Identifying Yield Strength of Steel 1

[andrew705]

Hello,

My question is in regard to determining the yield strength of steel I beams in existing buildings. In the past, I have used historical steel grades listed in the local steel design handbook as a basis for assumption, based on the age of the building. The problem I'm having is that for buildings constructed in the 1960's, there is quite a range of possible steel grades used.

Is there an easy way to determine the grade of steel used in an existing buildings construction? I know that a coupon test can be performed, but I was just wondering if anyone knew of an easier way to figure out steel grade. Is there any in situ testing equipment available? Any other ideas?

To give some background, I am frequently analyzing existing buildings for the feasibility of adding a solar array to the roof. In many instances drawings are not available for the buildings, so we send a field crew out to measure up the beams and joists, and analyze them to see if there is any excess capacity. Usually we can obtain the age of the building from the client, which narrows down the steel grade, but there are some grey areas. Sometimes having to take the more conservative value makes the building not have any excess capacity, which rules out the feasibility of the added weight from a solar PV system. It isnt practical to do coupon tests for every building we analyze, but if there is another way it would certainly be worth consideration.

Thanks!

 

[rb1957]

could you do an in situ hardness check ?

 

[andrew705]

well, what would that involve? We have full access to the beams (usually use a scissor lift to get up to them)...

Is there a hardness test procedure that is relatively simple and gives fairly accurate results?

 

[rb1957]

google "portable hardness tester"

 

[JStephen]

You might be able to determine yield strength of the material. However, some of the common specifications (such as A36) don't have an upper limit on specified yield strength, so it could be designed and built as A36, and then test out at 50ksi. So you still wouldn't necessarily know the original grade of material.

 

[weab]

A36 came out in the early 60's. I can't tell you an easy way to determine what you have. However, I would feel comfortable assuming A6 for all shapes, not A36.

 

[oldrunner]

You mean A7 steel.

 

[BAretired]

Can a hardness test provide an accurate measure of yield strength? I am under the impression that it cannot.

BA

 

[weab]

Sorry, yes I meant A7. I believe A6 is for plate.

 

[rscassar]

What I request for new steel is that in-lieu of mill test reports (which are generally unavailable with the abundance of steel imported from China for low cost). Or when these mill certificates are not available, the steel is to be tested by an independent accredited authority for chemical composition.

 

[andrew705]

BA, i think you are right. just logically thinking, it doesnt seem like hardness and yield strength have a direct relationship. theres probably a correlation, but i dont think you could calculate yield accurately enough.

thanks for the comments, all.

 

[frogit22]

if I was working on a building from the 1960's, I would use Fy = 36 ksi and not even think twice about it.

 

[JStephen]

What I recall from long ago was that there is a fairly clear relationship between hardness and tensile strength when dealing specifically with carbon steels. But if comparing materials in general, not so much. As to the actual accuracy, I have no information, the distinction between a 36k and 30k material might be hard to determine. I can come up with a conversion chart if one is not online.

 

[BAretired]

I think their is an approximate relationship between hardness and ultimate strength, but I don't think that relationship applies to yield strength. If anyone has evidence to the contrary, I would be interested in hearing it.

BA

 

[Eddycurrentguy]

Contact an NDE company and ask if they have PMI (Positive Material Identification) equipment. If you can access some of the steel in question, the machine should be able to determine, or provide two or three possible materials.

Another device known as a stress strain microprobe will do a better job than PMI, but is tougher to acquire. Also, trying correlate hardness to yield strength is possible, but factors such as material type, microstructure, surface finish, test equipment, technician experience, etc will influence the outcome. Correlating hardness to tensile strength is more common.

http://www.tppinfo.com/defect_analysis/yield_strength.html

 

[stanweld]

A-36 has been around far longer than the 1960's. It has been the common material for structural steel plate and shapes used prior to the advent of the "grade 50" structural materials. A-6 provides the "General Requirements for Rolled Structural Steel Plates, Shapes and Bars." It is additive to the A-36, A-588, A-709, etc. Brinnel hardness testing can be used to estimate tensile strength - not yield strength.