Brain Teaser for the Engineers (Angle Sizing)

[wilnj]

Referring to the photo above, I have this detail on my project where the engineer is insisting on structural steel (highlighted in blue) to support a lighting element (highlighted in yellow). The lighting element is 125 feet long and weighs 1.5 pounds per foot. The flashing shown over the top of the steel angle is part of my waterproofing system and is .040 aluminum that is doubled up with a hemmed edge.

They're calling for this support angle to be a 7x4x3/8. The attachment of the angle makes it a costly detail to construct (not shown in bid documents) and it is difficult to properly waterproof the facade with this angle attached to the building.

I think I could fabricate a cold formed steel angle to reinforce the aluminum angle at a significantly lower cost and it wouldn't fowl up my waterproofing system. If so, how thick should that angle be (18ga, 16ga, thinner or thicker?)

Show your work because I'll need to convince the engineer.

[mgatepi]

You can definitely reduce the thickness by using a HSLA steel. (High Strength Low Alloy) If I remember correctly the angle iron material is extremely low carbon with a Yield strength of about 35KSI at best. So going with a HSLA 60 or 70 steel, this will allow you to reduce the thickness by roughly 40% to 50%. You may be able to do some things with the shape of the steel to even reduce the thickness more. Such as formed ribs etc. if space allows

[wilnj]

Originally Posted by mgatepi

You can definitely reduce the thickness by using a HSLA steel. (High Strength Low Alloy) If I remember correctly the angle iron material is extremely low carbon with a Yield strength of about 35KSI at best. So going with a HSLA 60 or 70 steel, this will allow you to reduce the thickness by roughly 40% to 50%. You may be able to do some things with the shape of the steel to even reduce the thickness more. Such as formed ribs etc. if space allows

Thanks! Yes, hot rolled shapes are typically 36ksi, where as cold formed is 50ksi. I don't have room to deform the shape nor do I have time to engineer it but I could get cold formed in 18, 16, 14 and 12 gauge and essentially any leg dimension I need for it work.

[3rd_gear_Ted]

Global concern:
Having two dissimilar metals in a waterproof scenario is a precursor for corrosion.

Based on your dwg;
Found engineering math book picture:
"Cantilever beam with concentrated load @ free end" results = shear & moment

[wilnj]

Originally Posted by 3rd_gear_Ted

Global concern:
Having two dissimilar metals in a waterproof scenario is a precursor for corrosion.

Based on your dwg;
Found engineering math book picture:
"Cantilever beam with concentrated load @ free end" results = shear & moment

Doh!

I knew it was a simple answer. Too far removed from sizing any structures, I completely forgot to treat it as a cantilevered beam.