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Re: shear lug

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Daryl,
 
It is amazing that aluminum expansion coefficient is twice that of steel.  This is why I needed to allow for much greater movements.  It should be noted though that thermal stresses generated by the restrained aluminum are only 2/3 of those of steel - due to lower elastic modulus.  
 
Thanks,
 
V. Steve Gordin, SE
Irvine CA
  
----- Original Message -----
Sent: Tuesday, October 27, 2009 11:05
Subject: Re: shear lug

Steve,
 
        Just thought I'd mention something that you're probably well aware of; aluminum and reinforced concrete (or steel) have substantially different coefficients of thermal expansion.  If you're restraining an aluminum structure with either reinforced concrete or steel you may develop significant thermal loading if the temperature changes.  I've had problems with this before.
 
Regards,
 
H. Daryl Richardson
----- Original Message -----
Sent: Tuesday, October 27, 2009 8:52 AM
Subject: Re: shear lug

Guys,
 
Thanks to everybody who responded on the List and privately, your help means a lot.
 
The problem at hand was that I only could use an external shear-resisting component which had to be adjustable (to maintain the required gap and allow some movement at the slide support), could not be welded and should be lined with some inert material (the restrained structure is aluminum).
 
After I figured out that it could only be an embedded tube with adjustable SS plates, I tried to come up with a sensible way of designing it.  I could not find anything in the technical literature, so I finally modeled it in a FEA program.  The results made sense and were quite interesting.  The maximum moment in the tube was below the TOC (as expected), and steel did not govern - it was the bearing on concrete even for an 8x8 HSS.
 
This is something similar to what Andrew suggested, except that I could not make it work for a fixed support where the "blockout" would create a well that would eventually be filled with rainwater and debris in which the baseplate and anchors will be rotting away.
 
Thanks again,
 
V. Steve Gordin, SE
Irvine CA
 
----- Original Message -----
Sent: Tuesday, October 27, 2009 05:47
Subject: re: shear lug

Myself and other SEs I know use shear lugs all of the time for beam bearing connections in tie columns, tie beams and tilt panels. Gets you past the Appendix D headaches where headed studs used to work, though we use headed studs to resist the tension from eccentricity which causes a moment in the connection.

In a column application such as in a PEMB where you may have high uplift and shear forces at the base, we have used them also like you are suggesting. I think HSS8x8 may be a bit overkill for that force, but you just design the steel portion as a mini cantilever column, which the welds may in some instances be critical. The concrete you design as bearing just like a seated beam connection, which you can usually find design examples in concrete text books if not straight out of ACI. For cheap backup we would detail rebar or hairpins perp. at each side of the shear lug in the potential shear failure plane.

The kicker is that they will have to cast the base plate with the shear lug into the footing, which is unusual, then they would have to field weld the column to the base plate which I don't think with erection scheduling they will like. But perhaps you could do some type of field bolt connection from the BP to the column?

I would also think you could have them form a block-out in the top of the footing slightly larger than your base plate size, maybe 6" deep or so into the top of the footing (or whatever you need per your concrete design), and use the same type of design method for the concrete and thus eliminate the shear lug. Have them place non-shrink, high strength grout around the column after the column is set. Same deal as before, I would detail an extra bar in the potential shear plane.

Just some suggestions, others ideas seem like they would work too. Recessing the base plate may work well in conjuntion with the idea of welding horizontal rebar to the base plate and then just developing the bars into the top of the footing. That will take some coordination and precision on the part of the bar setting, which in a typical foundation has some slop so detail extra carefully if you go that route!

HTH,
Andrew Kester, PE

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