Relocating an Existing Lattice Tower 1

[JRay852]

Hello!

I have been tasked to design a new foundation for a 60ft lattice framework tower that will be moved from one property to another. The tower will be equipped with a platform and winch system to raise and lower a clay target thrower for 5-Stand competition at a local gun club. The tower was previously standing at another gun club for the last 15 years, and there is no record of engineering on the tower itself. From what I've gathered, the tower was erected from (6) 10ft lengths of framework that was left over from an electrical service provider jobsite ages ago. My scope is for a new foundation design but I will need to do a load determination for loads acting on the tower and subsequent calculations to find reaction loads on the foundation.

Is it safe to assume the tower is capable to resist loads for it's intended use since the use will remain the same as it has for the last 15 years or will moving the tower require new engineering?

The tower has a 30"x30" square cross section and is made up of steel angles. The base is secured to the existing foundation with (12) 1" anchor bolts. After determining loads using ASCE 7, will calculating the reaction loads be bearable using hand calculations? I have access to RISA 2D but I'm not sure how I would go about modeling this 4-sided structure and apply the necessary loads. Any advice?

Thank you for your time.

 

[skeletron]

SHORT ANSWERS

Q1: Is it safe to assume the structure (with no record of engineering) is capable of resisting the loads using 15 years of performance as evidence?
A1: No.

Q2: Will calculating the reaction loads be bearable using hand calculations?
A2: With some reasonable simplifications, yes.

Q3: How do I model the 4 sided structure?
A3: Using a 2D analysis could give you some indication of the overturning, shear, and axial at the base, which would allow you to design your scope (foundation design). However, it would depend on how you apply your loads and the 2D analysis wouldn't take into account that wind happens with different azimuths.

LONG ANSWER
Q4: Any advice?
A4: It is reasonable to acknowledge that it has the ability to withstand certain common loads. That does not give an indication of whether the structure can withstand current code required design loads. There is also no indication of whether this is necessitated given that the tower is target practice in an open field. I believe that leaves some room for engineering judgement which is really up to you. However, I wouldn't rely on the assumptions as you've stated. Rather, I would do some sort of inspection on the structure's condition and go from there. In any event, you will need to do this for your own analysis.
With regards to the 2D analysis, I believe you could probably do it with some dramatic simplifications on the applied load (re: shielding of members, ice loads, load hitting the corner of the tower at 15 degrees). Again, this would require engineering judgement and some understanding of how the structure behaves under load along the non-geometric axis. For a foundation design (which is your scope) you likely can have enough gravy on your design to not warrant a 3D analysis. Who's scope is the deconstruction, and re-assembly? Are they doing analysis on the tower? You might be able to save yourself some time and see if they have reactions (which you could confirm using crude hand calcs).


...but I can't recall if I have ever solved that problem yet.

 

[JRay852]

Thank you for your response! I appreciate your advice.

The tower will not be deconstructed beyond removing the nuts on the anchor bolts. In fact, the tower has already been moved and is laying on its side at the site it is to be erected in the future. This is a rural area and the gun club was able to get volunteer help from some local farmers who had a boom truck to take the tower down (in-tact), haul it to the new site and lay it down. Unfortunately, this means there is no analysis being done on the tower. Based on my previous work with the AHJ, I imagine the County building department will require seeing calculations for the tower prior to permit approval. I don't mean to get into ethics here, but should I even consider completing a foundation design if the clients don't pursue getting an analysis on the tower? Obviously, I will do a load determination as discussed but my scope is for a foundation design and not for verifying tower member capacities. What is your opinion?

 

[SlideRuleEra]

I have been tasked to design a new foundation for a 60ft lattice framework tower...
The tower has a 30"x30" square cross section...
...should I even consider completing a foundation design...

Absolutely, yes. This is a wonderful low-risk project that requires thinking and engineering judgement... not facts just handed to you on a plate. Take skeletron's advice and make conservative simplifying assumptions. It is not only the use of the tower that makes the project low-risk but also it's small size. There won't be much permanent material cost (primarily concrete) no matter how (reasonably) conservative your assumptions. Most of the project's construction cost will be labor, not permanent materials. By all means do the calcs by hand, that will help you get a feel how variations in your assumptions affect the design. Don't pass this up.

http://www.SlideRuleEra.net

 

[ElliottJames]

Even though this is not a telecomm tower I would use the TIA-222 standard. The equations, methods, etc. are all highly refined for just this application, a lattice tower made out of angle iron. Wind loading, and the resulting P-delta, are important for a 60-foot standalone tower. Don't forget to install a copper ground rod and connect a fat wire to a leg, for static electricity drainage and lightning safety. Inspect the tower for rust, consider painting it before erecting it. If humans are expected to climb the tower regularly to install the clay shooter, then consult TIA-222 for climbing safety requirements, like step bolts and tie-off anchorages. Advise the club to implement a policy that climbers must wear a harness and be tied off to the structure at all times.

Alternatively, to eliminate climbing and risks thereto, you could design a hinged base. Enlarge the footing and install an I-beam or lattice tower perhaps 20 feet high for use as a tilting fixture. Add a winch to it with aircraft cable and tie it to the tower. The clay thrower could then be installed with the tower in the horizontal position and tilted back up for use. This would eliminate climbing entirely.

 

[msquared48]

Kzt could prove to be a problem...

You also need to model it on TnxTower or a comparable tower analysis program.

The tower itself may not be adequate for the new location.

Mike McCann, PE, SE (WA)