Bridge load calculations (for the inept)

I’d be grateful for help with this

I have a bridge crossing a stream gully. It consists of a pair of I-beams planked with timber. Apparently it was originally constructed to take a motor car somewhere circa '70's. The deck is covered in mud and has grass growing and a few planks at one end have been bodge-replaced by another user. She regularly crosses it with a side by side ATV sometimes towing a small box trailer with a few sheep. I have made the point that it is at her own risk and posted appropriate signage. It's time to redeck it but I'd like to know what it’s proper load carrying could be.

The gap it bridges is 7 meters. To the outer sides of the I-beams is 90" so likely constructed in part in imperial measures 7.5 feet. The deck timbers are 2.4 meter long, 150mm wide and 45 mm thick. Unknown timber type.

The I-beams top and bottom width is 20cm and height (difficult to measure accurately) circa 33.5 cm. Thickness of the upper arm is 16mm but one exposed area has a depression underneath that makes it 13mm for a short way in. Presumably some variation when rolled?. There isn't much evidence of more than surface rust. The lower arm is even harder to measure but comes out thicker at the accessible bit.. Perhaps as much as 20mm. I can't manage a meaningful measure of the thickness of the upright but these are likely to have been the types of steels used in barn construction (farming country)

I have in mind to re-deck the thing in 75mm (or thicker) larch 250mm wide and 3m long to give room at the sides to attach a handrail. The timbers would be loose-laid with a longitudinal each side underneath to stop them shimmying off and by joining them create enough mass to keep them there.

The question is whether it could the support a 4x4 motorcar plus light trailer with say a 500kg load safely..

Intuitively the timber thickness should be fine since weight of a vehicle would be split between 4 wheels and no wheel could be more than a 30cm approx from the steel (assuming average car 5.8 feet) so nowhere near the centre of the timbers.

Where I fall down in any idea how to work things out is with the distribution of weight of the timbers on the steels and the weight distribution of car and trailer and the load carrying capacity of these steels with a safety margin with allowance for things like suspension bounce.

pgk

My immediate reaction would be to say get a professional - presumably a structural engineer?

If you do it yourself and there is an accident (structural failure) you may regret it big time! I doubt " I thought it looked strong enough" would stand up as an defense in court.

You want something like this **LINK**

You will have to put in your own data for the beams you have and know the maximum allowable deflection you can accept.

Martin C

The first things I would ask of your driving neighbour, are: Do you believe that you have any rights to use this bridge? Have you checked your deeds, as they may well make reference to any rights or obligations which exist?

You might also consider reminding your neighbour, if the reply is 'No', that when she uses your bridge she must take it as she finds it, and that you make no representations of how strong or how weak it might be. Say, if you wish, that you would be happy for her to continue use, as long as she responds clearly accepting the terms on which you are prepared to permit her use. You might also point out that work is needed to restore some of it, and invite her to contribute to the expense. And should you need to do work on it, be clear that you reserve the right to withdraw any permissions or apply additional conditions.

In the UK, you might also enquire of the County Council as Highway Authority if they are aware of any evidence that the route in question carries public rights (footpath, bridleway etc). If they say 'Er, yes ...' then come back to me - it gets complicated.

Regards, Tim

As a first pass I would use one of the online beam calculators as suggested by Martin.

Calculate the weight of the deck, add the ATV, rider, trailer and sheep, divide by two and treat as a point load in the centre of each I beam - see what sort of deflection it computes.

Then take the heaviest axle load ( ATV+ Rider /2 or Trailer + sheep), and treat as a point load in the centre of one length of the proposed decking. See what sort of stress and deflection you get.

If both deflections are acceptably small, then distributing those weights around the bridge will give you some considerable safety factors.

If anything looks dodgy - then as Bizbuilder says - get a structural engineer to run the calculations for you.

Just out of curiosity I ran the numbers for the I Beam through a couple of online calculators

I Beam Moment of Inertia Calculator (amesweb.info) give the second moment of intertia of an I beam. I got a cross section of 15sq in and a MI of 423.75

Using those numbers and putting a 2 ton point load in the centre of the 9M beam Free Beam Calculator | ClearCalcs gave me a deflection at the centre of the beam of 0.75" 

Sorry misread the length - had to recompute

Edited By Peter Cook 6 on 20/07/2022 17:43:25

Sorry, no help, but it's a definite cue for a bit of Kipling:-

THE careful text-books measure
  (Let all who build beware!) 
The load, the shock, the pressure
  Material can bear. 
So, when the buckled girder
  Lets down the grinding span, 
'The blame of loss, or murder, 
  Is laid upon the man. 
    Not on  the Stuff—the Man!

