Reuse of Old Lattice Tower as a Sign Support

[marinaman]

I’ve got a project where my client wants to reuse an old lattice power tower as a support for a sign. The old power tower was designed and fabricated and erected by the industrial company that once existed here but is now long out of business. This tower was likely built sometime in the early 1960’s. This old tower is fabricated of galvanized steel angles. The lower leg angles are 5” x 5” x 1/2” and the “X” bracing is 3 1/2 x 2 1/2 x 3/8” angles.....just to give you guys a feel for the sizes. The tower is 46’ tall and has a base that is 9’-9” x 9’-9” in plan size.

This old industrial company formerly created their own power. They had their own power plant and supplied their own power for their operations. As such, they designed their own towers, buildings, etc etc.

For me, this tower analysis became part of my project....as my project went along. It wasn’t part of the project at first, but as the client wanted more and more service, this tower reuse became part of the overall project. Thus its now a task for me to review.....seeing if an old tower can be reused as a sign support.

I began review of this tower by purchasing ASCE 10 regarding lattice towers. I developed the lateral forces per ASCE 7 and then developed a 3D model of the tower via RISA 3D. I’ve completed the anlysis and am now comparing calculated stresses to allowable stresses. I’ve run the tower as-is....allowing compression in the “X” bracing....and I’ve run the numbers with only tension members.

I’ve attached a photo of this tower.

I’m trying to work thru a couple items.....just to be sure I feel good about these things in my mind:

First, there are no intermediate horizontal cross bracing elements in the tower (diaphragms). Coming from a building design background, it looks as though there should be some....but per ASCE 10, it looks like they do not recommend any until a tower gets 75’ tall or changes in shape....which this tower does neither.

Secondly, in the analysis, I did not put in any eccentricities due to connection geometries nor eccentricities due to the difference between the load path and the angle leg shear center. ASCE 7, in the commentary page 30, says that the eccentricities are accounted for in the Fa calcs in chapter 3. My analysis fa is only 3ksi....but my calculated Fa is 18 ksi. Seem’s like I’m way good.

What do you guys think about my concerns 1 and 2? Also, do any of you tower guys have any thoughts about a reuse of a tower of this type?

Also, I’m likely going to recommend removing some number of bolts for testing to see their condition. Many have a considerable amount of rust not them....but they may be fine. I can’t tell without removing some and looking at them. I had thought about replacing them all to be safe, but I may not need to do that.

 

[JAE]

Two thoughts
1. Check the connection designs thoroughly
2. What about the foundation? Going from one use to a sign use may (does?) increase loads on footings considerably.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[Hokie93]

The member sizes seem stout for a 46'-0" tall tower. Is the sign large relative to the size of the tower? Are you designing a new foundation? From the photograph, it appears the tower has been separated from its original foundation. I agree with both of JAE's comments.

 

[azcats]

Just out of curiosity, how big a sign are they proposing?

Regarding the hardware, I echo your concerns. It's very tempting to just replace them all with new galvanized bolts while it's sitting on the ground. But I could be convinced otherwise if removal of several showed that they weren't significantly deteriorated.

 

[marinaman]

I appreciate all the comments. When I get back at a computer tonight I'll respond to all the questions. Again, I appreciate it.

 

[oldestguy]

How valuable is the sign? If it blows down, any damage of consequence? If no big deal, use it as is.

 

[marinaman]

The tower was taken down from the industrial site and going to be reused in a different location. I’m going to be designing a new foundation for the tower.

The signage is 12’ Wide and 6’ tall. The signs will be on all four sides and open in between. The signage is located very near the top of the tower. JAE, you are exactly right, in that the foundation forces more than doubled!

I’m wondering if the lack of horizontal cross bracing will increase the potential for the tower to twist if subjected to high wind loads.

What do you guys think about the leg loading and chapter 3 of ASCE 10, taking eccentricity into account in their provided equations for Fa? I’m just so used to calculating axial/bending combined forces in buildings, it seems odd to ignore it per ASCE 10.

 

[Andries]

Have the local authorities been contacted on this? There may be concerns about the new location regarding aesthetics, closeness to dwellings, etc.

Andries

 

[msquared48]

Probably originally designed under TIA 222 guidelines. Check everything...

