Discussion: View Thread

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

Craig, a free-body-diagram of the bolt would shed some light on the problem.

In this case, I would say the bolt is subjected to single shear based on the following:

    - it's conservative, justifying double shear would take some in depth analysis

    - I am assuming your corrugated metal is thin (likely gauge material)

    - the shear plane where the metal is in contact is easy to justify as single shear

    -  the shear plane on the opposite side has a gap...
                  is it less than or equal to 1/4"?  then you can ignore it and consider it double shear
                  is it between 1/4" and 3/4"?  then you have to reduce the strength and shim it
                  is it greater than 3/4"? then you have to fasten the shim to transfer the load

    -  going back to your free body diagram of the bolt...
        considering it as a beam, is your load closer to one support than the other? 
        would you be inducing bending on the bolt?  that is best to be avoided

We can conclude that one shear plane is loaded more than the other.  But, by how much?  Any justification of the amount would be subject to judgment and be questionable by the reviewer.  To find the true answer, testing would likely be required. 

So yes, it is likely double shear, but in practice you should only consider single shear.  You need to be able to justify any model presented in your calculations.  Designing for single shear is the safe choice. 

From what you are describing this is a different condition than a though-bolt in HSS, which is common and designed as double shear.

------------------------------
Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
------------------------------

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

I agree with Chad, i.e. I would design the bolts for single shear. In this case, each bolt is loaded eccentrically, so the loads on the bolt would not be P/2 where the bolt is in contact with the solid plates. The eccentricity would increase the load on one end of the bolt and decrease it on the other end. One could solve for the actual loads, but the effort is likely not justified. I would size the bolt for single shear instead. Elongation of the bolt hole in the corrugated steel due to crushing would likely occur way before bolt failure, maybe even tearing the connection out of the sheet metal. 

My $0.02 worth :-).

------------------------------
Stacey Morris P.E., M.ASCE
ETI Corporation
West Memphis AR
------------------------------

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

This is more or less where we ended up as well. The other PE's and SE's in the office conservatively used single shear in this application. However we all believe that testing would likely result in a larger load than just single shear. In the end I feel like the bolts themselves and the quantity for the connection will be single shear, but for  the plate calcs (tension rupture, bearing, etc.) the loads of double shear could be used.

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
Leesburg IN
------------------------------

Original Message:
Sent: 10-17-2019 09:44
From: Stacey Morris
Subject: Single vs. Double Shear Plane

I agree with Chad, i.e. I would design the bolts for single shear. In this case, each bolt is loaded eccentrically, so the loads on the bolt would not be P/2 where the bolt is in contact with the solid plates. The eccentricity would increase the load on one end of the bolt and decrease it on the other end. One could solve for the actual loads, but the effort is likely not justified. I would size the bolt for single shear instead. Elongation of the bolt hole in the corrugated steel due to crushing would likely occur way before bolt failure, maybe even tearing the connection out of the sheet metal.

My $0.02 worth :-).

------------------------------
Stacey Morris P.E., M.ASCE
ETI Corporation
West Memphis AR

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

I would not use a double shear model for the plate limit states.  The plate limit states are based on the thickness of 1 plate.  To best match the AiSC approach, you would determine the capacity of that single plate, then compare it to the load applied.  We just agreed, that the load applied is greater than P/2.  The model that you choose needs to be consistent throughout your calculation.  The load applied to a single plate should be considered P.  

I agree, It is unlikely that your plates/bolts will fail before your corrugated layer.  Should 1 plate fail (unlikely) the bolt would rack and apply weak axis bending to other plate, it would be considered a failure even if it is not a catastrophic one.  Consider the unloaded plate a form of redundancy, only.

I stay away from multiplying any capacity by 2 because in compression, the buckling is very dependent on the thickness and slenderness of an individual element.  Grouping 2 like members together can create confusion about the limit state geometry.  The geometry for the member is fixed.  The load is variable based on the distribution you determine.

------------------------------
Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
------------------------------

Original Message:
Sent: 10-17-2019 10:30
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

This is more or less where we ended up as well. The other PE's and SE's in the office conservatively used single shear in this application. However we all believe that testing would likely result in a larger load than just single shear. In the end I feel like the bolts themselves and the quantity for the connection will be single shear, but for  the plate calcs (tension rupture, bearing, etc.) the loads of double shear could be used.

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
Leesburg IN

Original Message:
Sent: 10-17-2019 09:44
From: Stacey Morris
Subject: Single vs. Double Shear Plane

I agree with Chad, i.e. I would design the bolts for single shear. In this case, each bolt is loaded eccentrically, so the loads on the bolt would not be P/2 where the bolt is in contact with the solid plates. The eccentricity would increase the load on one end of the bolt and decrease it on the other end. One could solve for the actual loads, but the effort is likely not justified. I would size the bolt for single shear instead. Elongation of the bolt hole in the corrugated steel due to crushing would likely occur way before bolt failure, maybe even tearing the connection out of the sheet metal.

