Service Moment (KNm)

[David WE Roberts]

Just looking at lintels and Travis Perkis has a table

shows loadings on lintels in KNm.

So how do you translate the weight of a roof plus joists etc. into KNm?

[Jim]

You need access to a copy of BS648 for the weights of various materials.

[dennis]

One KN is about the force you get from 100Kg weight.

Now all you need to do is workout the weight of the materials per meter and add in a suitable safety margin for snow, ice, falling volcanic ash, birds, etc.

[mark]

Multiply weight in Kg by 9.81 to get Newtons.

mark

[Bruce]

Beware, because none of the three replies that I have read so far on here have even recognised the meaning of "kNm". Even the data sheet gets it wrong, writing it as "KNm". Also, there is more to it than just "the weight of a roof plus joists etc.".

The figure given in kNm is the bending moment that the lintel can accept in service, in kiloNewton-metres. If you don't know what "bending moment" means, or how to work it out, you really should get a structural engineer or technician engineer to calculate it for you and select the appropriate lintel. He/she won't cost you a lot and could save you a lot of heartache and hassle, plus you will have the comfort of knowing that the finished job will comply with the building regulations and not collapse.

Your local newspaper classified ads will have a selection of people offering "plans drawn" and suchlike. This is the best place to start.

[Dave]

Anyone know why the average punter can't gain access to BS standards?

It looks strange that a standard can be laid down, but we can't see the details involved.

Dave

[Andrew Gabriel]

Anyone can buy a copy. Some libraries can give you access.

Or did you mean why can't you see it for free? We don't have the law which exists in a number of US states which requires that standards which are called up by law have to be freely available.

You can see it - buy a copy.

[Dave]

I am not an expert in this field, but I thought that the term was related to torque and not bendind.

Dave

[Roof]

Well, at least something has finally sunk in

[Lawrence]

You bend something by applying a torque to it. Beams have the added complication of distributed load. The calculations are not difficult but the concepts take a little thought. Anyone with A level Maths or a science based degree would find little difficulty. There are plenty of sources on the 'net. That's how I found out how to do the calculations!

Lawrence

[Roof]

Nope. You bend something by applying a load to it. If you apply a torque you twist it.

[Bruce]

A torque is a turning force that rotates something. A bending moment is a turning force that bends something such as a beam or slab.

Torque and bending moment have the same units (force times distance) but basically one is about rotating, therefore it is dynamic, and the other is about bending, which is static. One rotates, the other doesn't move, except to deflect.

Lintels have to be designed for both bending moment (along their length) and twist (across their cross section). The twisting force is often called a torque to differentiate it from longitudinal bending. although there isn't any significant rotation unless the beam fails.

Without wishing to complicate it, there is also the issue of shear forces.

[Dave]

Just as I hit the send button, It came to me. Sorry for the post.

Dave

[Roof]

Why is shear force a complication? What do you think governs the design of a beam or lintel?

[Steve Walker]

PDF version here -

[Roof]

You seem to be mis-reading the manufacturers data. They've provided a table giving the safe uniform working load a lintel can take for a particular lintel size and clear span. If the load isn't uniform then they've provided the equivalent service moment. If you don't understand the figures then it probably means you need someone to assist you.

[Bruce]

For the avoidance of doubt, f*ck off, and don't come back.

Meanwhile, congratulations on your restoration to my kill file.

[John Rumm]

Here you go roof, your chance to actually help the chap and explain it all!

[John Rumm]

What is your actual application here?

In many cases you don't need to deal with service moments since often the lintel will be carrying a uniformly distributed load - or at least a series of point loads that can be treated as a UDL e.g. floor joists spanning a room that are supported on a wall into which you are installing a lintel and creating an opening. Hence you can use the figure from the body of the table directly.

[Roger Chapman]

Is my memory finally failing or are all the experts who have responded to this thread more than half asleep to let such an obvious mistake go unremarked?

IIRC if you multiply weight in Kg by 9.81 you get Kgf. In SI metric mass and force are numerically equal.