Bolted connection design with Aluminum Alloy

[Robcat71]

I am currently working on a project where we are removing a portion of a curtain wall system from the first floor up to the third floor, and re-supporting on the third floor (bldg is 4 stories total - Concrete floor plate construction). The curtain wall gravity loads are currently carried by the foundation wall, and the system is laterally supported at each floor. I am working on a connection to the mullions to support the remaining gravity loads. My first stab at this involves some angle outriggers on each side of the mullion, with through bolts. The mullions are 5"x5" aluminum tubes with 1/8" thick walls. The bolt bearing forces will be away from the "cut" free edge, so I don't believe block shear failure is applicable, and the bolt bearing strength of the aluminum will control.

I plan on researching the aluminum design code to determine the bearing capacity of the aluminum, but am I missing any other critical checks for the aluminum? I do not have much experience in aluminum design, but I assume it follows a similar approach used for steel. Any recommendations are greatly appreciated.

 

[driftLimiter]

You might look into the bending moment at the bolt hole locations. The bolt holes reduce the section modulus and could be worth looking into if the mullion is continuous through the bolted connection.

The Aluminum code is somewhat daunting but if things are relatively simple and standard it isn't so bad. If you have a totally custom extrusion profile then it takes a significant effort just to determine the bending moment correctly.

Aluminum sections are often proportioned to be quite slender and as a result you have to investigate the buckling of different elements on the cross section to get the bending moment. Its not a trivial task especially for non-symmetric sections.

 

[jjl317]

Some key differences to be aware of -

a) strength of aluminum varies greatly between alloy/temper - hope you have information the alloy/temper of the mullions.
b) most aluminum curtain wall mullions are proprietary extrusions, often with relatively thin walls, so local buckling / local bending effects of the section often control.
c) depending on the spacing of the horizontal mullions, evaluating the vertical extrusions for overall buckling / LTB via typical analysis only can yield seemingly low bending values. Curtainwall engineers often have analysis tricks and/or consider system testing to help address these concerns.

Hope this helps

 

[BridgeSmith]

Another key difference between aluminum and steel is that, while the yield strength can approach that of steel (Fy = 35 ksi for typical structural alloys, the modulus elasticity is barely more than a third of what it is for steel. That means you can expect significantly more deformation at the holes for fasteners.

Rod Smith, P.E., The artist formerly known as HotRod10

 

[Robcat71]

Thanks all for the input. I do not have any information of the type of Alloy/temper of the mullions. I am going to have to use a super conservative strength of the alloy. Just to give you an idea of the loads, I am trying to support about 3,100 lbs. at each vertical mullion, so its not insignificant. I think its do-able but need to go through the calculations.

BridgeSmith - The deformation at the bolt holes is my biggest concern as I would consider that a failure.

 

[BridgeSmith]

The low bearing resistance at the bolt holes is likely why I've sometimes seen steel bushings at bolt holes in structural aluminum connections.

Rod Smith, P.E., The artist formerly known as HotRod10