HSS Section Reduction Question

[jdonville]

I am doing analysis checks for a HSS beam design where the beam is cantilevered and supported at the fixed end by bolts. The bolts are aligned through the minor bending axis. Stiffeners in the form of steel pipe will provide a sleeve for the bolts. In order to fabricate, a hole will be drilled in the top and bottom flanges of the rectangular HSS that matches the OD of the pipe stiffener. The pipe will then be welded to the top and bottom flange of the HSS beam.

What reduction for beam flexure should be taken for the bolt sleeve - HSS flange thickness x ID of the pipe, or OD of the pipe sleeve?

Any help appreciated.

Jeff

 

[csd72]

jdonville,

Need a little more info regarding bolt layout, HSS sizes e.t.c.

You are focusing on the bending stresses in the pipe, but there are other things to consider such as:
1. bearing on the thin walls of the outer hss.
2. crushing of the outer hss when bolts are tightened.
3. There is a joint weld on the inside of the outer HSS that may cause problems fitting the smaller one inside/

As far as the bending stress is concerned, it will normally only be an issue for beams when designed plastically. When elastic design is used then the stress at the bolts is usually much less than yield and can distribute to the adjacent steel either side. Use the most conservative assumption to check and you should be okay.

csd

 

[miecz]

I would not take any reduction for the bolt sleeves. The bending moment at the reduced section will be taken by a shear couple in the bolts, not by bending of the tube.

Agree with csd72 that the primary failure mode would be bearing of the tube steel at the interface with the pipe.

I would check the connection at the tension flange for all the failure modes of a pin connected member per Section D3 of AISC's 9th Edition, ignoring the weld. Be careful with AISC's 13th edition Section D5, as it no longer checks dishing of the thin plate beyond the pin hole. That is, the requirement that the effective edge distance be limited to 4 times the plate thickness has been eliminated, for no apparent reason.

 

[jdonville]

Thanks for the responses.

Jeff

 

[gwynn]

1. bearing on the thin walls of the outer hss.
2. crushing of the outer hss when bolts are tightened.

Whenever I see or use this detail, the pipe sleeves are sized such that the loads are taken by the pipe. The pipe is sized to have higher compression strength than the bolts have tensile strength. Generally a stiff plate washer is used as well, so there is no bearing stress on the HSS wall.

Something to keep in mind is this often results in slip-critical connections. Otherwise there is a fair bit of slop in the connection.

 

[jdonville]

gwynn,

The pipe will provide the resistance to local crushing at the bolt locations, and a large plate washer will be used to also transfer some of the bolthead stress to the "webs" of the HSS.

We will be using at least 1/2-inch thick HSS A500 Grade B.

I am concerned about a loose connection, and will require additional washers to keep it snug as needed.

I just reread my initial post, and will clarify:

-Beam is rectangular HSS aligned with long axis vertical.

-Cantilevered load is bearing on the top (short) side.

-Bolts are oriented through the HSS vertically.

-Pipe will be used to stiffen the beam at the bolts.

-Holes matching the OD of the pipe (or slightly larger) will be drilled through the HSS wall. Pipe will be welded in place around the pipe circumference matching the full thickness of the beam steel.

Jeff