Load carrying capacity of 15x50 s beams?

[rfc143]

Our private road is serviced by a bridge built in the 80's that consists of (6) 20' 15x50 S beams, covered with 4x6 hemlock cross members and topped with 2-1/2" x 12" planking. The bridge is about 15' wide. I inspected the underside and found several of the cross members broken but more important one of the beams' web where the beam sits on the abutments rusted out enough to allow the flange to bend upwards. There's some spalling of the concrete immediately beneath the failing beam.

We're thinking of leaving all the existing beams in place, but putting five new beams between the existing beams and replacing all the wood with Pressure Treated lumber. In the meantime is there a way to calculate what the safest maximum load on such a bridge might be so that we can post it until we get it rebuilt?

 

[SWComposites]

Is there a way to calc it? Maybe. Depends on the condition details and a lot of other stuff.
Is anyone on here going to do it? No.
Does the bridge currently have a load rating?

 

[rfc143]

Thanks. I have heard that when the bridge was built in the 80's it was to Vermont AOT standards at the time but who knows what those are? Do bridge standards in general specify a stress level or deflection for a given max weight (say 80,000 lbs; the weight of a fully loaded triaxle truck?)

 

[phamENG]

Based on your description of the condition, it sounds like it's on borrowed time already. The rusted web and bent flange suggests that one of your primary girders has already failed at the bearing, and is probably being held by an unreliable mechanism. In other words, determining exactly what or how much or when it will let go is as accurate as a literal crap shot.

If you plan to continue using that road, "until we can get it rebuilt" probably needs to be "now".

For a more nuanced answer, you need to hire a local engineer to come out and assess it in person.

 

[LittleInch]

So the beam are 3 feet apart. If one fails then with the broken criss members you could easily break through.

Which beam is broken?

What is the condition of the rest?.

But a 55 year old bridge has done its job. I doubt anyone will "rate" this bridge. If you can find the beans in decent condition limit the load to those and have a width restriction and limit to 2 tonnes.

What short of traffic does that bridge see?

 

[rfc143]

So the beam are 3 feet apart. If one fails then with the broken criss members you could easily break through.

Which beam is broken?

What is the condition of the rest?.

But a 55 year old bridge has done its job. I doubt anyone will "rate" this bridge. If you can find the beans in decent condition limit the load to those and have a width restriction and limit to 2 tonnes.

What short of traffic does that bridge see?

One of the two center beams. The others don't seem in too bad a shape. It's residential traffic, but there has been construction, with loads of hardpack occasionally crossing the bridge. Not looking to get it to AOT standards, but just safe to some limit. I thought that doubling up the steel with brand new beams (leaving the old ones in place) would do it. Two tons would never fly as the garbage and propane trucks weigh a lot more than that.

 

[JAE]

You should hire a structural engineer now.

 

[Ron247]

In the end, you need to hire a local registered structural engineer. But before you do, whoever thinks they control the traffic on the bridge needs to determine what weight it must carry as a minimum. If you told me say, 20,000 lbs for example, I may be able to let you know you are wasting your time whereas 2,000 lbs might take some calcs. Calculating max allowed takes way more hours than knowing a value to check.

Also, if you cannot control the traffic other than a sign, I would close the bridge if it is my responsibility. In my experience, the load limit on the sign just lets me know if I need to pray while crossing with a load beyond the posted limit.

Is the road and bridge yours or someone related to you? Can you control the traffic?

 

[rfc143]

In the end, you need to hire a local registered structural engineer. But before you do, whoever thinks they control the traffic on the bridge needs to determine what weight it must carry as a minimum. If you told me say, 20,000 lbs for example, I may be able to let you know you are wasting your time whereas 2,000 lbs might take some calcs. Calculating max allowed takes way more hours than knowing a value to check.

Also, if you cannot control the traffic other than a sign, I would close the bridge if it is my responsibility. In my experience, the load limit on the sign just lets me know if I need to pray while crossing with a load beyond the posted limit.

Is the road and bridge yours or someone related to you? Can you control the traffic?

It's a private bridge, serving four residences. We alone can not shut it down, nor would we, as three of the four go to work every day. We can recommend that effective immediately, no heavy trucks or equipment be allowed to cross the bridge. It's a difficult situation to say the least. Have a call in to our local bridge structural engineer and will see what he says.

 

[Greenalleycat]

This may take the cake as the stupidest post I've seen on here
You've given us enough information only to say that it sounds like your bridge is absolutely poked and you need an expert opinion ASAP

Even if you'd given more info, what were you expecting? A bunch of international engineers to give you a design for a heavily degraded bridge that we have no drawings of, can't inspect, and don't get paid to deal with?

 

[XR250]

I agree with Greenalleycat on this. I supposed you could add some cribbing under the bridge though as a temporary measure.

In my area, there was covered bridge that served 4 residences. It got taken out in the recent Chantal disaster. They are basically f'ed at this point.

 

[Ron247]

It's a private bridge, serving four residences. We alone can not shut it down, nor would we, as three of the four go to work every day. We can recommend that effective immediately, no heavy trucks or equipment be allowed to cross the bridge. It's a difficult situation to say the least. Have a call in to our local bridge structural engineer and will see what he says.

