Railings on concrete bridge section

[Curly]

I've been asked to quote for a job to place a set of railings along an old section of highway bridge that the customer is using to span his pond.

Anyone worked with these cast sections before? I have attached a quick sketch of the cross section. My real question and main concern is, where the posts will be placed there is only about 2-3 inches of concrete below, will that take being drilled without crumbling or breaking off or is it worth fixing the post to the side and coming up and around that bit? There is probably a good foot overhang. Not sure where the reenforcements are in these large slabs.

How would you go about doing this?

[John B]

Are you talking Cast iron section or cast concrete bridge slab section and what dimensions would help.

 

Are you replacing with steel rail or cast iron railings sections?

 

What traffic is there likely to be using the bridge, vehicle or pedestrian, or is it just visual?

 

Needs a little bit more information before answering that particular situation. Pictures of what is there would help.

[DSW]

Reinforcing is usually in the top and bottom of a concrete slab. General rule of thumb is that the reinforcing should be as far from the neutral axis of the member as possible for maximum strength. However concrete is a porous material. Usually you want the rebar at least 2" below the concrete surface to avoid water and salt getting thru and rusting the rebar causing spalling. Some codes want the bar even a bit deeper, say 3". generally you set bigger bar deeper. So if the slab is 4 to 6" thick, you might have rebar in the center. If it's thinner, I doubt it. Remember that applies to the sides as well as top and bottom.

 

 

How do you plan to set the posts? Are you thinking of coring and setting posts with anchoring cement, or are you thinking of plates and fasteners?  How you anchor will determine where and how far back you need to set these.  Tapcons can be set fairly close to the edge since the holes are small. Bigger concrete fasteners like the concrete bolts ( similar to tap cons but larger) take a bigger hole, thus need to be set back farther. Same goes with wedge anchors. You need enough mass to resist the side forces applied by the wedges. I'd set cored holes back even farther, and with these you'll probably go all the way thru if the crete is only 2-3" thick. The heavier the use, the heavier the posts need to be set. HAnd rails for a walking bridge don't need the sort of support rails do on a bridge to carry vehicles.

 

 

I probably wouldn't go thru the side. unless you plan to use tapcons or small fasteners. The thin section wouldn't be a great choice for side loads unless you spread it out with large plates.

 

Even if you hit bar, it usually won't end the world. A core bit will usually go right thru steel, but it is hard on the diamonds. At the edge, I doubt you will seriously weaken the structural integrity of the span. With rails up, no one will drive there anyways most likely. If it's not for road use, I doubt you have any structural issues. I'd avoid a hammer drill unless the holes are smaller than say 1/2".

[Curly]

Thanks for the replies, so to give a little more information. It's about 30ft long by 8ft wide, pedestrian only, although will have livestock using it from time to time. Mostly just sheep I believe. So it is cast concrete with currently no railings installed or preexisting fixings. The idea is to have simple box section posts with angle railings onto which hardwood boards will be bolted to.

DWS, I was planning on using plates and fasteners to set the posts. My original thought was to drill and set some threaded bar into resin and bolt the plates to that. If I were to bolt to the side as well as have a plate sitting on the top then I was hoping that would take the majority of the stress from the thinner part of the concrete slab.

[DSW]

Pedestrian bridge and light livestock  what you describe will work just fine. I'd set the rails back 6 to 8" from the edge and be done with it. That far back you should have plenty of "meat" to drill into to set threaded material with epoxy.

 

 

I'd be tempted to use anchor screws myself like the Redhead LDT anchors. They are a bit heavier than the tapcons I typically use for fastening to concrete and less work and cheaper than epoxies. I've used the standard tapcons to secure work platforms to the sides of concrete walls with no issues, but they are a bit thin and I have snapped a few driving them over the years. I've yet to snap one of the bolt type ones. I use standard tapcons to set plates for railings outside. They also sell Stainless tapcons if discoloration is an issue. I've used them on occasion as well. The only real "downside" I've ever had with tapcons is that they need the correct size hole. As the bits get worn, they sometimes get too small to set the tapcons right. Snap one off in too tight a hole and there's no way to get them out. If the material is super hard ( I've had some super hard concrete made using river jack stone that you can't easily drill thru down in the city) and the holes tend to open up and get too big as the drill hits the hard stone and wants to "wander". If the hole is too large, then the tapcons won't hold right.

 

 

http://www.itwredhead.com/screws.php

 

 

Epoxies don't loosen up over time though. They also seal out water and prevent freeze thaw issues. ( though in all the years I've set rails with Tapcons, I've never had a freeze thaw issue with them) Down side is that you really need to clean out the holes well after you drill them to remove the dust or the epoxy won't bond to the crete, just the dust. I spent 4 days drilling holes to set rebar with epoxy on one job. The guy behind me followed up with a bottle brush  we chopped the handle off and chucked in a cordless drill, and a blow gun I soldered a 16" length of 1/4" copper tube to. He bent the copper tube 90 deg so when he blew out the holes he wasn't getting all that dust back in his face. He'd blow the hole, brush, blow, brush and blow again... Then move on to the next hole. I'd drill about 2 holes for every one he cleaned. Then we had a guy who would come along behind and set the bars with epoxy once we had so many holes prepped so he wouldn't waste any of the epoxy.

[Curly]

Thanks for the info DSW, think I'll go down the anchor screw route as they look easy enough to install and like you say just set the plates back by 8 inches. If I can't get a decent hole drilled to take the screws I'll use the epoxy method.

