Bridging Drills

[kurnal]

Its all about forces on the ladder strings and the direction in which those forces are acting. The safety margin that determines the ability of the strings of the ladder to resist bending under load whilst not weighing a tonne is calculated at the normal angle of pitch. At angles less than this the safety margin will be eroded-  but on the fireground we dont have any data to show what the safe loading of a ladder is at these angles- or any knowledge of the loads we are placing on the ladder whilst bridging. The potential for a serious accident if it goes wrong is very high. Hence the current moratorium- but it really is high time a final decision was made

[Mike Buckley]

Fine but on the fireground there is not a lot of data on anything. How do you know what forces you are applying when you are extracting someone from a car? Again there is a potential for a serious accident if this goes wrong. Where do we stop?

[steve walker]

When it is very steep there is little load on the head and it moves easily. When very shallow the deflection increases, it is harder to move the head and if the head is resting on the rollers I worry about the heel kicking out.

If someone can suppliment this with some theory it would give me a better understanding.

How does the load on a ladder transfer to the head and the heel at different angles?
How does the ladder react to a load when at different angles?

[kurnal]

I dont know where this may lead ( hard sums were never my strong point)  but for starters I suppose we could start with the two extremes.

Ladder vertical - load of 1 person (50kg)  in the centre- load in line with strings 50kg. Load perpendicular to strings 0kg.( ignoring weight of ladder). Load on heel 50kg. Load on head 0kg

Ladder horizontal - load of 1 person in centre. Load in line with strings 0kg. load perpendicular to string 50kg. load on heel 25kg, load on head 25kg.

So the load on the head or on the heel, transmitted to the ground or wall  varies between 50% and 100% of the load on the ladder. The centre of gravity will move upwards as the load shifts up the ladder or the angle of the ladder is reduced, making the ladder less stable. The reduced load on the heal will reduce the friction between the ladder and the ground and at the same time the angle between the ground and the klladder will reduce making slippage more likely.

But the big issue for bridging is that the load in the centre of the ladder and perpendicular to the strings will increase from zero to 100% of the load depending on the angle.
So if we design the ladder with an angle of 60 degrees in mind, we would calculate the load perpendicular to the strings for  the standard firefighter in the middle of the ladder and multiply this by a safety factor.  Lets assume that for a 50kg firefighter at an angle of 60 degrees this load would be distributed with 2/3 on the heel and 1/3 on the head. So the load perpendicular to the strings will also be 1/3 ie about 16kg.

Our safety factor in designing the strings in this case could be say 3. So we make the strings strong enough to withstand a load of 50kg without failing. Otherwise they would be so strong we couldnt pick it up.

Now in the bridging situation we send our 50kg firefighter across the ladder and before he does anything we are at the limit of our safety factor. any additional load or if it is not evenly or centrally distributed will cause failure.

Well hope thats a start and that  someone cleverer than me can express it better!

[wee brian]

Blimey - a little knowledge is a dangereous thing!!!

I'll stick to being an engineer if you stick to being a firefighter.

The problem will be from bending. This will be most severe when the firefighter is half way accross the ladder.

[steve walker]

Thanks kurnal, a good stab at the question.

Does anyone know the workshop deflection test?

I have a vague memory that the 135 is fully extended horizontally and supported by a trestle placed at each end. Then weights are placed on the centre (114kg?) and the deflection measured.

Is it the manufacturers who say that it should not be used at certain angles or someother body?

When we were allowed to bridge ladders the maximum extention was limited. It would be interesting (to me at least) to know how it was all worked out.

The theory may be too complicated for me but I have an enquiring mind.

[wee brian]

Sounds a bit "elf and safety" to me. If you guys need ladders that bridge then you should be given them.

Just telling you not to do it, without giving you an alternative, is plain daft.

[kurnal]

Nah wee B
We do it anyway but just like to try and understand the risks involved.
Hey and as for sticking to being a firefighter I was once told I would never be a firefigher as long as there was an orifice in my posterior. so thats two career paths closed now then.

[wee brian]

If you want to find out for yourself then a loadtest would be a good idea. Its normal negineering practice to load something to about 3 times the working load.

If you have a firefighter weighing around 90 Kilos plus all his kit then you probably have a working load of 120K (I dunno you tell me).

So get 360 Kgs and stick it in the middle of a ladder between two trestles. It will definately bend a bit. If it snaps, shows signs of distortion or does not return to its original shape then dont go crawling across them.

Not sure how you explain to your boss what's happened to all the ladders though.

[Frankie]

I've written and obtained all the things i need to know now.

I'd really like to point out a couple of things:

1. Whenever I ask a question. I get a lot of people having a go

2. Many of those people seem to be in a different era to everyone else.

3. If you asked me a question on parade and I gave you an answer such as "that's not relevant" or "you don't need to know that" or even better "you should know that yourself" you'd beast me. similarly you'd beast me if I started ranting on about removal of drills when asked about one.

My point is this. If I ask a question and you know the answer then by all means tell me. Please don't try to make yourselves feel or look superior by putting me down. I am sure there are plenty of aspects of my civillian job that you don't understand and I would be only too glad to help you if asked..... However elementary the question seemed.

This seems to be the only place I know that when help is asked for the individual requiring help is ridiculed and mocked.

Shame on you.

[Martin Burford]

Steve walker

The deflection test involved extending the 35ft ladder  and attaching lines to the strings a approximately the dentre of the ladder. The weight of two men was applied, equally to both sides of the laddrer a two observers would be in place at either side to identify if the ladder returned to uts normal position.  Of course you may have done it differently.
Conqueror

[kurnal]

And wasnt it three men hanging on two lines for the 135 ladder, fully extended and poles stowed? Lines attached by rolling hitch to the strings at the  centre of the ladder.
This was the quarterly standard test on stations, but I think there was a further  deflection test carreid out on trestles as part of the annual inspection in the workshops.

Never saw the point of it myself.  

And what about jumping the rounds of a wooden ladder? What was that supposed to prove? Whether you could hang on with your hands if the round broke? The first time I did that the Sub said I looked like a blob of treacle trickling down the ladder.

Frankie- I had an old colleague who always had a bit of fun during the HMI inspection- if he thought the question stupid or patronising as most of them were, the answer was always apple or parsnip or some other vegetable. He managed to keep a straight face as well but the rest of us who knew what was coming were doubled up.

[Martin Burford]

Frankie

What a sensitive being you are!
Conqueror

[steve walker]

Yes, I remember all that hanging on the lines and jumping the rounds - character building.

[Kaiser]

Well I only recently joined this site but this one issue has made me very wary.  Have you lot never heard of the old Chinese saying.

"He who asks a question, is a fool for a short time, he who never asks is always a fool"

This, old style firefighter, new style firefighter thing gets on my nerves, it isn't the "New Style" NVQ firefighters who are at fault but the system that sends everyone out constantly putting up smoke alarms and sitting at a computer so much so that training records get falsified and they don't get the bread and butter training we old hands got.

Personally, when I get a probationer on my shift, I ensure that they have a lot of input on practical firemanship (yes I said it and stick by it, FIREMANSHIP). I make sure that they have plenty of training on issues that were always safety critical before the government said that we don't put out fires any more.  I believe that if the probationer isn't up to scratch, then it's my fault and my watch colleagues fault for not ensuring that they are an effective member of the team.

If the probationer or less experienced firefighter asks a question, don't jump on a soap box shouting out how great you are and how crap they are or they'll stop asking and one day the information they needed may be the information that could have saved your life.

Ladies and Gentlemen, I will now step down from the soap box, Thank You