Nothing to do with structural advice, but in terms of a private bridge that serves 4 residences, that could get real interesting and aggravating. Unless there is a binding contract about how maintenance costs are shared, who decides what etc., in my experience, at least 1 will try to get out of paying anything. I would be running those bases as much as I was doing on the structural end. Just a consultation by an engineer will cost someone. Get ready for some weird self-centered logic. "Well, the way I see it, my house was the last one built here. Had it never been built, you 3 would still have to pay for the entire bridge. So, why should I have to pitch in? Are all of the parties related?

I am not making this up, I have already heard that one from someone in a similar situation. 3 different parties independently screwed up at the same place in a building. Each screw-up would have had to be corrected even if they were the sole screw-up. While trying to put together a repair plan for the 3 to share in, one decided that they should not have to pitch in on the cost because "the way they figured it, even if they had done their job right, the other 2 would still have to make the repair and pay for it." In fact, they tried to change order for more money than the repair cost.

 

[rfc143]

Nothing to do with structural advice, but in terms of a private bridge that serves 4 residences, that could get real interesting and aggravating. Unless there is a binding contract about how maintenance costs are shared, who decides what etc., in my experience, at least 1 will try to get out of paying anything. I would be running those bases as much as I was doing on the structural end. Just a consultation by an engineer will cost someone. Get ready for some weird self-centered logic. "Well, the way I see it, my house was the last one built here. Had it never been built, you 3 would still have to pay for the entire bridge. So, why should I have to pitch in? Are all of the parties related?

I am not making this up, I have already heard that one from someone in a similar situation. 3 different parties independently screwed up at the same place in a building. Each screw-up would have had to be corrected even if they were the sole screw-up. While trying to put together a repair plan for the 3 to share in, one decided that they should not have to pitch in on the cost because "the way they figured it, even if they had done their job right, the other 2 would still have to make the repair and pay for it." In fact, they tried to change order for more money than the repair cost.

Thanks for the insight. On the plus side, the deed to all the properties clearly states that the cost is to be shared evenly by all the landholders. On the down side, there is a HUGE disparity between the financial wherewithal of the owners, which will put a huge strain on the "conversation". We know we need a structural PE to look at it; we're doing that. But until he/she comes, I feel we should alert the others to the potential problem and recommend some upper limit in the short term until we get a plan in place (advised by the PE); On the one hand, don't want to be a "sky is falling" type of neighbor, but on the other hand, if the bridge is further seriously damaged by heavy trucks, everyone involved is going to be pretty screwed.

 

[phamENG]

On the one hand, don't want to be a "sky is falling" type of neighbor,

Sure. But based on your limited description, the sky may be about to fall on anyone under that bridge. How long is this private road? Is it feasible to park cars on the road side of the bridge so you're not loading it?

Keep in mind that all buildings/bridges that have fallen down have one thing in common: they were standing up just moments before.

 

[rfc143]

How long is this private road? Is it feasible to park cars on the road side of the bridge so you're not loading it?

2500' to the furthest resident. Very unfeasible to park on the road side (which is a State highway). Likely immediate action will be to limit the weight of vehicles over the bridge (no one goes beneath...it crosses a stream) until an engineer comes up with a fix.

 

[phamENG]

no one goes beneath...it crosses a stream

I figured. That was more of a play on your not wanting to be a sky is falling neighbor.

Good luck. An unfortunate situation for sure. I'd recommend setting up a maintenance/inspection schedule moving forward. Perhaps an account that everyone pays into each month/year/whatever so minor problems can be dealt with and in 50 years time the money will be there to replace it again. Ownership in the account would pass with the property to new owners.

 

[Greenalleycat]

Thanks for the insight. On the plus side, the deed to all the properties clearly states that the cost is to be shared evenly by all the landholders. On the down side, there is a HUGE disparity between the financial wherewithal of the owners, which will put a huge strain on the "conversation". We know we need a structural PE to look at it; we're doing that. But until he/she comes, I feel we should alert the others to the potential problem and recommend some upper limit in the short term until we get a plan in place (advised by the PE); On the one hand, don't want to be a "sky is falling" type of neighbor, but on the other hand, if the bridge is further seriously damaged by heavy trucks, everyone involved is going to be pretty screwed.

From what you've described, your bridge sounds unsafe so I doubt there is a way to calculate a 'safe load'. So the best thing is probably to shut the bridge. Bridge failures can and do happen....I had a client with a similar sounding bridge to yours that got broken because a fire truck ignored the instructions to take the long driveway route and instead drove straight over the bridge and cracked a timber beam

In the short term, something that could help is to try and clearly define on the bridge where people should drive to avoid loading the completely corroded beam.

I had this on a job where we strengthened a pretty shitty bridge that, long term, will serve a new residential development. But in the short term, it's going to have massive concrete trucks etc driving over it. So we painted traffic lanes etc on the bridge to dictate where they can safely drive to ensure loads were directly over our main beams.

 

[LittleInch]

Got any pictures or drawings of this bridge?

It's always difficult being the bearer of bad news when the "well it's been OK for 50 years" rears its head.

 

[jerseyshore]

Keep in mind that all buildings/bridges that have fallen down have one thing in common: they were standing up just moments before.

I use this line way more often than I like to admit. I always tell people that things don't fall down, until they do. That's my Yogi-ism.

 

[MintJulep]

five new beams between the existing beams and replacing all the wood with Pressure Treated lumber.

Pressure treated lumber on steel beams is a bad idea.