[DSW]

Good luck and post up some picts when you are done.

[Frosty]

Curly: Epoxy anchors is how I' mount the railings. My favorite comes in glass tubes almost the same size as the anchor bolt use. You drill, drop in a glass glue tube, drop in the bolt and spin it with an air ratchet till it sets, about 30-45 seconds and there it is for good. The glass tubes have the epoxy is separate sections with a sharp gravel fill. The anchor bolt breaks the tube and spinning mmixes it thoroughly as it cleans the hole, te gravel jams the anchor bolt and the epoxy bonds it all permanently.

 

For your application almost any industrial anchor medium will do just fine, be it expanding anchor cement mechanical anchors or chemical glues or epoxies. all do the job. The only thing you need to do correctly is pick the one suited for the diameter and length anchor bolts you're using. It's simple arithmetic.

 

Frosty The Lucky.

[DSW]

Interesting Frosty. I'll have to look into those tubes. We usually use the 2 part Hilti epoxy like HIT HY 150 and the dedicated epoxy gun, and on rare occasions the  2 part type of epoxy that comes in a "standard" caulk tube for use with a regular caulk gun. I've seen the capsule epoxy from Hilti, but never used it. Usually I'm setting a bunch of anchors or bars, so the quantity in the tube doesn't matter all that much, but  the capsules might make sense for just one or two anchors vs planning on using a whole tube whether you need it or not.

 

 

Hilti used to be super easy to grab when Home Depot stocked most of the typical products. Saved me from having to drive all the way down in the city to the Hilti store when we needed something or having to wait for something to ship. The trip usually killed at least 1/2 a day with traffic even though they aren't really all that far away from me.

[Alan Evans]

Saved me from having to drive all the way down in the city to the Hilti store when we needed something or having to wait for something to ship. The trip usually killed at least 1/2 a day with traffic even though they aren't really all that far away from me.

 

Over here in the UK Hilti do a really good "deliver to site service", same day, which somewhat justifies their premium prices. I have used it once or twice when I have underestimated the resin quantities or forgotten to order in time!

 

The other source is the auction site, which means you can pick up a bargain of HY 150 that someone has surplus after a job. Double check its' use by dates of course.

 

Alan

[Frosty]

It's been years since I used the epoxy tubes, my greatest experience was testing them in the materials lab. We'd drill a test block of concrete or sawn rock cube and anchor a piece of thatever they needed tested, bolts, rock anchors, rebar, etc. Let them set and put them in the breaking press, it'd either press concrete test cylinders or pull using the attachments.

 

The stuff they used to anchor the rock along Turnagain Arm was in long plastic tubes with the catalyst in a stripe down the side. They'd drill, insert a tube in the hole, sometimes with a stream of water pouring out like a hose, shove the anchor bolt in and spin it till it stalled the air track drill rig. The couple times they overdid it the drill fatigued the 1 1/2" "Stressproof steel" anchor and snapped it, it NEVER broke the glue.

 

All that rambling and I have to say I don't know what the new stuff is like but my experience with epoxy anchor cements from 30+ years back says it's got to be better now.

 

Frosty The Lucky.

[DSW]

Yeah I know the epoxy itself seldom fails. Usually it's poor installation, as mentioned poorly cleaned holes, or failure of either the base material not being able to withstand the loads for the distance of the anchor, or failure of the fastener like you mentioned where they overloaded the material on instal. We covered some of this in our structures classes when I studied Architecture.

 

 

I find epoxies to be very useful, though the expense often means that it's not financially viable.

 

I mentioned in another thread a big job we did in February where we had to drill and set new anchor bolts and rebar in cold temps. When we did that big job, they screwed up all the layout on the footings when the surveyor set up his transit. That meant all the bar for the block was screwed up and all the column anchors were wrong. Since they had an engineer involved, and didn't want to rip out all the footings ( one footing took 67 yards alone IIRC and was 20ft x 20ft, 5ft thick with double mats of #5's top and bottom ( originally they wanted it to be 10' thick and we managed to talk them out of that since the footings for the walls weren't nearly that thick and only 10 feet away.) Instead we had the masons lay out the 1st course, so we didn't hit any of their webs, and then the Hiti rep and the engineer came up with a plan on how to reset the bar and all the anchor bolts for the columns. In a few cases we had to oversize the column base plates to miss old bolts we had to chop. Engineer wanted to watch us set the anchor bolts to make sure we cleaned the holes properly, but the cold temps chased him inside after a short while. :lol: He watched thru the window of the heated job site trailer instead.

[Crazy Ivan]

Safety warning: Some precast concrete bridge sections do not have rebar in them, but post-tensioning (high strength steel cable that is stressed to around 33,000 psi) drilling into one of those will ruin any appendage in the path of the whip (I have seen them snap, its horrifying) You will be able to tell if there is post tensioning used on it by checking the ends of the span (most likely front and back) for holes with anchor wedges in them. As I don't know exactly what you are working with, this may not be the case but in the off chance it is the case it could save your neck. There is a good chance its only rebar in there but for safety sake I figured I should throw that out there. Definitely epoxy coated (the green coated rebar) to prevent corrosion from road salt/water. Epoxy coated rods are used on bridge decks, roads, etc to allow the rebar to be placed closer to the surface (as DSW said, about 2-3") without rotting out. 

-Crazy Ivan