Uk Timber Rafter Design

[Wopshistos]

Can anyone advise what is the correct method of dealing with the concentrated load of 0.9 kN on a timber common rafter as, by calculation this always produces the worst load case for single span rafters up to around 5.0m. Should it be shared over more than one member or have I read that the concentrated load is ignored for a certain pitch range? If so, why is that the case??

 

[RARWOOD]

I will try to provided some insight into your problem based on my experience. However, I am not familiar with British design codes so my answer relates to what is common in the U.S.

In a system where you have a number of parallel rafters, which are loaded by decking or sheathing there will be some lateral distibution of a concentrated load on one rafter to the rafters on each side.

You could refer to ANSI/AF&PA NDS-2001 section 3.2.2 and section 15.1 for information on this topic. Section 15.1 gives some factors which can be used to estimate this distibution.

Also if you are dealing with closely spaced rafters, 16" o.c. (40.64 cm ?), the NDS allows you to apply a repetive member factor, which partly accounts for load distibution. The repetive member factor allows you to increase the allowable bending stress by 15%.

You can also refer to the NDS Commentary for more information. The NDS Commentary references a more complicated method. The reference is: National Forest Products Association "Alternate design procedure for wood floor joist system". You could contact the American Forest & Paper Association,formerly NFPA, for more information.

 

[tonyuk]

The 0.9kN (200lbs approx) concentrated load requirement represents the weight of one man. IMO you should be able to show that if this load is applied at any point the structure will safely sustain it. The envisaged situation is that of maintenance access so it is arguable that if you are considering a tiled roof with an attic space below (i.e. no finishes) then somewhat greater short term deflection should be acceptable.

 

[valleyboy]

I agree with tonyuk, but would add that the point load should be applied to a single rafter, rather than distributed over many members. I tend to use the Emin value for the deflection calculation also, as in theory the maintenance operative could stand over a single rafter.

In addition, to answer the other query in Wopshistos' post, the point load of 0.9kN cannot be reduced regardless of roof pitch. However you are allowed to reduce the minimum uniformly distributed load of 0.6kN/m2 for roof pitches over 30 degrees. I believe this is due to the fact that roof imposed loads (snow) are less likely to stand on steeply pitched roofs.

I hope this assists.

Valleyboy

 

[Wopshistos]

thx for the replies.
The UK code allows snow load reduction to zero depending on roof pitch but the point load is to be considered as an alternative case ie. not occurring at the same time in any event. At 0.6kN/m2 snow load effects dont match those caused by a concentrated load but the latter is generally disregarded/marginalised where i can find reference to it by other designers.
As a man load it could be considered that crawl boards might be used to spread out the load during maintenance but that is not mentioned in any code as far as I know. Part 3 of the BS applies to Trussed Rafters (the part that introduces the point man load, water tank load etc)which is not quite the case I am thinking of. The Part A of B.REGS discusses the point load but rafters sizes dont seem to bear this out in those tables.
Load sharing and duration factors help but IMO not enough to dismiss the case entirely as I am seeing. The question really I think is whether that load has to be applied at all in accordance with the standard (for common rafters)OR is load sharing (not k8) acceptable through spreaders.
Sorry to labour the point.
I have looked quickly at the eurocode ( are there any euro engineers in the forum?) which I think makes the point clearer. I will endeavour to look up US codes as suggested.

 

[Wopshistos]

Bs5268 part 7 apparently.

The point load need not be considered for rafters over 30 deg pitch!!!!!!

In case anyone else is interested.