What is the proper anchoring design againest a uplift force 2

[Magikgihan]

I am designing a roof top supporting structure for a four legged telecom tower to resist a uplift force of 770kN per one leg.Beams of the above structure has been designed assuming simply supported and no moments will be transfered to existing columns. More over chemical anchors will be used to resist the uplift force.What is the most suitable approch for a chemical anchor designing for supporting columns of size 250mmX250mm? Will this existing column is strong enough to carry such a loading.

 

[phuduhudu]

That sounds like a very large uplift force to take with chemical anchors into 250 sq columns. Remember the column itself then has to take that force too. That implies that it needs enough reinf to take that force.

 

[hokie66]

It is unclear from your post if the 250 x 250 columns are the legs of the tower or the supporting structure. Whichever is the case, 770 kN is an enormous load to support with post-installed anchors.

 

[Magikgihan]

Thanks all your comments, actually this is a roof top of six storey building, and the column is a structural member of the building.This is a 30m high lattice tower with a total weight of around 7 tons. I did not have the analysis report of the tower and i managed with the details provided by a third party. Can a uplift force of a 30m high lattice tower be 770kN? I thought that may be assumed as 400kN top at a wind speed of 160KMPH

 

[ChipB]

You need to push to get the loads from the tower vendor. Wind @ 45 will likely control maximum uplift.

Sorry, but it sounds like you need more supervision. Hire a structural engineer. Not trying to be ugly, just truthful.

 

[Magikgihan]

can anyone explain me how the uplift force is going to act on a four legged tower, precisely two towers legs under tension while other two under compression?

 

[hokie66]

That's right. Uplift is probably the wrong word. The tower legs resist the overturning wind force by forming a couple. But as ChipB says, you have to resist the force at 45 degrees also, which makes one leg take most of the force.

This is not just an issue of bolting down the legs. The building structure needs to be verified as well, and this sounds a bit scary. I hate telecom towers.

 

[rowingengineer]

Just to show you how complicate this subject could get, the bolts and plates will need to be designed for fatigue I would suggest. This could even lead to your post installed anchors needing to be pretension to some extent, and a maybe even some free length required. This will cause a nice big head ache. Then there is the building like hokie pointed out, always a bit scary if you are not the original designer.

Thus in summing up I agree with everyone general comments about getting a structural engineer on board that has some experience with this kind of application.


When in doubt, just take the next small step.

 

[racookpe1978]

If the column holding (resisting) the upward (and downward!) loads from the 45 degree wind load is part of the structure, why go to the (assumed concrete and steel!) column with chemical anchors?

Build all the way back to the structural steel inside all four of the columns, use multiple-through bolts to the reinforcing and internal column steel members with shear bolts, develop a complete steel pad fastened to the vertical steel inside the column on the roof (penetrating the nominal roof deck of course) then fasten the tower to the exposed steel.

The theorectical wind could come from any direction, will come from any direction actually, so all four attachment points have to be equally strong against the full up and down worst case forces. As mentioned, pre-tensioning is essential against the vibrating wind loads.