Old Existing Truss
Old Existing Truss
(OP)
I have a project where there is an existing truss that appears to be overstressed. I am currently having the GC take coupons from the truss to have testing done to determine the yield strength of the material and to determine the weldablilty of the steel.
I am not taking coupons out of every member, as that is not practical in this truss.
This truss was fabricated in 1923 (A9 Steel). A9 had a minimum steel yield strength of 30ksi. If the coupons reveal 36ksi yield, does it make sense to use the 36ksi as Fy, even though thats above the specfications for that year.
For members where I am not taking coupons, I can have a Rockwell Hardness test completed at several locations. I am told that the Rockwell Hardness test will correlate to tensile strength. Could I then take 50% of the tensile strength to get yield strength for the members where no coupons were taken, as the code of the day took 50% of the tensile for yield?
I am not taking coupons out of every member, as that is not practical in this truss.
This truss was fabricated in 1923 (A9 Steel). A9 had a minimum steel yield strength of 30ksi. If the coupons reveal 36ksi yield, does it make sense to use the 36ksi as Fy, even though thats above the specfications for that year.
For members where I am not taking coupons, I can have a Rockwell Hardness test completed at several locations. I am told that the Rockwell Hardness test will correlate to tensile strength. Could I then take 50% of the tensile strength to get yield strength for the members where no coupons were taken, as the code of the day took 50% of the tensile for yield?






RE: Old Existing Truss
I would use a discounted fy of 30. E.g., 0.85 x 30 = 25.5 -> say 24 ksi, to account for deterioration, stress history, etc.
Others with more experience with historical structures may have a different opinion.
Jeff
RE: Old Existing Truss
RE: Old Existing Truss
The steel has already been "discounted". The old data calcs yield as 50% of the tensile strength. The yield (Fy) when used in calcs to determine the allowable, will be multiplied by a factor of safety.....for example, in tension, per AISC, we'd use (0.6)(Fy) on the gross area.
But does testing allow Fy of 36ksi to be used?
And then the correlation via hardness testing?
RE: Old Existing Truss
marinaman - I'd be leery about arbitrarily assuming all members have the same Fy. It's unlikely that all of the members were manufactured in the same heat.
Are all of the members overstressed? If not, you could obtain your coupons from just the overstessed members; this way you minimize the number of samples. Rockwell hardness can be used for a correlation to tensile strenght, BUT, you still need a sample. I don't think the portable gages can be used for what you intend- better talk to a metallurgist.
Since this is a truss, reinforcing various members is not difficult except gusset plates, which can be done but it's a detailing headache.
RE: Old Existing Truss
I am taking coupons of some of the members. I was thinking of taking hardness tests of the coupons before they are pulled (and are to be checked for weldability/carbon content). Then, once the yield data is returned, if I have a good yield result, hardness testing the rest of the members, and if other hardness tests come back similar to the coupon hardness tests, I would know that the tensile strengths are similar, and ultimately, the yield strengths would be similar.
I do not want to take coupons from the overstressed members. They are at their allowables at dead load only, and, I'd have to stiffen them before taking the coupons....problem being, stiffening would require welding, and I dont know the weldability without taking a coupon.....I've got a dog chasing its tail!
RE: Old Existing Truss
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My comments;
- Stiffening a member to allow coupon removal isn't logical, if you are stiffening it anyway just use the A9 properties.
- You should have enough coupons from elsewhere to determine weldability. The above link flags that weldability is likely to be poor.
RE: Old Existing Truss
BA
RE: Old Existing Truss
RE: Old Existing Truss
BA
RE: Old Existing Truss
Also, workmanship. Field welding is done all the time but you need a qualified welder, as well as proper surface preparation. The truss is probably covered with lead paint - which adds another annoyance.
Drill a few holes, slap a plate on, and be done with it.