Prying connection of angle vs plate

[Norgad]

Hi all,

I'm currently doing a design and I would like some opinion on it.

Basicaly, i'm designing a steel to concrete tension connection but for multiple reasons, my bolts / anchors are distant from each other and the tensile load is applied in between the 2 of them.

When the load is low, I don't have any problem checking the prying action with a flat bar. When the load is high or the bolts are really far from each other, the prying action additionnal forces are getting too high and a flat bar can't be consider anymore.

I though about switching the flat bars with an angle so the inertia would be much higher and it will be able to resist greater loads but do I still have to consider the prying action and if so, how do I do that?

Thanks for the tips!

 

[rb1957]

isn't prying more affected by the distance between the fastener and the end of the bar/angle ?

wouldn't you start assuming a pinned condition on the bar/angle, rather than a fixed end (and have the moment carried as a couple between the fastener and the prying force at the end of the bar/angle) ?

 

[Norgad]

From what I've learned, the distance between the fastener and the end of the bar/angle determined the prying force that must be applied to the fasteners in addition to the main tensile load. It's the distance between the application of the load and the fasteners that determine if prying occurs (a shorter bar would mean lower deformation so lower prying).

Also, in my case I think that I must consider that the fasteners acts as fixed end since they'll be pre-tensionned at the installation. No snug thight bolts.

 

[rb1957]

"From what I've learned, the distance between the fastener and the end of the bar/angle determined the prying force that must be applied to the fasteners in addition to the main tensile load." ... i'd agree with that.

"Also, in my case I think that I must consider that the fasteners acts as fixed end since they'll be pre-tensionned at the installation. No snug thight bolts." ... i'd agree with that too.

"It's the distance between the application of the load and the fasteners that determine if prying occurs (a shorter bar would mean lower deformation so lower prying)." ... not logically, given the previous two statements. Strictly, prying always occurring. having the two fasteners closer together means there's a smaller fixed end moment to apply as a couple. Or smaller deflection, but it's not like the smaller deflection elminates prying (IMHO).

having a longer edge distance would reduce the couple (by increasing the distance between the forces). Usually i'd use 2/3rds of the edge distance (assume a triangular reaction).

having a more flexible bar (instead of the bending stiff angle) would also reduce the prying moment, as the increased flexibility of the bar would permit small deflections (instead of the ideal fixed end).

 

[delagina]

i dont get it. i thought you should increase the thickness of the plate to eliminate prying action.

so why would having a more flexible bar be better in this case?

 

[rb1957]

a thicker plate would reduce the deflections (and stresses) due to prying.

a more flexible overhang (from the fastener to the edge) would reduce the moment and reduce the couple forces, but it would increase the deflections relative to the fully fixed and probably higher stresses too.

 

[Norgad]

I think i see your logic rb1957.

As a matter of fact, I'm calculating the prying action as describe by the AISC Steel Construction Manual (p. 9-10 in my version of the book). To see if prying action occurs, we must consider the geometry of the plate (aka stiffeness) and the distance between the load and fasteners.

Basically, given the dimension between the load ans the fasteners, there's a range in the stiffeness of the plate that varies from full prying action (flexible plate) to no prying at all (really stiff plate). Once we know the that, the additionnal force to be add (if prying occurs) is determined with a ratio of the distance from load to fasteners on distance from fasteners to edge of plate. So as you were saying, a more flexible plate can be use but the edge distance should be increased to reduce the additionnal load on the fasteners.

In my design, I'm stuck with a load to fasteners distance and I can't extend the edge distance as much as I should to reduce the prying load. My only option left would be to increase the stiffeness of the plate, that's where the angle idea come from. Since the angle is like a bar with a stiffener on it's side, that's why I was wondering if I could neglected the prying action. They'll be some deformation, I agree, but they'll be much lower then even with a really thick bar.

 

[rb1957]

i suspect your code is conservative ... assume fully fixed ends when designing the prying load, assume SS when designing the fitting (as the end moment reduces the moment in the fitting).

It sounds like you're having trouble with either the stresses or the displacements of the fttg. a stiffener (ie angle) would be more efficient in reducing both of these (compared to a thick plate).

 

[azcats]

IMO, when you switch to the angle, the prying action changes direction by 90 degrees and is mainly due to the eccentric loading between the application point and the bolts.

The angle is likely/possibly stiff enough in the other direction to neglect the prying.

How about a channel of some sort to get rid of the eccentricity?

 

[Norgad]

Thanks for the tips guys!

I never though about a very small channel. It can be a good idea for the purpose and it would fit just right. I'll see what I can design with those.