Aluminum vs. Steel 6

[LPPE]

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

 

[gwynn]

Neither do I. Generally, most building structures see even the specified loads few times over thier lifetime. Bridges by far have greater concerns with fatigue and there are a significant number that were built in the 1960s (back when costs were equivalent to steel in some places) that are still aound and doing fine.

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:

http://en.structurae.de/structures/data/index.cfm?id=s0007233

 

[JStephen]

I think in a typical architectural application, fatigue would be a pretty minor consideration. If it is an issue, you design around it, just as you do with corrosion in steel or cracking in concrete, etc.

 

[JedClampett]

We work in water and wastewater treatment and use a lot of single aluminum members. On occasion, we use aluminum framing (where one aluminum beam frames into another), but I try to avoid this because of connection concerns.
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.

 

[civilperson]

Fatigue calculations for aluminum are similar to those for other materials. For instance, a fillet weld in detail category F and having Fatigue Design Detail similar to 18, (Aluminum Design Manual, Table 4.8-1 STRESS CATEGORY and Table 4.8.1-1 Constants for S-N Curves), will have fatigue stress limit of 3.4 ksi for one million cycles of constant amplitude stress loading. Lesser number of cycles will have proportionally higher fatigue stress limits. Building components usually have many fewer cycles than I used as an example.