Triangular Lattice tower with Pipe Sections

[engagarcia]

I´m trying to design Triangular Lattice tower for Communications with Circular Hollow Sections or Pipe Sections. I would like to know if ASCE 10-97 is the right code to check this kind of structure. I used to design transmission tower with this code, but there´s no reference to Pipe Sections(3.7 COMPRESSION MEMBERS:ANGLES and 3.9 COMPRESSION MEMBERS NOT COVERED IN SECTIONS 3.7). Should I read the ASCE 48-05 (Design of Steel Transmission Pole Structure)?

Thanks

 

[transmissiontowers]

Comm Towers loads are covered by the EIA/TIA 222. You might check the PLS-CADD web site

http://www.powline.com/products/tower.html

to see what codes are supported. The major Comm Tower suppliers usually have their own programs, but PLS-Tower is easy to use and will analyze guyed and self supporting lattice masts with pipe or solid round legs for TIA loads.

ASCE 48 is mainly for tubular pole structures, while ASCE 10 is for angles and/or light gage shapes.

GTStrudl has a module for Comm Towers that costs extra.

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

 

[engagarcia]

I used to design Angle Lattice Tower for Overhead Transmission Line and check them by ASCE 10-97 Code. But right now, i would like to design Triangular Telecom Tower with Pipe Sections. I have read EIA/TIA 222, but there´s nothing regarding Pipe Sections. I used to handle PLS TOWER, MSTOWER and Staad Pro, but I´m looking for new software.Is ASCE 10-97 the right code to check the structure of the tower???
Thanks

 

[transmissiontowers]

I looked over the manual for PLS-Tower and there are loading sections for the EIA/TIA where you can automatically generate most of the loads from wind and ice on the structure. The members in the tower can be angles or pipes or solid round. Maybe what is confusing you is that the member properties for the structure is the .ANG file which can be angles, as well as channels, structural tubes, and the pipes. You input properties for area, rx, ry, rz and w/t plus wind width. The way Tower determines that a member is round is the w/t is 1.0 for round members. If you select a round member property, the bolts calculation are not used and members are considered to be welded at the connections.

IIRC, TIA Rev F is based on ASD while TIA Rev G is LRFD based. PLS-Tower will check for either method. You will have to remember that PLS-Tower is a truss analysis program and any moments in the members are not considered in the allowables and many warnings are given when the moments in the members become large.

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

 

[engagarcia]

I have questions about ASCE 10-97 and Transmission Towers.

Today, I have been reading AISC ASD and AISC LRFD, what is the main difference between them??? Because I have heart that for Overhead Transmission Towers with Pipe Sections you have to check them by ASD code.

Thank you.

 

[transmissiontowers]

AISC ASD and NESC/ASCE 10 are very difficult to co-exist. ASD is Allowable Stress Design and was taught in school for many years until LRFD came out. ASD was used for buildings and other steel structures. LRFD came out but most practicing Engineers stayed with the old ASD that they knew.

In ASD you apply the loads in some specified combination (I don't do ASD so forgive my errors if I make them) and the steel members are limited to some allowable stress like 0.6 x Fy for bending. The safety factor comes from reducing the allowable stress.

The NESC requires certain overload factors on the applied loads and codes like ASCE 10 give the capacity of the members from lots of testing of single angle members for compression and tension.

In LRFD, you apply overloads to the loads and design up to a Phi factor times the yield. The LRFD is more suited to 3 leg lattice towers with pipe or round legs, in my opinion.

EIA/TIA Rev F uses ASD (IIRC) and the older Fastest Mile wind. EIA/TIA Rev G uses LRFD and the more modern ASCE 7 wind maps with 3 second averaging period winds.

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

 

[engagarcia]

I have been studying EIA-222-G (LRFD), i have found many similiarities between LRFD and ASCE 10-97 in terms of how to calculate Design Stress for Compression Members.
Could you tell me why do you apply overloads to the loads, and then design up to a Phi Factors (0,85 or 0,9).
As you told me, are different wind calculations defined in 222-F and 222-G?

 

[transmissiontowers]

I don't have the EIA rev F and G in front of me right now, but the Rev F used a "Fastest Mile" method to rate the wind at a particular location and the newer Rev G uses the 3 second method.

The Fastest Mile is a rather old method and the averaging period is a variable. It was used by structural engineers in the 1930's until the change was made to a 3 second wind sometime in the 1970's. If you imagine a "mile of wind" passing a fixed point and you measured the wind speeds as they fluctuate from lulls to peak gusts, the average speed for that "mile of wind" was recorded for many weather data locations and published. At 120 MPH, a "mile of wind" would pass a fixed point in 30 seconds but at 60 MPH average the "Mile of Wind" would take 60 seconds to pass, thus the variable averaging period.

A 3 second wind report gives the average wind speed for many 3 second time periods and you publish the maximum of those measurements on a wind map.

The ratio of peak gust to the "fastest mile" average speed is always much greater than the 3 second wind average to peak gust ratio because the averaging time is so much shorter. IIRC, the gust factor used to be around 1.3 for fastest mile and it was squared with the velocity to get the pressure on a stationary object.

The LRFD is just a method of doing an analysis where you apply overloads to the loads and reduce the allowable with a relatively small Phi Factor based on the material. For codes like the NESC that specify what minimum OLF's to use, the LRFD method is better.

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