Does a leger angle anchor produce tension AND shear or it tension OR shear? 1

[see_gen_notes]

I'm struggling to wrap my head around this concept. Suppose I have a ledger angle with a load acting downward (in the direction of gravity) at the end of its leg. When using anchor design software, should I account for both the tension caused by prying and the shear from the load? Or is failure evaluated as either prying or shear, but not both simultaneously? My boss insists on the latter approach but didn’t provide much explanation—more of a statement than reasoning. I feel like there’s some shear happening in reality, but is it negligible compared to the prying force? And vice versa?

 

[driftLimiter]

The bolts need to resist shear and tension to achieve equilibrium. Some designers use a thick enough plate that it can be idealized as a rigid body. Countless examples of angle ledger anchorage exist that account for this. There are examples for steel ledgers, and wood ledgers alike.

As far as prying ... In structural steel the term, prying action, refers to a flexible plate where the tension load is amplified due to the deformed shape. Prying action is not the same as the rigid body equilibrium I mentioned above, it is in addition to that effect.

 

[Celt83]

Taking it back to basic statics you have a vertical load at the angle tip.

Sum Fy = 0 -> satisfied by vertical reaction at the bolt

Sum Mz = 0 -> you have a P*e generating moment if you have nothing resisting this then by statics you have unrestricted rigid body motion/twisting

Sum Fx = 0 -> you’ll need a T/C couple to satisfy moment equilibrium to prevent rigid body motion, this typically presents as bolt tension and some form of bearing between the angle and the backup structure.

In summary from the statics the bolt will experience both shear and tension concurrently to satisfy static equilibrium.

 

[Enable]

I see it as the gents above. And unless we are incorrectly picturing your scenario, I would suspect that your boss probably meant something quite a bit different. My money is on that they were trying to say that for design purposes you can figure how many anchors you need for shear only, and then figure how many you need for the tension force only. Add both of these together to obtain how many anchors you need in your ledger (in total). This is effectively just a quick way of computing a linear (in the exponent) interaction equation, and what I have seen older engineers sometimes do for a quick and dirty.

 

[dik]

Prying action is not the same as the rigid body equilibrium I mentioned above, it is in addition to that effect.

With wood, I don't use prying action... the material, IMHO, is not stiff enough.

 

[driftLimiter]

With wood, I don't use prying action... the material, IMHO, is not stiff enough.

Agreed. I've always been a bit worried assuming the ledger rotates about the bottom so I use 5/6 x that distance. I need to check resources to see where that came from though.

 

[dik]

I generally use an eccentricity close to the contact with the angle (Not overly conservative), assuming the leg deflects and there is no load on it.

 

[driftLimiter]

Dik this is the what I mean about the 5/6 factor.

 

[Celt83]

Not a fan of that angle orientation

Tension on Bolt of a seated angle - Structural engineering general discussion

Learning the basics sometimes is frustrating. Everyone uses it but no one knows where it really comes form. On the left diagram, What is the proper name of the triangle in sketch below. Why do we assume it to be a triangle. What is the supporting document that allows this assumption. Why isn't...

www.eng-tips.com

 

[driftLimiter]

Thanks celt that's insightful.

I've been using your 'strong' orientation for heavy loads like joist seats. I use the weak orientation for metal deck support.

My sketch above was mostly to illustrate how I approximate the resultant of the compression field.