Attaching traffic signal heads to underside of post tensioned deck

[t230917]

Hello

I am working on a project where the traffic folks want to attach new 8" traffic signal heads to the underside of a post tensioned deck.

Has anyone done this before, and how should I proceed?

Can I make connections to the underside of a post tensioned deck? i am going thru the as built plans for the deck, and the PT strands are 5" to 8" above the bottom of the deck?

What will this connection look like? Any standards that I can look up?

 

[JLNJ]

PT strands often drape, and the spacing isn't necessarily nice and uniform. You will need to be vary careful, maybe even get the area scanned prior to fastening.

There are strict requirements for using overhead post-installed anchors. You might want to contact your local Hilti rep for fastener recommendations.

 

[t230917]

Yes, I understand. the as-built drawings have some details on the profile of the post tensioning strands, which i am studying carefully.

I will make sure that they scan the area prior to fastening.

Thanks for the heads up regarding talking with Hilti. i will definitely give them a call.

 

[STrctPono]

This is a bridge deck that you are talking about?

Is this a bridge deck on girders?

How thick is the deck?

I ask these questions because if anchoring to the bridge deck will not provide you enough capacity, I've utilized a spreader beam before where I span from girder web to girder web and attach the traffic signal head to that.

Either way, it's pretty straightforward with anchoring to any concrete with PT... You need to be careful and you need to scan the concrete so as to layout where any reinforcing might be. We have several different types of concrete scanners in our office and they are very useful for something like that. Once you know where the ducts and other mild reinforcing are, it's a pretty standard post-installed threaded rod anchoring system. I would not use any type of wedge anchors. Use a threaded rod and adhesive.

 

[Ingenuity]

...and the PT strands are 5" to 8" above the bottom of the deck?

To have tendons that are 5” to 8” from bottom of slab is very unusual. At some point along any given span the tendons will be at a low point with 1” min bottom concrete cover.

What is the floor framing system - one way slabs and beams, 2-way slab?

As JLNJ stated, definitely scan the slab first before drilling etc.

 

[r13]

If there is wall or column around, shall consider cantilever the signal out, rather than risk the PT slab.

 

[t230917]

Yes this is a bridge deck. It has a 13" slab with 14" concrete "ribs" acting as girders every 32.5'. So at the "rib" locations, the total depth is 2.25'.

I talked with the folks who are designing the signals, and they mentioned that the weight of these signal heads vary from 10 to 20 pounds.

Ok, I will stay away from wedge anchors and look for threaded rods with adhesive (I believe hilti has a few of these). From your previous experience, how deep do the threaded rods need to be inserted into the concrete? With the low weights of the signal heads, they dont need to support a lot of load.

The flooring system is a one-way slab with concrete beams spaced at 32.5' centers.

Yes, there are columns around. Would a better option be to cantilever a small steel section from the column of edge of bridge deck ad connect the signal heads to this cantilever?

 

[r13]

Yes, there are columns around. Would a better option be to cantilever a small steel section from the column of edge of bridge deck ad connect the signal heads to this cantilever?

For such small a weight, I guess the surface area subjected to wind is also small too. In that case, the cantilever beam maybe heavier than the signal, thus make it unattractive. Seems there are quite a few thing you can attach it to, do the best to avoid touching the PT slab, also, try to avoid overhead installation on existing concrete structure, as it is not reliable/durable. If you want more suggestions that in the right direction, a cross section will help.

 

[jayrod12]

I agree with the others indicating anchors in permanent tension are undesirable. Is there any way to mount it to the side of one of the beams? I'd feel more comfortable with the anchors in shear.

 

[t230917]

I have attached a section of the deck. The signal heads need to be under the westbound lanes (so between C to G).

Would there be any way a connection can be made at an expansion joint?

 

[r13]

Maybe there are better methods, but at this moment, I would back up your idea - one end to the vertical support, the other end hanging from the slab. Due to the uncertainty with the hanging mechanism, the support at the vertical support should have some moment capacity, so when slab support fails, the support member will deflect but not fall/fail immediately. Carefully review the profile/pattern of the PT cable will help to select a correct location to receive the inserts. Scan remains necessary.

 

[STrctPono]

Well that is a very strange bridge design. Perhaps it's normal in your neck of the woods but I certainly have never seen a bridge like that.

I was initially thinking that you could maybe span a structural steel spreader beam from one "girder" to another but they are too far apart.

Some on this forum may be hesitant to directly anchor the signal head to the soffit of the deck but I'm not. It sounds like you are mounting a small signal head. Not sure how far you are extending it down below the deck. Sometimes visibility is an issue so the signal head needs to be extended down below the beams. The weight of the signal head is not an issue. But as retired13 mentioned, you should check it for wind load.

Many people are gun shy of installing post installed anchors in an overhead scenario due to the big-dig incident some years back. Even though the adhesive manufacturers all produce very high quality creep resistant epoxies/polyesters/acrylates.... you still have the uncontrollable variable known as the Contractor. For this reason, don't afraid to install the rods deeper than is needed for the design. I wouldn't go any less than 6" embedment. I would be inclined to install it far enough to develop the yields strength of the threaded rod. Although I have never personally used it, HILTI makes the HIT-Z rod for the cases where cleaning of the hole is a concern. No matter what, you should call out in the specs/plans for the installers to be ACI certified.

One last thing, your bridge is crowned and the soffit of the deck follows the crown. However, the extension tube for the signal head needs to be plumb. Therefore, it should be noted on your drawings that the end of the extension tube will need to be beveled at the end slightly prior to welding to the baseplate.