Aspect Ratio of Small Tower

[jike]

I am designing a small, 4 legged, open structure with braced frames on 4 sides to support a couple of industrial platforms, 45 feet in height. No significant dead load other than the steel weight. Wind will be limited because it is relatively open.

I am looking for suggestions on a starting point for the distance between legs. I have my own ideas, but want to see what you guys think.

 

[civilperson]

Fifteen foot as a minimum between columns.

 

[DaveAtkins]

I guess 22'-6" (one-half the height) is a good starting point, but I think a closer spacing could work, as well.

DaveAtkins

 

[jike]

Wow! 15 feet is an aspect ratio of 3:1! 22'-6" is an aspect ratio of 2:1!

I was thinking more of an aspect ratio of 6:1 which gives about a 7'-6" base.

The piece of equipment that we are accessing has a similar aspect ratio (although more DL and more WL) and I don't want the platform structure to look out of place.

I guess it has more to do with "looking right" than engineering.

 

[LOKSTR]

Jike,

Your aspect ratio will depend upon your panel height. Panel ht. and bottom width will influence your bracing angle( depending upon X or K type bracing you may have). Try to keep the angle between 30-55 degrees to avoid fabrication and erection problems.

Also keep in mind the requirement of ladder slope. EIA/TIA-G and CSA S-37 will not allow too steep slope for safety reasons.

LOKSTR

 

[warrenw]

jike

I agree with you. 7'-6" seems just fine for a starting point. I would assume that the overall size of the structure will be governed by the platform geometry and then the members will be sized for the wind and D.L.

 

[Dinosaur]

I would probably start by analyzing an aspect ratio from 3:1 to 5:1. I assume the aspect ratio will be controled by the amount of tension you are willing to develop in the legs of the tower.

 

[MiketheEngineer]

Actually being in "OPEN" space increases the wind load - probably Categoty "C"

 

[rn14]

I have been doing a lot of trussed towers and bridges for a flyash pipe. For ones that are about 11m tall the base is 2.5 x 1.5m but they taper in the2.5 meter direction to about 2m. Some of the foundations have been upwards of 6m sq. by 1.5m thick to resist overturning but other than that they have been working fine.
Make sure to actually calc the open area as many times trussed structures are not as open as you think as far as code is concerned.

 

[SLTA]

MiketheEngineer, I believe the reference to "open" was to the fact that it's not considered a "closed" structure, not to the exposure category of the location.

ASCE 7 gives force coefficients too for trussed towers, so that may also help determine size and bracing requirements.

 

[chencfp]

For analysis point of view, you need to consider wind speed in the tower site per ASCE 7. you might also need to consider "P-Delta" effect from dead load. For fabrication porint of view, the straight section is easer to fabricate than tappered section. Your tower base width can go as small as 3-ft based on the platform wind area, wind speed and deflection limits.