old existing steel beam, in good condition, do you reduce capacity?

[AskTooMuch]

I have a steel beam built in the 70s, was hot-dip galvanized so very minimal rusting if any. If checking this beam capacity for additional new loads, should I put more safety factor just because it's very old?

 

[Agent666]

Not really, provided you know the original grade just check it like you normally would to the latest code. If you don't know the exact grade (or can't take a conservative stab at it) then most codes have a procedure that states it should be treated as unidentified steel with a correspondingly low allowable yield stress.

 

[KootK]

I don't feel that there should be an "old penalty" just because the thing's elderly. That's ageism which I cannot abide. Obviously, you need to be accounting for accurate material strengths and any corrosion but that's a different kettle of fish.

From a structural stability perspective, one could actually argue that the beam is better than a new member because it's proven itself free of serious defect out in the wild. Seriously.

HELP! I'd like your help with a thread that I was forced to move to the business issues section where it will surely be seen by next to nobody that matters to me:

http://www.eng-tips.com/viewthread.cfm?qid=456235

 

[JAE]

For the 1970's - most likely an A36 steel. 50 ksi steel was used and available but only for "special occasions".

Otherwise - just like a new beam unless fatigue is an issue.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Agent666]

One point to add, if the older code under which this material was originally designed/supplied had a different strength reduction factor on the material that is/was function of the standard distribution of strength of that materials population then I would use the lower strength reduction factor but in conjunction with the latest code requirements. Primarily because the statistical distribution of the original material properties might differ from newer materials and they are calibrated to give strength reduction factors that produce an acceptable risk of the remoteness of a failure occurring. But in most cases the reduction factors are historically kept the same, because the material is fundamentally the same (might be slightly higher grade specification for example) so its not an issue in practice.

 

[AskTooMuch]

I have the drawings and it's A36.

 

[BridgeSmith]

Not surprising it fails under ASD and passes under LRFD, especially if the dead load/live load ratio is high. Be sure you're using any applicable resistance factors with the LRFD load factors. The effective load + resistance factor under ASD is 1 / allowable stress (1 / .55 = 1.81). Under LRFD, the load + resistance factor for live load is similar, but it's quite a bit less for dead load. Obviously it's your design, and you're taking responsibility for it, so it's ultimately about what you're comfortable with, but I wouldn't have any qualms about saying it was adequate for the proposed loading.

 

[Ron247]

I would not reduce the capacity. Old steel is not the same as old people. Old steel reduction is zero and old people reduction is what they claim divided by 3.

 

[SlideRuleEra]

I have a steel beam built in the 70s...should I put more safety factor just because it's very old?

I have the drawings and it's A36.

Based on FEMA 356, available documentation for the project, and field condition assessment (testing of samples not required), you can up-rate 1970s A36 yield strength to 110% of the specified value (i.e. from 36 ksi to 39.6 ksi). "Expected-Strength Steel Properties", since 1961, reliably exceeds both nominal tensile & yield strength.

Make sure beam connections are satisfactory for increased loading.

http://www.SlideRuleEra.net

 

[ThomasH]

The only reason I can think of where I would reduce strength due to age (assuming no damage) is if fatigue is involved. But than you need a load history which is often difficult. The 70's is not that old, 1870's that would be more of a challenge .

Thomas