Steel connection elements subject to combined loading?

[abusementpark]

On page 9-3 in the 13th Edition of the AISC Manual, it is stated "Connection design has been traditionally based on simple stresses, such as shear, tension, compression or flexure, not taken in combination. This simplification is adequate because connection elements are usually small or short enough that an interaction-type distribution cannot form." They follow that up with an explanation involving Von Mises criteria, which doesn't fully satisfy me.

Thoughts? Is everybody neglecting interaction in their connection design?

RAM Connection seems to make this assumption. I guess I like would like to see a little more background information before tacitly accepting this simplification.

 

[normm]

Although I was not aware of this AISC statement, I have always ignored interaction in connection design as this gives more conservative answers and of course much simpler to calculate.

 

[BAretired]

Interesting. Unless it has changed recently, the Canadian Code CSA S16 considers an interaction type formula for bolts in combined shear and tension in bearing-type connections:

(V[sub]f[/sub]/V[sub]r[/sub])[sup]2[/sup] + (T[sub]f[/sub]/T[sub]r[/sub])[sup]2[/sup] <=1

And CSA has another formula for combined shear and tension in slip-critical connections.

BA

 

[abusementpark]

I have always ignored interaction in connection design as this gives more conservative answers

How is it MORE conservative to neglect interaction? Interaction usually reduces your capacity, especially if you are near the stress limit on one state alone.

Canadian Code CSA S16 considers an interaction type formula for bolts in combined shear and tension in bearing-type connections:

I should clarify that AISC does consider interaction for bolts similar to the formula you posted. I believe the statement I quoted refers more to connection plates (e.g. a beam web connection plate subject to shear and axial load).

 

[normm]

When you produce simple connection design, you will have a plate that will resist axial stress alone; another seperate plate resisting shear stresses alone. In both cases stresses will be limited by what is allowed in the Code. The limiting stress in shear is less than the limiting stress if you performed combined stress calculations. I work in the UK and use AISC only occassionally. But last time I used AISC ASD method, I was using 0.4 Fy in shear and 0.7 Fy as combined stress limiting value. I think that if you perform combined stress calculations, no doubt the numerical value of stress will go up, but because of higher allowables, it may not be more severe.

 

[kingnero]

NBN 51-001 (not valid anymore) used to have a formula describing stress state in bolts with combined loading,
if necessary I can still look that one up.

I'm not overly familiar with the eurocodes, but that probably will have something about combined loading too.

However I also recall reading something about max. allowable shear stress not being influenced by any (tensile) preload.
I'll have a look for that publication.

 

[structSU10]

Speaking of von mises, if you look at page 10-103 for extended shear tab connections, you reduce allowable bending stress using the von mises criterion.

Also starting at page 10-131 they have a discussion on special considerations for simple shear connections, which discusses some other cases and their appropriate checks.

 

[RFreund]

Attached is a good discussion for combined stresses in gusset plates.

EIT
www.HowToEngineer.com

 

[tony1851]

Interesting. Unless it has changed recently, the Canadian Code CSA S16 considers an interaction type formula for bolts in combined shear and tension in bearing-type connections:

(Vf/Vr)2 + (Tf/Tr)2 <=1

UK equivalent for bolts in combined shar and tension is

applied shear/shear capacity + applied tension/tension capacity <= 1.4

sorry, can't find the symbols!)

 

[JoshPlumSE]

I have been looking at interaction equations for connection (specifically shear tab connections) a lot lately. Some thoughts:

1) As STRUCTSU10 points out, the 13th edition manual (pg 10-103) gives a method of combined stress interaction for shear and flexure. The method is based on Von Mises criteria and is a bit cumbersome to use.

2) AISC 14th edtion gives an equation (eqn 10-5 on page 10-104) which is a good bit easier to use. Unfortunately, the method does NOT address the presence of axial force in the connection.