My $0.02 worth :-).

------------------------------
Stacey Morris P.E., M.ASCE
ETI Corporation
West Memphis AR

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

Chad,

The two things on why we feel more comfortable with using double shear on the plates ONLY are:

1. These are tension only members so there are no concerns about buckling.
2. There are multiple rows of bolts on the corrugations. So some bolts are: Metal, Metal, Gap, Metal. With other bolts being: Metal, Gap, Metal, Metal. This balances out the TOTAL load on the plates, although each individual bolt still seems like it may be in single shear.

Even though, I will admit, it is odd to use the two methods mixed, it seems to make sense in this particular case. That said, the difference in plate grade and thickness may be so small of a difference between double and single shear that it may be good to keep conservative and use single shear for everything.

------------------------------
Craig Smallegan EIT, A.M.ASCE
------------------------------

Original Message:
Sent: 10-17-2019 11:33
From: Chad Morrison
Subject: Single vs. Double Shear Plane

I would not use a double shear model for the plate limit states.  The plate limit states are based on the thickness of 1 plate.  To best match the AiSC approach, you would determine the capacity of that single plate, then compare it to the load applied.  We just agreed, that the load applied is greater than P/2.  The model that you choose needs to be consistent throughout your calculation.  The load applied to a single plate should be considered P.

I agree, It is unlikely that your plates/bolts will fail before your corrugated layer.  Should 1 plate fail (unlikely) the bolt would rack and apply weak axis bending to other plate, it would be considered a failure even if it is not a catastrophic one.  Consider the unloaded plate a form of redundancy, only.

I stay away from multiplying any capacity by 2 because in compression, the buckling is very dependent on the thickness and slenderness of an individual element.  Grouping 2 like members together can create confusion about the limit state geometry.  The geometry for the member is fixed.  The load is variable based on the distribution you determine.

------------------------------
Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI

Original Message:
Sent: 10-17-2019 10:30
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

This is more or less where we ended up as well. The other PE's and SE's in the office conservatively used single shear in this application. However we all believe that testing would likely result in a larger load than just single shear. In the end I feel like the bolts themselves and the quantity for the connection will be single shear, but for  the plate calcs (tension rupture, bearing, etc.) the loads of double shear could be used.

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
Leesburg IN

Original Message:
Sent: 10-17-2019 09:44
From: Stacey Morris
Subject: Single vs. Double Shear Plane

I agree with Chad, i.e. I would design the bolts for single shear. In this case, each bolt is loaded eccentrically, so the loads on the bolt would not be P/2 where the bolt is in contact with the solid plates. The eccentricity would increase the load on one end of the bolt and decrease it on the other end. One could solve for the actual loads, but the effort is likely not justified. I would size the bolt for single shear instead. Elongation of the bolt hole in the corrugated steel due to crushing would likely occur way before bolt failure, maybe even tearing the connection out of the sheet metal.

My $0.02 worth :-).

------------------------------
Stacey Morris P.E., M.ASCE
ETI Corporation
West Memphis AR

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

Agreed.  Never change models.  But if you do, describe the reasoning explicitly as you have done.  Too often, we think the reviewer is a mind reader.

------------------------------
Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI
------------------------------

Original Message:
Sent: 10-17-2019 16:46
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Chad,

The two things on why we feel more comfortable with using double shear on the plates ONLY are:

1. These are tension only members so there are no concerns about buckling.
2. There are multiple rows of bolts on the corrugations. So some bolts are: Metal, Metal, Gap, Metal. With other bolts being: Metal, Gap, Metal, Metal. This balances out the TOTAL load on the plates, although each individual bolt still seems like it may be in single shear.

Even though, I will admit, it is odd to use the two methods mixed, it seems to make sense in this particular case. That said, the difference in plate grade and thickness may be so small of a difference between double and single shear that it may be good to keep conservative and use single shear for everything.

------------------------------
Craig Smallegan EIT, A.M.ASCE

Original Message:
Sent: 10-17-2019 11:33
From: Chad Morrison
Subject: Single vs. Double Shear Plane

I would not use a double shear model for the plate limit states.  The plate limit states are based on the thickness of 1 plate.  To best match the AiSC approach, you would determine the capacity of that single plate, then compare it to the load applied.  We just agreed, that the load applied is greater than P/2.  The model that you choose needs to be consistent throughout your calculation.  The load applied to a single plate should be considered P.

I agree, It is unlikely that your plates/bolts will fail before your corrugated layer.  Should 1 plate fail (unlikely) the bolt would rack and apply weak axis bending to other plate, it would be considered a failure even if it is not a catastrophic one.  Consider the unloaded plate a form of redundancy, only.

