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# Converting moment into couple for two connections?

## Converting moment into couple for two connections?

(OP)
Hi there,

I need to check an existing connection and I want to make sure I'm going about this in the correct way.

To provide some background there is a metal pole that is connected to a hollow masonry CMU block wall with some mechanical anchors. I would like to check if this existing connection is adequate for the pull out loads.

I've drawn a sketch of the system below:

I guess my main question is to understand the structural system. It seems logical to me that if a bending moment is applied to the base of the pole from a wind force, connection 1 would be fully in tension and connection 2 in compression. Therefore, can I just convert the moment of 1.2kNm into a couple :

1.2/.711 = 1.7kN tension and 1.7kN compression.

As in both bolts in connection 1 will take the 1.7kN (0.85kN each) tension and both bolts in connection 2 will take 1.7kN compression?

Any advice would be appreciated if I am going about this analysis in the wrong way or if there is a better way to do it I would be happy to learn.

Thanks

### RE: Converting moment into couple for two connections?

analyze the pole as a beam (simply supported) on two supports ... which is the same as a cantilever built in at the midpoint of the two supports.

but I think the real question is is the moment a couple on the fasteners or is the "tension" force of the couple taken by the bolt and is the compression force the result of a distributed load against the wall ? If the pole is clear of the wall, is there a spacer around the bolts ... which would then provide a localised loadpath.

### RE: Converting moment into couple for two connections?

1) I think that you've got the right idea in general.

2) I get slightly different numbers than you. I'm sure that you'll be able to parse out my reasoning based on my math.

3) As rb1957 intimated, your compression connection may well be a direct bearing of the connection plate on the substrate rather than passing the force through the bolts there. That depends on the detailing of course.

(OP)

### RE: Converting moment into couple for two connections?

the overhand is 4000-852 = 3148mm
the max moment is at the upper fastener. The airload on the overhang is 600N *(3148/4000) = 472N, assume UDL,
and the moment is 472*3148/2 = 743243 Nmm

but this is IMHO a bad way to approach the problem. Always start with the reactions. Take moments about the lower fastener ...
600*(2000-141) = Ru*711, Ru = 1569N, Rl = -969N

my numbers slightly different to KootK's ... he overlooked the 141mm offset

### RE: Converting moment into couple for two connections?

#### Quote (rb1957)

my numbers slightly different to KootK's ... he overlooked the 141mm offset

That was intentional. I believe that is most correct to take the moments about the center of the lowest fastener group, not the lowest fastener.

### RE: Converting moment into couple for two connections?

sure, but then the load isn't applied 2000mm away

### RE: Converting moment into couple for two connections?

I see the system as a cantilever beam with a short backspan. The two outriggers are the supports and the bolt groups support each outrigger reaction. I took the liberty of using a 100mm cantilever below the lower outrigger based on your initial sketch which is why my numbers are slightly below KootK's.

I'm making a thing: www.thestructuraltoolbox.com
(It's no Kootware and it will probably break but it's alive!)

### RE: Converting moment into couple for two connections?

why 100mm, when OP has 141mm ?

### RE: Converting moment into couple for two connections?

Because the OP's sketch appears to show a 141mm spacing between fasteners, not from the lower bracket to the bottom of the tube.

### RE: Converting moment into couple for two connections?

I'm making a thing: www.thestructuraltoolbox.com
(It's no Kootware and it will probably break but it's alive!)

### RE: Converting moment into couple for two connections?

oh, ok ... I read as 141mm to the bottom of the tube.

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