Aluminium when to consider the HAZ effect in a truss frame ?

[SilverBeam75]

Hi,

I have a verification to do on a traffic light truss (span of 90feet) The truss is made of 6061-T6.

What will be the value of the Fy for the calculation? Will you use a value of 35 ksi or will you consider it welded and reduce it to 15 ksi ?

 

[MiketheEngineer]

If you weld it then you must look at those areas at 15 ksi. I believe the 15 or 35 ksi is ultimate - need a Safety Factor

 

[gtaw]

I tell people that 6061-T6 is time at temperature sensitive. That is, the mechanical properties degrade over time if the base metal is held at a high temperature. The HAZ is usually overaged.

That means a single pass weld on thin plate that cools rather rapidly will not experience the same degratation of mechanical properties as a thicker plate that is welded with multiple passes with no attempt to control interpass temperature.

The values published by AWS D1.2 for UTS and YS, in the as welded condition, are for situations where the procedure is qualified by testing and the manufacturer does everything correctly.

It can be challanging to attain the required values even when the manufacturer/fabricator is expereienced with aluminum alloys.

The HAZ can extend a considerable distance from the toe of the weld depending on the material thickness and the interpass temperature. The HAZ is overaged, allowing the alloying elements to precipitate to the grain boundaries, thereby weaking the base metal in those areas.

Best regards - Al

 

[SilverBeam75]

Gtaw,

thx for the help, but still I'm not sure about my verifcation, should I consider the HAZ ?

 

[connectegr]

If the truss panel points are welded, then the material strength is reduced within the HAZ. I agree with gtaw's comments, and the reduction may be conservative depending on the actual condition. But, as the designer you may not be aware of all the weld parameters and must simply reduce the material strength. The reduction is a local effect and does not effect he entire member. It may be necessary to analyze a member as a combination of members with varying strengths.

http://www.FerrellEngineering.com

 

[Ron]

SB75...not sure of your location, but in the US, the criteria of the Aluminum Association is "code referenced", meaning they are inclusive. While I agree with GTAW's comments, you must use for your analysis, the reduced values for the welded condition, so yes, you must consider the HAZ.

In testing I've done, the welding reduction is very conservative; however, the properties in the HAZ are highly variable, thus the overall reduction in allowables.