I stay away from multiplying any capacity by 2 because in compression, the buckling is very dependent on the thickness and slenderness of an individual element.  Grouping 2 like members together can create confusion about the limit state geometry.  The geometry for the member is fixed.  The load is variable based on the distribution you determine.

------------------------------
Chad Morrison P.E., M.ASCE
Professional Engineer
Greenville RI

Original Message:
Sent: 10-17-2019 10:30
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

This is more or less where we ended up as well. The other PE's and SE's in the office conservatively used single shear in this application. However we all believe that testing would likely result in a larger load than just single shear. In the end I feel like the bolts themselves and the quantity for the connection will be single shear, but for  the plate calcs (tension rupture, bearing, etc.) the loads of double shear could be used.

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
Leesburg IN

Original Message:
Sent: 10-17-2019 09:44
From: Stacey Morris
Subject: Single vs. Double Shear Plane

I agree with Chad, i.e. I would design the bolts for single shear. In this case, each bolt is loaded eccentrically, so the loads on the bolt would not be P/2 where the bolt is in contact with the solid plates. The eccentricity would increase the load on one end of the bolt and decrease it on the other end. One could solve for the actual loads, but the effort is likely not justified. I would size the bolt for single shear instead. Elongation of the bolt hole in the corrugated steel due to crushing would likely occur way before bolt failure, maybe even tearing the connection out of the sheet metal.

My $0.02 worth :-).

------------------------------
Stacey Morris P.E., M.ASCE
ETI Corporation
West Memphis AR

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

​Craig,

I've appreciated the discussion thus far. I think one of the most salient points is that there is no diagram or details to really help answer the question. The remaining points made are all good ones based on the assumptions made. My contribution is that any solution needs to be compatible throughout all parts of the design. There cannot be incompatibilities between forces assumed for the bolts and forces assumed for the connector plate design.

If your initial statement is correct, that the connector plate forces are each P/2, then the bolt group must be in double shear. I believe you suggested that on average through the entire bolt group you feel that the P/2 assumption is correct. If that's accurate, my approach would be to design the connection overall as if the bolts were all in double shear. Without knowing details, it seems reasonable to assume that inelastic connection behavior and redistribution of bolt forces will make this a reasonable assumption.

If you investigate the bolts directly and feel that they have important unbalanced shear forces at each connector plane, creating a concern for single shear, then the assumption of P/2 in each plate cannot be correct. What you have is analogous to shear lag. You should design the connector plates accordingly for the force you think may be offset through shear lag.

Best of luck.

------------------------------
Brett King P.E., S.E., M.ASCE
Senior Structural Engineer
GHD Inc.
Lake Oswego OR
------------------------------

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
Leesburg IN
------------------------------

Original Message:
Sent: 10-18-2019 13:58
From: Brett King
Subject: Single vs. Double Shear Plane

​Craig,

I've appreciated the discussion thus far. I think one of the most salient points is that there is no diagram or details to really help answer the question. The remaining points made are all good ones based on the assumptions made. My contribution is that any solution needs to be compatible throughout all parts of the design. There cannot be incompatibilities between forces assumed for the bolts and forces assumed for the connector plate design.

If your initial statement is correct, that the connector plate forces are each P/2, then the bolt group must be in double shear. I believe you suggested that on average through the entire bolt group you feel that the P/2 assumption is correct. If that's accurate, my approach would be to design the connection overall as if the bolts were all in double shear. Without knowing details, it seems reasonable to assume that inelastic connection behavior and redistribution of bolt forces will make this a reasonable assumption.

If you investigate the bolts directly and feel that they have important unbalanced shear forces at each connector plane, creating a concern for single shear, then the assumption of P/2 in each plate cannot be correct. What you have is analogous to shear lag. You should design the connector plates accordingly for the force you think may be offset through shear lag.

Best of luck.

------------------------------
Brett King P.E., S.E., M.ASCE
Senior Structural Engineer
GHD Inc.
Lake Oswego OR

Original Message:
Sent: 10-16-2019 10:26
From: Craig Smallegan
Subject: Single vs. Double Shear Plane

Got into a discussion around the office yesterday about single vs. double shear on bolts. It's usually quite straight forward, but what if the center piece of the connection is corrugated? On the global scale, it is still in double shear since there are two outer flat plates pulling one direction and an inner corrugated plate pulling the other. So P/2 + P/2 = P. But on a local level looking at an individual bolt, it now has a stack-up of metal, metal, gap, and metal. Does this gap now put it in single shear?

------------------------------
Craig Smallegan EIT, A.M.ASCE
Design Engineer
DeMotte, IN
------------------------------