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

 

[PEinc]

This topic has been discussed recently. Search the forums for aluminum design.

 

[PEinc]

thread507-215430

 

[csd72]

E for sluminum is approximately a third that of steel.

Aluminum weakens when welded.

Bolt connections to aluminum are weaker.

You need to get an aluminum code to design this properly.

 

[LPPE]

I searched for "Aluminum vs. Steel" and didn't find what I was looking for, sorry. I will try that other thread.

 

[LPPE]

Anyone have any cost information? Are the same sizes of steel HSS sections also available in aluminum sections?

 

[Lion06]

As csd noted, the allowable strength for aluminum goes WAY down when welded (approximately 1/3 that of the unwelded portion. It only affects around 3" on each side of the weld, but it still kills you.

 

[MiketheEngineer]

Real rough rule of thumb - AL is 3x cost of steel and you might save 1/2 the weight

 

[csd72]

LPPE,

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.

 

[rb1957]

IMHO, you poor bastard ... having to work with such a dumb $hit ! (hopefully no-one will red flag that !)

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.

 

[csd72]

LPPE,

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.

 

[civilperson]

Aluminum is a great material if the total concept is to save dead load weight. Aluminum 6061-T651 weighs 169 pcf, (34.6% of steel), and has minimum tension/compression yield of 35 ksi, shear yield of 20 ksi and Modulus of Elasticity of 10100 ksi. Price is 240% of steel by weight which makes the finished product a wash in total cost compared to 50 ksi steel and a little cheaper in initial material costs compared to A36. The lesser painting and maintenance costs for aluminum is the tie breaker. (Steel coated with aluminum is now competitive with aluminum for looks and maintenance.)

 

[lkjh345]

The question to add to your response to the architect is:

'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.

 

[LPPE]

rb1957 - "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."

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.

 

[JStephen]

If you haven't checked steel prices lately, they are running very high and might make AL a good bit more competitive than it would have been a year ago.

 

[frv]

The price of all commodities is rising. I don't know whether aluminum has risen at the same precipitous rate as steel, but I know it has also risen quite a bit.

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.

 

[Bobber1]

Contact the Aluminum Association and get their Aluminum Design Manual. It has the allowable stresses, design methodology and member shapes for aluminum.

Bob Garner, S.E.

 

[ZachEIT]

Don't use aluminum. Period.

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.

 

[drawoh]

ZachEIT,

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

 

[rb1957]

weellll i don't know about that ...

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.