Bolted Connection - Plate Snet 1

[TylerM94]

Hi,

I'm trying to learn about connection design. I'm a new structural engineer working on a lot of CA and shop drawing review. The engineer has calculated a number of limit states for connections involving Snet including Coped Beam Flexural Rupture. Plate Flexural Buckling, and Coped Beam Lateral Torsional Buckling.

Would someone be able to help me understand the procedure for calculating Snet for a plate? I have attached an AISC connection example for reference.

I know the connection design calculations are ultimately the responsibility of the fabricator but I would like to be able to verify some of their standard connections and advance my own understanding.

My understanding of the net Section modulus is that it's My/I, taking into account the effect of the bolt holes but I'm having trouble putting this into practice.

 

[r13]

The net sectional moment of inertia is bh³/12-Σ(A_i*d_i²). Where "Ai" is area of the respective bolt hole, and "di" is the distance of the respective bolt hole to the neutral axis of the section. Then you can get S_net thru I/y.

 

[Blackstar123]

My understanding of the net Section modulus is that it's My/I, taking into account the effect of the bolt holes but I'm having trouble putting this into practice.

My/I = M/(I/y) = M/S is the flexural stress of an elastic material, where I/y is the elastic section modulus, S.
Snet of a rectangular cross-section with holes can be calculated as explained by retired.

 

[TylerM94]

Thank you very much for your help, and
I have taken your formula and put it into practice on the problem attached. I am still getting an error on my calculation - could you take a look at my work?

I'm getting 6.98 when I should be getting 6.53.

 

[r13]

Are you programming in Mathcad? Looks good.

 

[TylerM94]

*Screaming into pillow* wow I can't believe I missed that. Thanks so much for taking a look! And I used Latex for this.

 

[human909]

Uh phrasing?

 

[TylerM94]

I guess my only other question is that the corrected math looks right but Risa is still getting 6.53 for Snet. Any idea why I might be even further off?

Tyler

 

[r13]

Wait a minute, I made a mistake too. A=b*t, for bolt hole, A = 0.375*(0.75+1/16) = 0.305 in². Sorry for miss that.

* We are calculating S_net for the cross section, not the plane.
** Once the correction is made, I got S_net = 6.71. The difference could be caused by rounding. Another thing is, does the RISA consider diameter of the hole = d_b + 1/16", or 1/8"?

 

[TylerM94]

Thank you so much! . That makes more sense now - considering we start by taking the moment of inertia of the gross section using the thickness of the plate and then subtract the areas due to bolt holes.

 

[r13]

I figured RISA uses d_b + 1/8" for bolt hole diameter. After this correction, I got 6.54 in³.

 

[r13]

It is coming back now. For fabrication, d_h = d_b + 1/16", but consider the irregularity of shape and stress around edge of the hole, the effective hole size for design therefore is, d_h = d_b + 1/8". I think it is addressed some where in AISC manual. Check it out.