Tower connection design

[beagleboy]

Hi everyone,

Just wondering if I can get some guidance in tower connection design. I'm working on a tower reinforcement project (all steel) that may also require checking the strength of gusset plates, splices, block shear and maybe develop a new connection altogether. As far as connections are concerned, which method is more appropriate for towers- ASD or LRFD? There are overload factors for various loading conditions for towers/poles and capacity reduction factor (phi factor) 1.0 for steel but is it applicable to connection design as well? I found the thread below but would like to know more.

Thanks in advance!

thread608-397912



"There is no TENSION, it's always a COMPRESSION," said a wise man

 

[MotorCity]

Use whatever you are more comfortable with. If you need to convert ASD to LRFD, its real simple in the 14th edition of the AISC manual.....safety factor, omega = 1.5 / resistance factor, phi

 

[transmissiontowers]

I replied to the referenced post, and just read this one. (I don't check here often enough). ASCE 10 is the standard for T-Line towers and all things latticed. IIRC there is a newer edition available and a buddy at work is on the committee revising the standard.

In general, the gusset plates are designed for the same load as the member it is connected to without additional factors. The member loads are derived from forces on the structure that already have factors applied. The gusset plates are 1/16 inch thicker than the thinnest member in the connection so it the member passes the bearing stress, the gusset plate is OK. If you have a PLS license, the TOWER program allows you to check members and connections for block shear and all the other things that go into tower analysis.

I am curious if the tower you are working on exists in the field or are you analyzing an old design for heavier loads and making the connections stronger. If it exists, it is hard to retrofit stronger connections other than making the members larger, thicker, or both because the number of existing bolt holes is fixed and they cannot be easily added by drilling up on the tower. The best you can do is to change to high strength A394-T1 bolts or the really exotic A354-BC galvanized bolts to get the shear strength you need.

If it does not exist in the field, you will need to contact an experienced tower detail firm to layout the new connections.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[beagleboy]

Engineers, sorry for the delayed response!

@transmissiontowers thank you for the clarification! These are old towers and require reinforcements per revised design criteria. Regarding the phi factor being 1.0 for connections design, is there any codel provision that you can point towards? Just not sure if AISC formulae for connections are applicable to towers as well.

Thanks in advance!

"There is no TENSION, it's always a COMPRESSION," said a wise man

 

[transmissiontowers]

If you are doing T-Line lattice towers, you can ignore the AISC because it is for buildings. The ASCE has produced a Standard (ASCE 10-15) that carries the weight of AISC for lattice towers. The ASCE 10 code has their own tension and compression formulas and everything needed to analyze and design towers (and test them too). Our industry is pretty unique in that we test our structures in full scale tests and many times we test to destruction to verify the assumptions made in ASCE 10.

You won't see the building and bridge guys build a structure and then test it to destruction just to verify the AISC had the right compression curves. I guess they could but it would be expensive.

For your old tower, the NESC code says that you can use the code in place when the tower was originally designed. If you are adding more wire or turning a larger angle, you can check the tower for the code in effect at the time it was originally designed. You can go conservative and check it for the current NESC-2017 code if you wish. (this assumes you are in the USA).

I would look at the NESC of your choice, look at ASCE 74 to develop loads, consult the T-Line owner to see if there are any special loads to consider, and check the old tower according to ASCE 10-15 to see if the members are OK. If you use PLS-CADD, it will check the tower for your load criteria and tell you which members or bolts fail. You can do a manual member up-size and it will tell you what members will work. You can replace lacing members but replacing legs is almost impossible. If it is a compression problem you can try to add redundants to cut down the unbraced length for the leg but you will have to tell the field people where to drill the legs to attach the redundants.

On an existing tower, if the bolts are failing in shear, you will need to find stronger bolts because you can't add more bolts to a connection of an existing tower. A394-T1 are high strength bolts and A354-BC mechanically galvanized are as strong as you can get readily.

_____________________________________
I have been called "A storehouse of worthless information" many times.

 

[msquared48]

Stronger bolts in shear will not necessarily improve the connection capacity.

Remember to check the app0owable bearing stress on the hole too...

Mike McCann, PE, SE (WA)

 

[transmissiontowers]

Very true, Mike. But in general when you can replace the lacing or gusset plate, you can get a thicker member with higher yield so that the bolt shear is the weak link. In rare cases, I have considered reaming out the hole and going to a bigger bolt diameter. I have never done it (in over 40 years), but it might be possible. The usual trouble with bigger bolts is the bigger head which may interfere with the OSL of the lacing. This is why T-Line towers use A394 with smaller heads than A325 structural bolts.

_____________________________________
I have been called "A storehouse of worthless information" many times.