Calculating purlin

[Crashsurfer]

Hi, can anyone point me in the direction of some span and load tables for a purlin.
I'm planning on taking out a second floor wall which obviously currently bears half the weight of the approx 45 degree clay tile batten and felt roof above, I'm planning on installing a purlin prior to removal obviously to take the weight and extend new rafters from that down to an existing ground floor wall. This will enable me to remove the wall and means I can extend floor joists across as we'll and use the existing flat roof as internal second floor space.
If that makes sense, may have just been rambling, it is getting late.
Current rafters span 2.7m and will have to meet new purlin at approx that point and new rafters will have to extend an additional 3m to cover the current 1.8m flat roof, span is 4.7m. But unable to find span and loading charts.
I've found dead loads for roof materials etc which if my calcs are correct work out about 1000kg for total m2 of current roof covering and this will obviously double with the extended rafters as they are pretty much the same length again?
Any help greatly appreciated

 

[limerockconstruction]

Hi, can anyone point me in the direction of some span and load tables for a purlin.

Try the TRADA tables or the BUilding Regs book has them in.

 

[tony1851]

If I've got this right, your purlin will be supporting 4.7 x (2.7+3.0)/2 sq m of roof, = 13.4m^2.
Say dead load of clay tiles 1.0kN/m^2;
live load (say) 0.5 kN/m^2,
total load 1.5 x 13.4 = approx 20kN.

max. bending moment = 20x4.7/8 = 11.75kNm.

If you use a 75x300 purlin (which is about the biggest practical timber) the bending stress would be about 10.4N/mm^2, which is almost double the accepted limit of around 6N/mm^2

Your deflection be approx 30mm, whereas the maximum allowable deflection for roof timbers would be around span/250 = 19mm.

Conclusion? Use 2 75x300 C16 timbers bolted together, or consider a steel beam instead.

 

[limerockconstruction]

If I've got this right, your purlin will be supporting 4.7 x (2.7+3.0)/2 sq m of roof, = 13.4m^2.
Say dead load of clay tiles 1.0kN/m^2;
live load (say) 0.5 kN/m^2,
total load 1.5 x 13.4 = approx 20kN.

max. bending moment = 20x4.7/8 = 11.75kNm.

If you use a 75x300 purlin (which is about the biggest practical timber) the bending stress would be about 10.4N/mm^2, which is almost double the accepted limit of around 6N/mm^2

Your deflection be approx 30mm, whereas the maximum allowable deflection for roof timbers would be around span/250 = 19mm.

Conclusion? Use 2 75x300 C16 timbers bolted together, or consider a steel beam instead.

Now that was a good answer!

 

[Crashsurfer]

Many thanks tony, would anyone know how to calc for a steel?
I have just been looking a trada tables and did see I couldn't really achieve the span I'm after in wood.
And yes tony you did manage to decypher my gibberish perfectly! Thank you

 

[Crashsurfer]

May have found some steel span and load tables, does a UB 254x146x37 sound sufficient..??

 

[tony1851]

May have found some steel span and load tables, does a UB 254x146x37 sound sufficient..??

That would be OK.

 

[hurforddj]

Gentlemen do not forget all the other components to make the roof, change material type to suit. For a single calc you may want to get somebody to calc this through, shouldn't be expensive.

LOAD DATA (kN/m2)
Roof Main 45deg
Tiles 0.74 (concrete)
Felt & Battons 0.05
Rafters 0.12 (C16)
Insulation 0.02
Plasterboard+Skim 0.15 (if vaulted)

Dead load 1.08
Dead Load to roof pitch 1.53
Live Load 0.75 (this can be reduced no lower than 0.45)

Combined Load 2.28