Aluminum vs. Steel
Aluminum vs. Steel
(OP)
I have two arched trusses (arched top chord, flat bottom chord) that span about 46 feet. Trusses are parallel to each other, about 3 feet apart. Architectural details show tube steel members.
I've designed them using steel and ASD. Now the architect wants to try aluminum. I told him he's nuts, as aluminum is more expensive than steel (cost is an issue on this project), but he insists on an aluminum design also.
First question - how much more expensive is aluminum than steel? My preliminary research shows about 2x more expensive.
Second question - 6061 is the most common aluminum alloy to use for structural applications? What are the design strength properties, and can you use ASD for designing?
Thank you
I've designed them using steel and ASD. Now the architect wants to try aluminum. I told him he's nuts, as aluminum is more expensive than steel (cost is an issue on this project), but he insists on an aluminum design also.
First question - how much more expensive is aluminum than steel? My preliminary research shows about 2x more expensive.
Second question - 6061 is the most common aluminum alloy to use for structural applications? What are the design strength properties, and can you use ASD for designing?
Thank you






RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
Aluminum weakens when welded.
Bolt connections to aluminum are weaker.
You need to get an aluminum code to design this properly.
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
All aluminum sizes are different from steel. The tubes are square cornered and are available up to 6x6x1/2" at least (I think even up to 8x8).
The aluminum sizes required WILL be larger.
RE: Aluminum vs. Steel
the only benefit of Al is weight at the cost of just about everything else.
It sounds to me like he wants Al "just because", so it's your job to make the resulting section unworkable ! you're already wasting time and money discussing it.
RE: Aluminum vs. Steel
Also, the buckling equations are completely different to steel.
I havent gode my aluminum code handy, but from memory, E=10100 and allowable stress when welded = 11ksi for 6061-T6 aluminum. This should give you a ballpark for a preliminary design.
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
'Do you want me to bill these extra services on an hourly basis, or will you need a lump sum fee proposal from me to perform the design in Aluminim.'
If he wants it, he can pay for it.
RE: Aluminum vs. Steel
If you only knew how many times I've said "We're already wasting time and money discussing it."!!
Yes, this particular architect has an affinity for Aluminum. Maybe has a brother-in-law in the business or something, I don't know.
Thanks all for the valuable info.
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
Aluminum manufacture is very energy energy-intensive. It stands to reason that as worldwide energy costs are rising, so too will the cost of aluminum.
RE: Aluminum vs. Steel
Bob Garner, S.E.
RE: Aluminum vs. Steel
I joined this forum just to say that. One thing that many, many people miss is that aluminum's fatigue strength equals zero. Just like a coke can tab, you bend it back and forth enough it is going to break, no matter how small the force. Steel reaches a minimum strength limit, aluminum does not. I've done aluminum design many times before and a rough rule of thumb is 5 times the total, installed cost if the limiting factor is not self weight and 3 times the cost if it is. Just don't use it unless you have a really good reason to do so. If your Archie buddy wants something fun, design with high grade steel or something like that.
RE: Aluminum vs. Steel
I was about to ask about that. I understand that steel has a limit at which it stops fatiguing, and aluminium does not. At some point in time, all aluminium structures have to be removed from service. A steel structure can be designed to last indefinitely, if it does not rust.
I have never worked any of this out. Is the fatigue limit of steel a practical design stress?
JHG
RE: Aluminum vs. Steel
we use alot of Al in our line and so long as you take care of it it'll be ok. sure steel has an endurance limit which makes it good for rotating machinery pieces, but i'm not sure how that translates into structural applications (ie buildings). again, in our business we can define limit loads and in-service fatigue loads, and fatigue allowables design a bunch of aero-structures.
RE: Aluminum vs. Steel
Extruded sections for bridge decks are commonly used throughout Europe. As long as detailing and design account for fatigue, this should not be overly restrictive.
Example:
h
RE: Aluminum vs. Steel
RE: Aluminum vs. Steel
I generally try to avoid any absolute terms in using aluminum. It's pretty good over liquids. But my point is that I recently talked to a pretty sharp supplier of clarifier mechanisms who likes to use aluminum trusses to support walkways. He said he prefers this to single steel members and trusses. I didn't pick his brain enough to get his complete reasoning, but I'd bet it's pretty sound.
RE: Aluminum vs. Steel