Guy Wires for a Stainless Steel Chimney 1

[KarlT]

I am looking at a project in which some existing stainless steel chimneys are projecting 25 ft above a roof. The chimneys are for some summer boilers, and the client figures the exhaust may be around 800 degrees F. They have requested that we look at the possiblility of extending the existing chimneys another 20 feet.

During a site visit, I noticed that the existing guy wire support brackets have pulled upwards and have lifted the lag screws out of the parapet. I am concerned about the thermal expansion of the stack (I believe that's what caused the failed brackets, not wind loads).

The existing guy wires have an adjustable threaded turnbuckle at the bottom to allow for tightening. I'm wondering if there are some spring loaded connections available to allow the pipe stack to move upwards, but still maintain some tension in the wires when the boilers are not operating?

 

[ChipB]

Remember F=1/2kx^2. As x gets larger, so does your force. To my knowledge, you won't be able to get away from this using guy wires.

Can you add stiffeners to the stack such that it would be self supporting? I've always used channel framed toes out around stacks, with T's welded to the stack for guides. Maybe build a small tower like this then guy wire the tower. The tower will undergo some thermal expansion, but not nearly as much as the stack.

Chip

 

[GregLocock]

"To my knowledge, you won't be able to get away from this using guy wires."

No, his suggestion of using springs, preferably travel limited, is entirely sensible.



Cheers

Greg Locock

 

[ChipB]

KarlT stated:
"During a site visit, I noticed that the existing guy wire support brackets have pulled upwards and have lifted the lag screws out of the parapet. I am concerned about the thermal expansion of the stack (I believe that's what caused the failed brackets, not wind loads)."

While using springs to aid in restraining the stack is a viable option, the increased loads due to thermal expansion wil not go away. The supports must be designed to accomodate this and he is clearly already concerned on the ability of this connection to transfer forces.

Newton's Third Law:
For every action, there is an equal and opposite reaction.

 

[VoyageofDiscovery]

Hi KarlT,

Another take on this, calculate the force required to restrain this thermal expansion and design a new connection at the parapets and guys if necessary.

You will probably need to redesign anyway as you have been asked to look into extending the stack. The possibility of having another level of guys due to the natural frequency of the new stack also exists.

Regards

VOD

 

[KarlT]

I think I may have caused a bit of confusion with my post. The original brackets failed because they were overloaded by the guy wires. I am guessing that the guy wires were all pre-tensioned. Then when the boiler fired up and the stack eleongated upwards, the guy wires stretched and the weak link in the chain was the connection, which gave way to relieve the build-up of stresses.

I guess my thought was that if the guy wire was pretensioned and then the wire had to elongate an additional amount, then that amount of elongation may break the guy wire. (Delta = PL/AE, P may become greater than Pultimate).
If a spring is installed in the system, then my thought was that the delta is taken up by the spring, and the guy wire no longer elongates. However, I guess what ChipB is saying is that the tension in the guy wire will still increase, since it is still part of the overall system?

 

[warrenw]

KarlT;
I don't know if you have the room, but if you can move the guy rope connection location further away from the stack it will help reduce the expansion load on the ropes. In other words try to make the guy ropes more horizontial.

 

[ChipB]

KarlT,

Correct. The use of the springs will aid in the reduction of the thermal loads by decreasing the strain in the cable by the amount the sprong compresses, however, you'll still get an increase at the connections.

You can calculate the amount of vertical thermal growth at the guy wire connection if the wire wasn't there, then, using the guy wire angle, calculate the vertical component of your PL/EA of your wire, and model it in as a spring support, and figure out the overall strain and force in the guy wire.

 

[VoyageofDiscovery]

Typically, guys for these types of structures require some initial tension. Make sure you account for this.

Regards

VOD

 

[msajjadh_imported]

KarIT

I would suggest you to modify the connection at guy wire support. To allow elongation, you can make a slot so that guy wire can adjust its length by sliding in the slot. During normal operation, when the temp. will be high, the guy support will shift closer to the chimney. But in shutdown condition, there should be a spring to pull the support back to its original position.

Regards,