See here:-

Hymn of Breaking Strain

For those that need more details.. The bridge is part of a right of access (not a right of way) across my land to a field over the stream and I own a derelict property with access rights across that further field too. The woman has been made very aware that crossing the bridge is at her own risk (in writing) but she is one of those people you can't have a polite discussion with. Life has become more complicated because the deeds state that maintenance is in proportion to usage and her usage is multiple times mine since I only pop across to check the derelict 2-3 times a year on foot. There are other disputes as well but they have no bearing here. I have a need to do some fencing work around the derelict's land and essentially refuse to take stuff across the bridge until it is made safe. I don't fancy carrying all the equipment back and forth on foot.

Peter Cook... helpful. Using the first calculator and using the lower figure of 13mm for steel thickness comes out as cross section 14,508 sq inch so not much less. Your further calculation with a load of 2 tons centrally is on one of the beam pairs and strongly supports the concept that this bridge was indeed designed to take a motor car (back when they weighed less) with a good safety margin.

Further I had indeed offered the option of a structural engineer calculating things but the witch woman is pretty resistant to putting her hand in her pocket to the point where everything is being documented carefully by me and I’m insisting that once details have been agreed any repair will need her to put money in escrow before each stage: possible engineering calcs, materials, labour costs. And I repeatedly make her responsible for her own liabilities. I've even tried to make her understand that if I arrange it all then I cannot issue a vat invoice and she’d be better organising it to my satisfaction herself and paying bills directly - but you can't reason with some folk..

pgk

If it were me i would get good legal advice before beginning any work on the bridge, and having a structural engineer check it, difficult neighbours can turn it into a real nightmare,i know.

8in by 12 inch I beams are massive and they would easily carry a 4x4 and trailer over a 20 foot gap, I would be more concerned about corrosion particularly in areas which cannot be seen,particularly and in in areas where the beams rest, also what do the beams rest on,are there brick built built piers? or do the beams rest on rocks or worse rest in the dirt of the bank, The question must be will the girders stay in place after repeated bumps from a vehicle/trailer combination as it goes from a dirt track onto a semi rigid structure, I would tie the girders together with a couple of tie bars and then ensure the timber decking is secured to the girders with bolts and clamp plates,never drill I beam flanges. Plus some guide rails to stop wheels slipping over the edge of the deck. When I get something like this bridge I have a look round at what other people have done and if that structure is sound then copy that, when I needed a two ton capacity gantry spanning 12 foot for lifting stationary engines and machine tools I looked at catalogues of commercial gantries and copied their specs,as they had already done the calcs. Though nowadays with shall we say awkward neighbours ready to make claims one has to protect your backside,and really a structural survey is required rather than just calculations of the beam strength. This could prove expensive though better/cheaper than an accident/negligence claim.

It would seem the state of the timber, that is the problem, not the beams unless they are corroded through in areas you can't see.

You first paragraph is illuminating That (and later explanations) make fairly clear this all your property.

" The deck is 2 [in a poor condition and " a few planks at one end have been bodge-replaced by another user. She regularly crosses it with a side by side ATV sometimes towing a small box trailer with a few sheep. "

So the bodging is hers? Then if they fail and dump her, ATV and sheep in the stream, it is her fault, surely? Though her "defence" might be that you allowed her to "repair" it then failed to maintain it.

Is there another, legal route this woman can use to reach her sheep, avoiding the bridge?

Do you need it to be vehicle width?

Can you practicably narrow it to be negotiable by a person or a flock of sheep in single-file?

I'd be tempted to tell her that you consider your bridge on your land to be unsafe for further vehicular use, and that her "repairs" have proven unsatisfactory; and that you will give her a month to find new accommodation for her animals, negotiate a new route or share the costs of professional inspection; after which you will narrow it to sheep-width footbridge only.

