Doubling Sole Pads - Double the bending stress??

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HJA

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I have seen several statements in calculations that doubling a sole pad doubles the bending stress, trebling..trebles the bending stress so that allowable bending moment of a sole pad is stated as 0.468kNm for single, 0.936kNm for double & 1.404kNm for treble pads

Section modulus for a rectangular section is not directly proportional depth but depth^2 (since z=(bd^2)/6)

My question is - do any designers have any calculated proof to support some designers' assumptions that allowable bending moment doubles...trebles when used in a loosely layered configuration (i.e. not nailed, glued, banded).
 
I have read the blog regarding bearing pressure.

There are other approaches to this. The approach relative to my question is where the timber sole pad acts as a uniformly supported timber beam (timber bedded on level ground). Rather than the approach taken by idh, for load acting at centre take moments about the centre line to give max bending and this is found to be W/2 x L/4 (=WL/8), providing this doesn't exceed the bending stress of the timber, then its whole length x width (Ae) may be assumed to distribute the load rather than the typical 2:1 with & 1:1 against grain areas.

For something like a timber sleeper this is fine, the contention comes with multiple layers of timber.

I have read argument that without constraint the uniform deflection of all layers (as in a suspension spring) may uphold the case of doubling/trebling, but cannot find anything in calculation to support this.
 
Well in that case you'll have to wait for somebody who actually works all that out as myself and my company don't.

We don't see the point. If we are told to keep it under a certain bearing pressure then we do, either by using a single layer of boards or a suitable load spreading base. (Beams & Sleepers)

I'd guess that the friction caused by the weight bearing down upon it (if multiple base boards were required, that would be quite a substantial load) would be enough to cause the friction between the board to hold it all together without the need for banding or nailing.
 
why is there a bending moment on your solepads? the ground should be capable of withstanding the bearing pressure without any settlement, therefore there's only compression on the timber, not bending.
 
HJA said:
I have read the blog regarding bearing pressure.

There are other approaches to this. The approach relative to my question is where the timber sole pad acts as a uniformly supported timber beam (timber bedded on level ground). Rather than the approach taken by idh, for load acting at centre take moments about the centre line to give max bending and this is found to be W/2 x L/4 (=WL/8), providing this doesn't exceed the bending stress of the timber, then its whole length x width (Ae) may be assumed to distribute the load rather than the typical 2:1 with & 1:1 against grain areas.

For something like a timber sleeper this is fine, the contention comes with multiple layers of timber.

I have read argument that without constraint the uniform deflection of all layers (as in a suspension spring) may uphold the case of doubling/trebling, but cannot find anything in calculation to support this.
Click to expand...


HJA - Your method is right but it over-complicates matters. The 1:2 approach ignores bending in the board. For bending to occur we need differential settlement to occur over the length of the board. To assess this we need to know the ground properties which we seldom do so it is easier to apply a direct stress approach ignoring bending / differential setlement. This is what we get when we know the ground won't settle - i.e. tarmac / concrete. I agree it is not perfect or as clear with softer materials.

As for doubling section modulus / 2nd moment of area, this is fine for layered materials of the same section size and elasticity. It doesn't work with say ply over boards or boards nailed together as you now have a composite section.

This is atleast my take on it but remember all opinions expressed on this site are on the whole wrong.
 
IDH, now you are beginning to sound like a designer.:laugh::laugh:
 
In addition; sole boards are usually so short that you can say bending is not a primary concern. Unless you are talking about continuous sole boards? As IDH says, it's a tough one to analyze because we rarely know the site ground conditions so its all on assuptions.
 
as most scaffolds now need a design engineer let them worry about stresses, when i started scaffolding many yrs ago, no one ever thought about calculating the stresses the supervisor took you to the job pointed to where the person wanted it then left, then it was down to the tophand(now a advanced scaffolder) how it went up , and try as i may i cannot recall any major incidents regarding of scaffolding falling , we used to use flattened singles for ties , no t bars on droppers, no harness , no gloves safety hat safety boots ,nothing , but the job got done and we all went home safe and happy(after the pub)
 
....and you also didn't have the media and internet reporting every scaffold collapse mate. Same sh1t different decade that is all apart from we now have more Pokski's
 
I have been recently asked to design a Base plate /sole pad arrangement for a client, who advises that the bearing capacity of the bearing stratum is 100kN/m2, the axial load in the standard is in the region of 55kN which results in a required Area of base = 0.55m2.

Based on timber scaffold boards I need to provide a 900x900mm Base; and based on the 2:1 & 1:1 load distribution rule (of thumb) the base would need to be 6 layers deep to ensure the load is adequatley distributed (see IDH blog on Bearing Pressure). This arrangement requires 21 No 900mm long boards and i believe will be very problematic. One of our advance scaffolders tells me it's over the top and that two layers will be enough, but I can't put down on my calcs that my supervisor says it will be fine as a justification for the base arrangement.

So I was wondering if anyone could advise on this as to how to reduce the number of boards/layers required.

I have read the alternative method above by HJA and I am not convinced to this; as the Moments calculated on this are for a beam wich is simply supported at either end, if it was uniformly supported along its length then the applied load would be distributed straight down (with some distribution through the beam) to the bearing strata below, bending would not be induced as implied. {am I interpreting this wrongly?}

It was suggested using a steel soldier type beam to combine this leg with outher legs to spread the load, but again it depends on the rate of distribution through the beam member????
 
Roadplates Roarz, Simplest Solution, rather than messing around with piles of boards.

Also what are you erecting your scaffold on, a beach?
 
denbo said:
as most scaffolds now need a design engineer let them worry about stresses, when i started scaffolding many yrs ago, no one ever thought about calculating the stresses the supervisor took you to the job pointed to where the person wanted it then left, then it was down to the tophand(now a advanced scaffolder) how it went up , and try as i may i cannot recall any major incidents regarding of scaffolding falling , we used to use flattened singles for ties , no t bars on droppers, no harness , no gloves safety hat safety boots ,nothing , but the job got done and we all went home safe and happy(after the pub)
Click to expand...


Not sure when you started Denbo, however, unless it was pre 1909 someone has always been calculating the stresses in scaffolding you just may not have been aware of it.

With regards everyone going home safe in times past, unfortunately the figures seem to demonstrate an alternate opinion. As VLLB has pointed out with the internet an media this detail is far more in the public eye which is not always a bad thing. Far fewer operatives are now being hurt which is always a good result.

Regards
Alan
 
You could also consider timber sleepers. You can also calculate the boards (and base plate) in bending rather than using the 2:1 and 1:1 distribution from BS 5975 which will give you better figures.
 
Follow the design thats been agreed, if out happens and you've done everything correctly then your covered........meerkats.
 
absolutely astonished that such a conversation like this is taking place when a qualified designer will calculate the stress factor of the total weight compressed down on per square mtr depending on the ground structure a basis scaffolder will tell you if the ground level is fit for purpose on lets say a general building site.
And as for spreading stress on the downward point if the sole boards are insufficient use boards double up or railway sleepers find the stress calculations on a railway sleeper and i,am sure this will suffice and you will be able to sleep at night.
And do us all a favour and take your b.ollocks somewhere else your boring the scaffs bellend.
 
Only one formula that needs to be discussed on the eve of a bank holiday.

4 Ways to Calculate the Amount of Alcohol You'll Need to Entertain a Group

^^^^^^^^^^^^Some people have far too much time on their hands^^^^^^^

Or for the more logical thinkers in the forum, i like to use the tried and tested method as follows:-

when your head becomes level or worse, below the level of your feet. stop drinking.
 
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