Signal pole within span 1

[GoDucks]

Has any of you ever placed a cantiler mast arm signal pole within the span of a bridge? The signal designer on a project that I am working on is pushing for this.

I feel that inducing approximately 240kip-ft into the deck and Bulb-T beams is unreasonable.

This is complicated by the fact that the pole will be placed in a location where it may be impacted by vehicles, which would increase the forces that the deck would need to resist.

Any comments or thoughts?

 

[civilperson]

Place the base of the pole at the location of a diaphragm that is parallel to the mast arm, or add a diaphragm to connect as many bulb tees as are necessary to handle this induced moment.

 

[Qshake]

agree with civilperson, this is usually how we handle such situations.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[GoDucks]

I guess my question was poorly worded. I understand that it is possible...given the money and extra cost.

Do you have a history of performance of such installations?

How did you handle the case of the possible impact and ensuring that the pole fails before the deck?

 

[Qshake]

If you are working for a state DOT or equal (in other countries) the standard detail is a breakaway detail.

If you're not working for a DOT then you'll need to search the DOT's website to get familiar with what is used in that area and or discuss with a supplier of mast appertences.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Dinosaur]

This is not routine, but it is not uncommon in my area.

Consider first that a typical bridge overhang of 2.5 to 3 feet has to be able to absorb the crash load described in Chapter 2 of the AASHTO code. This is a 10 kip load normally applied 2'-8" above the deck at the top of a typical concrete barrier. The effects of this load are distributed over a five foot width as the load gets absorbed into the bridge. So the slab should have a capacity of no less than five ft-kips per foot width to resist this concentrated load of 10 kips creating a total moment of 26.7 ft-kips. This is usually much less than the concentrated moment at the base of a cantilever overhead signal or sign. At this point, I guess if the load were close to a 26.7 ft-kips load, I might consider using the next larger bar size over a five foot width of slab reinforcing, but that hasn't been my experience to date. So, I provide some type of bracket below the post and I make this bracket part of a transverse beam that ties the three exterior beams together.

I once was required to design such a mount for an overhead sign structure, cantilever supported, in a 100 mph wind zone with a cantilever arm of some 20 ft (to c.g. of panel) and two panels, one of 12 ft x 15.5 ft, and a second panel about 2 ft x 6 ft. I don't recall the precise numbers, but I do recall the final magnitude of the applied moment was about 10 times the crash load I described earlier. I detailed a steel beam under the slab, with stud shear connectors, and made it act with other steel to tie the three exterior beams together to help distribute the load. The DOT guy was outraged that I would propose so much reinforcing and said, "Just put a few extra #6 bars in the slab and forget about it." I then carefully described how I computed the moment and made the comparison to the crash load. I clearly pointed out that this load was 10x the crash load. He shrugged his shoulders and acted like I hadn't said anything. Then he went on about how we just needed to add a few rebar. If I didn't know the guy, I would have recommended the board take action against him. But I know he is vary capable and must have been having a very bad day.