates welded to beam and to column, then bolted together.
The forces here range from 20 to 100 kips.
I use ASD. I am guessing I can use formulas for stiffeners to prevent web crippling. This is the closest approximation I know of -- but I am not a guru of the Green Book!
What formulae would you use?
-DD
RE: Failure of HSS rectangular tubing
RE: Failure of HSS rectangular tubing
RE: Failure of HSS rectangular tubing
You're talking about the HSS face punching, and other out of plane effects. What about the in plane effects, such as compression & bending etc.? Are those not existant, or have they been omitted from the analysis?
You use code specified HSS face slenderness (and web crippling expressions for that matter) only for IN-PLANE compression of your HSS face, not for the out-of-plane effects you describe.
Please be careful with this, it sounds plain wrong.
tg
RE: Failure of HSS rectangular tubing
RE: Failure of HSS rectangular tubing
Thank you for your concern. All cautions are appreciated. I expect I am okay with this, here's why:
The forces involved are an axial load and a moment coming from the column-beam connection above. There is no wall attached to the column, so only the 8" width of the column itself takes an out-of-plane load. This is small compared to the load at the haunch, coming from a roof.
There are at least two engineers doing this locally. Both design HSS buildings regularly and have been doing so for years.
And, it makes sense. If an evenly loaded beam deflects too much, an extra support midway is an effective solution. And that is what happens here...
punching is not the concern here. There is no shear tab or other connection to the column.
UcfSE,
I am ordering my HSS connections book TODAY. I also found a handy website, Bull Moose Tube, http://w
This page shows HSS connections and considerations, with graphical displays of failure types, including yield lines and shear punching.
Peace, DD