Mike McCann, PE, SE (WA)

 

[marinaman]

This reuse has been worked thru all of the local building officials, zoning, etc etc.

 

[transmissiontowers]

Prior to ASCE 10, there was ASCE manual 52 but the content was about the same IIRC. The 1960's was probably too early for EIA 222 and the structure was probably designed as a T-Line tower with a hand method of joints. Many assumptions were made to allow a hand analysis. (I did a few in my early years before computers) I'm not familiar with RISA and the capabilities to check the member capacities. I use PLS-CADD's TOWER to do analysis (about 90% of the USA use it for T-Line work) and GTStrudl has a tower analysis module but either one is not cost effective to buy just for one analysis. The model you have could be used for just the analysis and use hand calculations or a spreadsheet for the tension and compression capacities. The bolt shear values should be based on the bolts you will use and the bearing values are in ASCE 10. You might want to consider an oblique wind direction to find the maximum load on the foundations.

46' tall is relatively short for a T-Line tower and it may have been a 1 circuit horizontal configuration.

For the loads, I would use ASCE 74 to develop the wind on the new signs and wind on the tower members. Typically for WOT, you calculate each panel separately and apply a concentrated point load at the top of the panel points. For the wind on the signs, find the total wind area and apply the concentrated load at the attachment points. IIRC, a drag factor of 2.0 for flats is in ASCE 74. The members behind the signs will not get wind on them. Keep in mind T-Line towers were designed as a truss and you cannot apply a point load in the middle of a lacing or leg member because angles are terrible in bending.

The key to the capacity values is to read ASCE 10 carefully and get the right KL/r for the x, y, and z directions and get the correct end conditions. The compression equations are there for the legs and lacings and are based on many test values since we test our towers full scale.

Good luck

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

 

[Lomarandil]

Star for TT, and I'd give another star for his signature if I could.

----
The name is a long story -- just call me Lo.

 

[marinaman]

Here’s another interesting twist to this analysis:

Looking at quartering wind:

If we look at diagonal wind, then an interesting thing happens. Just talking in general terms....In order to get the projected area on the diagonal, it seems to me we would need to multiply the face area by 0.707. Then, once that area was determined, we would determine the lateral force on that face area. Then, we would multiply that force by 0.707 again, to get the force back into the “X” and “Y” axis.....and....we would need to do this procedure for each face to each side of the corner.

When you multiply all this out, 0.707 x 0.707 x (2) sides....you get 1.0. In other words, in analysis, it appears one would apply the full lateral wind load, of the X and the Y direction, onto the tower at the same time...and this would be the X and Y components of the diagonal wind.

Not only that, but per ASCE 7, diagonal wind must be multiplied by a direction factor not more than 1.2. So in reality, you’d have full X and y forces, acting on the tower at the same time, multiplied by 1.2.

Am I interpreting this correctly?

 

[transmissiontowers]

I try to stay away from ASCE 7 and there is a place there where they say they do not apply to T-Line structures, but since you are using a tower as a sign support, they may apply to your case. Anyway, read over ASCE 74 for a description of yawed or oblique wind. There is a Cosine squared term in the force equation so at 45°, you get a 0.50 factor for the perpendicular pressure to the face. I would take the wind pressure at the 45° angle to the tower and apply 1/2 the pressure on each face at the same time (times the member area) and calculate the force to be applied. The question now is what type of overload do you apply to the pressure.

In my world of T-Lines, we use pretty low factors since it is an extreme event.

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

 

[msquared48]

ASCE7 will control here due to the useage.

I agree with TT on the wind application though.

This structure may have been designed for wind + icing which could make it stronger than anticipated, at least the verticals.

Mike McCann, PE, SE (WA)

 

[transmissiontowers]

Mike is probably right. The NESC load factors were 1.67 on wire tension and 2.54 on wind back in the old days and we designed the steel up to yield. If bolt shear is a problem, change them out to ASTM A394-T0 which is an A325 with limited thread length to place the solid shank in the shear plane and not the threads. If the tower is really old, you may use A-7 steel properties unless you have documentation on what was used. If you don't trust the analysis, you might erect the tower in place and calculate the effective load from the signs and pull on it to see if it stands up.

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