Old Existing Truss 1

[marinaman]

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?

 

[jdonville]

"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."


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

 

[Ron]

If you test A36 steel today, it's minimum yield is 36 ksi, but any individual test might be 40,42,38...etc. Would you use those test results for design? No. Use the testing to confirm that it met the minimum A9 requirement and then use the A9 minimum for your analysis...remember, you are not sampling and testing every member, so variability is expected. That's why you start with the minimum.

 

[marinaman]

jdonville,

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?

 

[bridgebuster]

Jeff - normally (in bridge work) we don't discount the yield stress. If there's deterioration we calculate section properties based upon filed measuresments. Deterioration can be repaired by adding plates, angles etc.

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.

 

[marinaman]

Bridgebuster,

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!

 

[apsix]

This discussion may help;

http://www.modernsteel.com/steelinterchange_details.php?id=232

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.

 

[BAretired]

If in doubt, beef up the truss with additional members and quit worrying about members which "might be adequate". In the end, it will be more cost effective and more reassuring to all.

BA

 

[bridgebuster]

is it possible to stiffen a member by bolting a plate? To me that's more reliable than welding a plate.

 

[BAretired]

It is possible. Why is bolting deemed to be more reliable than welding? I don't agree with that.

BA

 

[bridgebuster]

The age of the steel; 1920's vintage steel is not the best for welding.

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.