Double angle bracing connections in transmission towers 1

[Zafery]

Sir

I have used a double angle back to back cross bracing connection in a transmission tower. In the structural detailing we make two members
(one for each group) continues and other one discontinues.

Now my consultant say "if you provide one continues and one discontinues in same compound, it will create unequal bearing stress distribution on the bolt because of bolt -hole tolerance of 1.5mm and unequal member length.

Therefore i want to check the actual bearing stress in the connection to prove the provided connection is ok.
(I have attached a reference drawing).

Please advise accordingly.


Thanks & Regards
Zafery

 

[PAstructural08]

Perhaps this is just me, but I find this post confusing (especially the first two sentences). I'm not to sure what your saying and what you are actually asking.

Are you asking about the connection where the two double angles cross? What do you mean when you say "compound"?

Some clarification could help.

 

[paddingtongreen]

The bolts in the continuous angles only see the stability force from the discontinuous angles and usually have fewer bolts. The bolts and the gusset plates carry the full load of the discontinuous angles on one side to the discontinuous angles on the other side but don't transmit force into the continuous member except for stability forces. The oversize of the bolt holes is ignored industry wide, member lengths also have a tolerance which is ignored, this is not clockwork, exact fit is assumed for design purposes.



Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[KootK]

Now my consultant say "if you provide one continues and one discontinues in same compound, it will create unequal bearing stress distribution on the bolt because of bolt -hole tolerance of 1.5mm and unequal member length.

This is probably true. One brace will transmit its load through two bolted connnections with slip; the other brace will transmit its load through four. In theory, the continuous brace would pick up load first, before the discontinuous brace kicks in.

As Paddington has suggested, this is stuff that we typically ignore in practice. Me make buildings, not pianos.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[Bendog01]

My experience with double angle bracing is to have both continuous angles in the same "group" or direction, with the discontinuous angles in the other "group" or direction. I believe this method is also easier for the detailer.

 

[paddingtongreen]

I missed that each direction had a long and two short angles. I missed it because I would never arrange it that way except in extraordinary circumstances. If the two long angles are together, the force goes directly through the member at the joint. It is easier to erect if the long angles are parallel too. Any concerns with different shears on the bolts go away too.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Zafery]

Dear Sirs.

We have arrage is that way to get some support at the crossing point.If we provide two long angles contineus and other to discontineus ,it means the short angles are supported to the longer angles, in this case the long angles should be designed in compressing (L/R) for full length.
But as per practice in transmission industry and ASCE 10-97 recomendation if one group of angles are in compressing and the other group carries at least 20% of tension, the corrosing point can be considered as support, and in comprsssion we design only with half length.

Therefore i need to know if i can provide some supporing calculation to verify that.
Can somebody help.

 

[mtu1972]

Fortunately, I have never worked in the transmission industry. I've worked on buildings for 40 years and would never do X-bracing as you have proposed.

In fact my preference is to use 2 sets of double angles (where double angles are necessary) with one piece passing in front of and the other behind a plate that connects them at their intersection. Then the field erection is 2 pieces instead of 3. I also noted where necessary, because in buildings the X-braces are often considered tension only, and the brace that would be in compression is neglected, so single angles may prove to be sufficient.

As to the comment "in compression we only design with half length", I do not see how that works for the out-of-plane stiffness. And with the arrangement that you described, I am not sure how one would work with the combination of 1 continuous plus 2 half-length angles for the out of plane buckling/compression. Seems that you have a weak-link where the continuous angles are by themselves.

gjc

 

[paddingtongreen]

In the arrangement you show, you would be using "tension only" or tension/compression bracing. If it is tension only, you have no problem except the "compression" does need to be designed for some reduced amount, we used to use 20%.

If you are using compression/tension bracing, the double angle has much more lateral resistance than the single angle and with approximately half the load travelling through the tension members, they are considered to stabilize the compression members at the joint.

Years ago, I did work on modifying some old towers but I didn't see this arrangement.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Zafery]

Dear Mr. Peddington/Mr. GJC

Sir, your reply was very helpful, However i just want to make a few point's as follows:-

1) Since in the transmission tower most of the crossing diagonals/lattices are compression/tension type and are critical predominantly in compression. Therefore to get additional strength in compression we choose double angle bracings.

2) Usually Out of two sets of crossing diagonal one is in compression and the other one is in tension, Because of this situation ASCE-10-97 has recommended that if the tension member in a set of crossing diagonal has at least 20% force that of compression, then the crossing point can be considered as support, therefore for L/R capacity calculation length can be considered from the crossing point.
3) As the main load on the transmission tower is wind on structure and conductor, and its direction is not certain.
Therefore in a set of crossing diagonal one member is in compression if wind load is coming from X direction, but the same member can be tension if the wind direction is -X. Because of that we design both the set of crossing diagonal for compression as well as tension.

4) Also since the quantity of similar structures are more in transmission line weight need to be optimized very much.

Based on your earlier reply, i tried to justify my connection by a calculation, as attached; i am not very sure about that,
Therefore i request if you see the attached calculation, please refer the attached calculation and advice accordingly.



Regards
Mohd. Zafery

 

[paddingtongreen]

Your attachment didn't show.

I am not up to date with the latest codes so I can't help with that. We can apply some logic, though. Failing that, perhaps someone who is current on the code will help.

The two long angles together are much stiffer and stronger laterally than they are separately, this alone says it is a better arrangement. It seems to me that the code clause that allows you to consider that tension in the one diagonal allows you to consider the center point pinned for the other direction, must also apply for both arrangements.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Zafery]

Dear Sir

I have attached the calculation now, kindly look in to it and let me know if itseems ok or advice accordingly.

Thanks & Regards
Zafery