Effective width using AISC 13th edition

[nuche1973]

I am using Table B4.1 to determine my effective width. I have a 12guage steel plate, 120”x120”. The plate is stiffened with 3”x 1/4” bars at 24”on center. The final section will resemble the example detail in the table for cases 7 and 8. However, my question is: do I double the b value when I calculate my final section properties? I am confused because in Section B4, paragraph 2, note c, for diaphragm plates, it states that the width (b) is the distance between the adjacent lines of fasteners or welds. This gives me a large section modulus and thus a large capacity. And the (b/t) ratio exceeds the limits noted in the table. Any clarification would be greatly appreciated. Thanks.

 

[ishvaaag]

You should check as per the case 14 IF the stiffeners were to prevent entirely against local buckling out of the plane of the plate buckling at the seams; since it may not be the case,said otherwise, the stiffness of the stiffeners has a say in the ability of the stiffened plate to bear load (along the length of the stiffeners). Essentially you need to check an orthotropic plate in compression and maybe the code is not giving the more convenient info to that purpose.

Yo can see how to proceed in Galambos IV p. 155, p. 255, that I find of more direct applicability than what quoted in Galambos V.

To make the point I attach jpg printout of one Mathcad 2000 worksheet by one method based in that reference. Were concomitant shear stresses present the limit stress in the check should be reduced.

Note also that you can quite easily model the thing in FEM, RISA and onwards, include some initial imperfections if required and if the thing remains stable, P-Delta included, (with the proper material stiffness reduction if required) you will get a safe design.

I may add some other worksheet later on related issues.

 

[nuche1973]

thanks for the information.

 

[BAretired]

nuche1973,

I don't think you have the correct case for the 12 ga plate. It seems to me it is more like Case 10 or Case 14 where a plate under compression is stiffened at each edge. In those cases, b/t is limited to 1.49[√](E/Fy) and b is the clear distance between stiffeners.

BA