Factor of Safety on Macomber Joist 1

[waytsh]

Hi all,

I am looking at an old Macomber joist for some revised loading and I am curious about the published safety factor of 1.65. Does anyone know if this was a code requirement or could this safety factor potentially be reduced with engineering judgement? Appreciate your thoughts and thank you to all those who took the time and effort to make the load tables available online!

waytsh

 

[JedClampett]

Back in the bad old days, steel was commonly designed to 21.6 ksi vs. it's yield point of 36 ksi. If you do the math, that's a 1.67 ratio. Now it was much more complicated than that, with lateral bracing, out of plane bending and a lot of other issues that needed to be considered. But the majority of steel in strong axis bending was designed to 21.6 ksi.
But I think it's a misnomer to call that the factor of safety. The steel very seldom yielded at exactly 36 ksi, usually it had a lot larger yield point. This was the point where the far edge yielded, so there was a lot of member left to yield. There was strain hardening. So the steel has a lot of excess capacity.
I suspect that Macomber did calculations and used a similar design method. If it was good enough for a steel beam, it was good enough for them. But who knows what controlled? It could of been compressive buckling of the top chord. I wouldn't want to eat into that F.S.
My point is, without some inside information on what controls in these joist designs, I wouldn't mess with the F.S.

 

[waytsh]

Thanks Jed, your point is well taken. According to the literature the moment strength was controlled by the resisting moment of the chords. I think I could reinforce the chords for the additional load but the problem I am running into is with the allowable end reaction. It would seem to me that there are a number of failure mechanisms that could be controlling at the end of the joist. If the joist would not be reinforced for shear the "factor of safety" for the end reaction would drop down to 1.35. The areas I would see as needing to be checked would be the buckling of the webs, the web-to-chord welds, and the end seat.

 

[waytsh]

Does anyone know if the material yield stress on these 1968 era Macomber joists would have been 36 KSI?

 

[JedClampett]

Almost all steel milled in the 1960's was 36 ksi, but I wouldn't stake my life (or the people underneath those joists) on it. If you're designing close to the limit, you'll have to take coupons and test the steel. There's an excellent chance you'll find out it exceeds 36 ksi by a substantial margin.

 

[woodman88]

This thread has a link to useful information from SlideRuleEra site on them.

thread507-287669

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.