Capacity calculation 36 ksi/50 ksi

[Shylesh]

I have a steel section which consists of a hollow pipe (grade 50 ksi yield) and it is reinforced with an angle (yield strength 36 ksi). The angle is continuously welded on to the pipe. My question is how do you calculate the compression capacity for such a section with different grades of steel.

Thanks.

 

[dougantholz]

Shylesh,
For all practical purposes I would design your built-up section for 36 ksi allowable stresses. If this compression member has any length to it, then yeild strength won't play that big of a factor (Euler buckling will).

 

[Taro]

Also check that you are using the correct properties for the pipe. A53 Gr. B pipe has a yield stress of 35 ksi.

If you are using round HSS tubes instead of pipes, you can get a high-strength tube in 50 ksi (A618 or A847), but the most common type is A500 (42 ksi).

 

[Shylesh]

Douganholtz,

Thanks for the reply. The section has a length of about 6 ft. I am a little confused though ... isn't the buckling point ultimately related to the yield strength ? I feel that assuming the entire section to be 36 ksi may be too conservative. Your thoughts ?

 

[dougantholz]

Shylesh,
First, yield strength and column capacity are related in the plastic & elastic/plastic range; in the elastic range is more dependant on kl/r. To prove this to yourself look up pate 3-25 in 2nd Ed. LRFD manual (if you have available) and you will see the column capacity for a W12x40 is 64 kips at 32 ft for 36 & 50 ksi steel.
For you, if your column has a large enough kl/r; then still using 36 ksi may be accurate. If you have a section that will fail plastically, then proportion the area of steel may be accurate (w/n 10%). You will have to decide where you are at and if you work it both ways (as all 36 and proportionally) and you get close answers- then you have it. If not make a judgement call because all 36 is conservative (but fast).