3) AISC has somme design examples which may extend this concept out further. Specifically example II.C-5 (which is a chevron brace connection) which shows a direct combination between axial stress and flexure to compare against yielding. I don't like that very much since flexural yielding doesn't necessarily mean that you have achieved failure. They address this by assuming a uniform flexural stress profile (rather than the linear stress profile we normally assume).

4) My personal view is that a general interaction equation can be arrived at by combining items 2 and 3 together... but, only if you look at item three as instead being related to P/Pyield + M/Mplastic. That allows for an easy extension of AISC 14th edition equation 10-5:

Interaction Equation for Flexural, Shear, Axial yielding of a shear tab or gusset plate:
(Vr/Vc)^2 + (Pr/Pc + Mr/Mc)^2

At least, that's what I've decided to do. It's relatively irrelevant for most shear tab connections, but becomes important for vertical brace gusset plates or drag struts where the connection's axial force may be significant.

 

[RFreund]

When AISC says that the Von Mises is not applicable because we don't know the stress perpendicular to the section - why is this? Does this mean you can no longer treat the plate as a single dimension member or in other words plane stress is not applicable? For example if a plate is subject to axial load along the z-axis and bending about the z-axis and shear along the y-axis.

You would find:
Normal Stress the the x-z plane: P/A+Mc/I
shear Shear: V/A (approx)
The normal stress on the other planes would be 0. But I guess this is the assumption that is no longer valid?
This seems so basic yet I'm struggling to understand this.




EIT

http://www.HowToEngineer.com

 

[nutte]

I've always thought the AISC comment in the original post deals with plate elements in simple shear connections. Not connections resisting shear and axial, just shear connections. For example, for a shear tab, you'd have shear stress in the plate. You'd also have some flexural stress based on the connection eccentricity. It is these two stresses, flexural and shear, that are typically checked independently, rather than using some interaction which may or may not more accurately reflect the actual stress limit in the plate.

For shear-and-axial connections, like a shear tab with axial load, I would combine the axial stress with the flexural stress to size the plate. I would not factor in some interaction with the shear stress.

 

[Venturini]

RFreund,

It is not basic, bolts present complex mechanical behavior. If you use the theory o solid mechanics that would be correct, but in real world situations there would be plane strain conditions allied with plane stress conditions due to misalignment, yelding of the bolt and the vicinity material and some geometric imperfections.

You can do rough aproaches like adopting criterea as von Mises that does over designing to prevent failure proven kind of effective over the years or you do mechanical testing to asure how much abused your bolts can be. There is no direct and correct way to correlate a combination of tensile/compressive stress and shear stress to the measured yelding tensile stress.

I'd sugest to keep with the code's recomendation and perform some laboratory mechanical testing to be completely safe.

Sorry if the message is unclear. English is not my first language and I find it challenging to discuss engineering related subjects in english.

 

[RFreund]

Venturini - Thanks that's along the lines of what I was thinking.

Abpark - Sorry don't mean to threadjack.

EIT

http://www.HowToEngineer.com

 

[abusementpark]

I've always thought the AISC comment in the original post deals with plate elements in simple shear connections. Not connections resisting shear and axial, just shear connections. For example, for a shear tab, you'd have shear stress in the plate. You'd also have some flexural stress based on the connection eccentricity. It is these two stresses, flexural and shear, that are typically checked independently, rather than using some interaction which may or may not more accurately reflect the actual stress limit in the plate.

It certainly seems like a general statement to me. They really should clarify it if this is the case.

For shear-and-axial connections, like a shear tab with axial load, I would combine the axial stress with the flexural stress to size the plate. I would not factor in some interaction with the shear stress.

I guess I should I see if I can find a worked out shear-and-axial beam connection and see how they handle interaction. I'm curious.

 

[nutte]

There is an example in the AISC Seismic Design Manual of a shear tab with axial load.

 

[nutte]

I meant to add, I believe there is an error in the Seismic Design Manual example. This thread (

http://www.eng-tips.com/viewthread.cfm?qid=237677)

explains the error.