Shear tab design

[GalileoG]

Hi all,

I recently received shop drawings where many of the shear tab connections for the shallower beams (W8) had two columns and 2 rows of bolts (2 x 2 bolt pattern.) Presumably this was done because the connection designer thought that they can divide the factored shear by 4, ie. less load per bolt. However, simple statics is telling me that the 2 bolts closer to the end of the beam will (because of this configuration) carry greater load than the applied factored shear. This seems simple enough, but can others here confirm my understanding before I bring this up with the connection designer? Thank you.

Clansman

If a builder has built a house for a man and has not made his work sound, and the house which he has built has fallen down and so caused the death of the householder, that builder shall be put to death." Code of Hammurabi, c.2040 B.C.

 

[hokie66]

Never known a W8 beam to need so many bolts, unless it is a really heavy one. If you need 4 bolts, better to use an end plate. What is the reaction force?

 

[dhengr]

Clansman:

I agree with Hokie, and/or maybe use two larger bolts in one column (not two columns of bolts) and a heavier shear tab. However, in your current arrangement....

Isn’t it possible or even likely, within fabricating tolerances, that the two bolts furthest from the beam end (largest canti. distance on the shear tab) or even only one of them might take the most or the largest individual load? Several of the bolt hole/bolt interfaces must yield in bearing before all of them come into play in a more equal fashion. It’s a different story when the bolts are fully pretensioned for a slip-critical connection. Furthermore, the deflection of the shear tab is quite small over its canti. length along the beam axis; but the beam deflection and thus the rotation at the connection is such that it will load the two bolts furthest from the column, and maybe even start unloading the two bolts closest to the column.

See JoeBaseplate’s thread “HSS Col-Beams from four sides,” thread #507-307746, only a few day old, and still going.

 

[JedClampett]

I agree with hokie. Plus if the four bolts are necessary as you surmise, the forces on the weld of the shear tab to the main member will be even larger. This is due to the increased moment arm for the four bolt pattern.
You might also end up with some inadvertant torsion on the main member.

 

[connectegr]

This is not a "shear tab", but rather an extended plate connection. The shear requirement of the bolts is not simply R/4. A traditional shear tab with eccentricity less than 3 1/2" considers the bolts to be in straight shear. For the double column of bolts the design must consider eccentricity to the center of the bolt group. The result is that very little capacity is gained by the additional column of bolts.

http://www.FerrellEngineering.com

 

[connectegr]

Clansman
I provided a sample single plate standard for 3/4 A325N bolts LRFD, in the post referred to dhengr. It can provide a reference for the capacity of the two row connection. Note the capacity can be limited by the cope capacity of the beam, if this is a beam to beam connection. More of these single plate capacities are printed in the 13th Edition manual. As mentioned above, these shallow beams rarely require two columns of bolts.

Another note - for a thin web W8, bearing may negate the advantage if increasing the bolt diameter.

http://www.FerrellEngineering.com

 

[nutte]

Clansman,
Did you provide a shear requirement for the connection designer? This connection has a capacity less than four times the permissible bolt shear. The question is, is this capacity more than what is required?

 

[csd72]

As others have suggested, I would design this from first primciples. Ie the shear in the bolts is one quarter tha applied beam reaction plus the additional shear reauired to resist the eccentricity between the supporting beam and the centre of the resulting bolt group.

This may well give you less capacity than a shear tab with 2 bolts.

 

[ToadJones]

Are you certain there are only shear loads present?
If this is a column line strut in a braced frame there will be axial loads present.

 

[civeng5150]

I agree with connectegr and Toadjonnes. When we design shear tabs for that famous "1/2 UDL" clause, instead of having the real loads, you get to a point, most often than not, where you need just one more bolt than that which would fit within your beam height, so you're stuck to use an extended shear tab where your bolts are not worth what they would be worth in a normal shear tab, due to the excentricity.
Also, sometime, to save shop work or overcoping a beam, when framing a beam into a girder with a very large flange width, one could go with an extended shear tab, depending on the loads, this would also permit the erectors to simple drop the beam in place within the receiving girder flanges.