Appropriate use of Fy 1

[Lion06]

If a structure is designed with A36 and someone made a very BIG "woops" (not me), and the steel is tested and comes back with Fy > 50 ksi I believe it is ok to use A572 or A992 for a second look (provided the other requirements of the standards are met). Is it appropriate to use Fy = say 61 ksi if that's what the test shows or are you limited to a max of 50?

Any opinions?

 

[Ron]

If you have enough test data, use the procedure that JAE noted. If not, correlate the data you have with a field nondestructive method such as a portable hardness tester,then check the uniformity of your test results.

Uniformity of the rolled products is an issue in this evaluation. Different shapes may have come from different mills, and at the least would be from different "heats" of steel. This means that their mechanical properties will vary and should be assessed for their uniformity and statistical minimums.

 

[swivel63]

max of 50.

i guess if you're in a pinch and it's that particular member after there's a problem that's been caught. maybe.

just like in concrete breaks.

 

[swivel63]

whoops, you're checking a old building.

i think meicz hit it on the head. that's what i did when i would do structural evals.

 

[swivel63]

what does the original engineer's calcs look like?

 

[BAretired]

When you stand up in court to explain to the judge why you believed that all of the A36 steel in the structure had a yield strength of 50 or 60 ksi, keep in mind that there will be no shortage of experts prepared to testify to the fact that this was a completely irresponsible decision.

I seem to recall a similar situation respecting the collapse of the Save On Foods Store in Burnaby, B.C. back in about 1980. The engineers breathed a sigh of relief when tests showed that the steel had a higher yield strength than expected so nothing was done to repair a seemingly underdesigned cantilevered steel beam (Gerber System). On opening day, a section of the roof (also used for parking cars) caved in on the shoppers below. Not a lot of fun for the engineers involved.

The locals joked that it was the "Cave In Food Store".

Best regards,

BA

 

[msquared48]

Being from the 90's and with the overstress with A36 mentioned, I wonder if the mistake was discovered early in the construction phase, like real early, and, to quickly rectify the situation, the steel grade was increased to solve the problem. Although the stresses may have been mitigated in this scenario, it would not have solved any deflection problems.

Mike McCann
MMC Engineering

 

[DRC1]

Again I would not use the coupon strength. The steel may have been mill ordered, bought from a service center, or a combination of both. If the building records do not have the certified test reports for the material, you really do not know what you have. At that time grade 50 was phasing out but still available. My concern would be that depending on availabilty and budget, the steels may be intermixed. At that time A572 gr 50 was assumed equivelent to A36 except it was stronger, so it was freely substituted. As Mike pointed out E is the same regardless of strength, so deflections would be the same. With a 200% overstress, I would expect significant deflection problems. Also, I am not sure how going to grade 50 helps you. Allowable for A-36 assuming full bracing is 24 ksi. Allowable for grade 50 is 34 is only a 141% increase. LRFD will give you slightly better numbers, but I doubt it is enough to make it okay. Again, you can not use the 61 ksi as that is not the yield over the entire section. If it is taken at the web, as is done for mill testing, the flanges, where the yield becomes important, will be lower. As BAretired points out we can almost always get the numbers to justify a design if we make certian assumptions. The problem is that occasionally gravity does not cooperate. I think you should be looking at a strategy to reinforce the existing building.

 

[civilperson]

Stress is a calculated guess using many assumptions, strain can be measured and known. Saying 200% over stressed is hard to interpret as a calculated quantity. If the 83% of yield for maximum stress due to dead loads and the 71% of yield for maximum stress due to live loads are exceeded then the structure needs strengthening or reclassified. (Percentages are the inverse of the load factors and maximum stress is phi times measured yield).