Made-up section!

[jonastp]

Hello,

I need some help figuring out something.

I need to calculate the section modulus for a made up section, namely a circular HSS of 16''x0.500'' with a W10x39 inside (centered), and filled with 5 ksi concrete.

I need this for a soldier pile and lagging temporary retaining wall.

I think I should add the section modulus of both steel section (since it is about the same centered point), and neglect the concrete, am I right?

Thank you very much for your help!!

Jonathan

 

[TehMightyEngineer]

Dump it into autocad as a region with the centroid on the 0,0,0 coordinate and you'll get 99% of your section properties through the massprop command.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[BAretired]

Neglecting the concrete may be a reasonable approach, but you should not add the two section moduli together. The concrete fill will force both steel sections to deflect in unison, so you could add the two values of moment of inertia together as an approximation to the likely behavior of the pile.

If you believe the concrete will bond to all steel surfaces, a better way would be to calculate the section properties of the transformed section.

BA

 

[KootK]

I think I should add the section modulus of both steel section (since it is about the same centered point), and neglect the concrete, am I right?

- You will have composite behavior between the two steel members.
- You'll likely have composite behavior between the steel and concrete but, unless I needed it badly, I probably wouldn't bother with the effort of trying to account for the concrete with cracking etc.

Whether or not your strategy is appropriate will depend on whether you're using the plastic modulus or the elastic modulus.

- Plastic: add the two moduli.

- Elastic: add the two moments of inertia and then divide by DIA/2 to get the combined modulus.


I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Guastavino]

Can you sketch is out and upload it?

As I think I understand it, I think you are correct, it would be the Sx of the HSS and the W10x39 combined, Assuming they have the same centroid (IE the W10 is centered in the HSS). Assuming that you are OK assuming they will act compositely and that the internal shear forces will transfer between the two, which likely isn't a bad assumption, albeit I haven't done this before so I'd have to ponder it some.

 

[Lomarandil]

too slow on the draw. Removed to avoid confusion

 

[Guastavino]

Yea, forget what I said. ADD the MOI, not the section modulus's together, and then divide by 8, (IE d/2). Brain fart for a second....

I just applied parallel axis theorem to Sx...my excuse is that it's Friday...

 

[jonastp]

Thanks a lot guys, I usually roam more in the earth retention and foundation forum, but you guys are awesome!