The problem of course might be that she'd simply find some fording-point nearby and cause a lot of damage to your fields by it.

Hi Nigel, I've never heard that you should never drill I beam flanges and in my industrial fabrication jobs, I've had to drill them as per the drawings and have seen many structures where the flanges have been drilled and even tapped for fixing hand rails and such.

Regards Nick.

Tricky!

As I understand it the bridge is co-owned. One partner believes it's strong enough to meet her needs and happily crosses it with an ATV and light trailer. The other wishes to cross it with a much heavier load, a 4x4 with 500kg trailer, and is doubtful about the condition of the deck and the weight bearing capacity of the girders.

There are two separate problems!

1. Who pays and how much? Difficult because one partner appears to be content with the bridge, and can be expected to resist spending any money whatsoever on it! pgk says 'the deeds state that maintenance is in proportion to usage and her usage is multiple times mine since I only pop across to check the derelict 2-3 times a year on foot.'. Unfortunately, not true: the usage causing the dispute is a 4x4 with heavy trailer. It can be argued the partner who needs a stronger bridge should pay for it. Might be the easiest and cheapest answer: rather than get into an expensive in itself legal dispute that might end badly, pgk simply gets an engineer in, and replaces or upgrades the bridge himself, without bothering the neighbour at all.
2. Is the existing bridge strong enough? I think a qualified engineer needs to look at it. With luck the girders can be reused because they're big enough to take the load, have adequate foundations, and aren't corroded. However, old-age is the deadly enemy of bridges: it's unlikely to be in as new condition. The deck sounds suspicious, but should be easy to replace it. Looking at the web, Larch is clearly a better choice than Pine, but I wasn't able to confirm Larch is suitable for a load bearing bridge.

Always worth considering the 'Do Nothing' option. The neighbour has demonstrated the bridge copes with the weight of her ATV and trailer, so maybe pgk could use one to do his job. Work within the limitations of what's available - don't insist on a 4x4!

Different of course if the plan is to develop the derelict property and safe access to it is required on a permanent basis. This also changes the usage covered by the deed, and - I think - would dramatically increase pgk's share of the cost.

Engineering is often much easier than economics and politics!

Dave

Edited By SillyOldDuffer on 21/07/2022 10:00:46

It would be inappropriate to give you guys the full details of the disputes. The bridge is the property of the owner of the derelict building (me) who has rights to have a bridge that can take a motor car. Routes via the bridge are rights of access to the derelict in my case and the fields in her case. The calculations so far imply that the original bridge was indeed built to take a motor car with safety margin and I thank Peter Cook for pointing me in the correct directions for that.

pgk

How deep is the stream gully and does it carry a flow at any time. I've seen a few youtube videos where landowners have laid large diameter pipe in the flow then covered it with rubble to make a road

I liked Calvin's Dad's answer, **LINK**

Indeed, and I have another bridge just like that. All part of the disputes I can't engage here. Winter flow can rage And vthe stream is officially named as a river though never more than 4 foot wide when raging at this location.. But would require consent from River authority

Pgk

All I will say is reading this thread has helped me pass the time while I wait for some patching mortar to reach a consistency where it won't slump.

Thank you.

Neil

A preliminary report from structural engineers states that the bridge can carry a 3.5T Land Rover plus 1T trailer but is subject to sensitivity testing and a final report.

pgk

Convert it to a drawbridge, get a book on 'How to avoid spells.'

Bear............

Just thinking aloud:

Structure aside, this is a difficult legal situation. If the bridge were to collapse as the "lady" was crossing it, no doubt she would sue you for damage to her vehicle, and/or personal injury.

I wonder what the legal definition of 'access' is in this case. Footpath, bridle path or vehicle roadway.

A possible option might be to remove the decking completely, and replace it with a person-wide path on one of the beams. You and her sheep could then cross the river, (on foot). When you start renovating the derelict, you can temporarily replace the full decking.

A shame that you (presumably) cannot charge a toll.

Depending on the legal view of the 'access' in this case, I wonder if you could put a locked gate across the bridge with "Unsafe Structure" signage, and tell the "lady" that funds are needed to replace the decking, and she needs to contribute a reasonable proportion of that cost.

Edited By John Doe 2 on 20/10/2022 10:52:38