rtcosic
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Another ‘simply supported beam’ question.
I’m designing a lean-to pergola in SHS/RHS with a lightweight twin wall polycarbonate roof.
It’s now 45 years since I last consulted Timoshenko & Young so I’m just a tad rusty.
Firstly I’m seeking re-assurance that my Design Load basis is sensible.
Dead load - Polycarbonate neglible so self weight of the steel rafter only.
Live loads – various websites suggest around 1 kN/m2 snow load. Span is 3m and rafters are on 1m centres so snow load is 3mx1mx1kN/m2= 3kN uniformly distributed.
Alternatively point load of 100kg at midspan to represent a lardy maintenance worker and tools (me).
Wind load is an uplift issue, but as it is located at the sheltered rear of the house I’m unconcerned (until it lands in the neighbours garden in small pieces).
I found an Australia publication giving load capacity for all the common sections as simply supported beams with uniformly distributed load:
http://www.austubemills.com.au/publications-and-resources/technical
Design Shear Capacity
40x40x2 (2.31kg/m) = 53.8kN
Design Moment Capacity
40x40x2 (2.31kg/m) = 3.47kN
L/250 Deflection
40x40x2 (2.31kg/m) = 0.474kN
So I will be deflection limited only? Seems sensible given the very modest loads.
If so Baldrick has a cunning plan shamelessly copied from all the pergolas I saw recently in Dalmatia – more later.
The rafters will be supported by a wall plate at one end and a lintel or beam at the other. This will also be 3m long. So a rafter will bear directly on the columns at each end and deliver point loads at the 1 and 2 metre points on the beam. Can I just treat that as a uniformly distributed load instead?
The total snow load would be 3mx3mx1kN/m2 = 9kN but only half this would bear on the beam so 4.5kN.
But half of the load of the 2 outer panels would bear directly on the columns so I can subtract the load equivalent to one full panel of3mx1mx1kN/m2 = 3 kN leaving just 1.5 kN in total.
This seems very low, so where have I gone wrong?
The point load of 100kg at midspan would also need checking.
That cunning plan? All the ones I saw use rebar in tension to add stiffness as in the sketch below:
That’s 40x40x2 SHS 3m long. 8mm rebar at 5 deg angle. Calculating deflection for the two loads is beyond me.
Anyone got access to programs to do that?
A pergola is Permitted Development and an ‘exempted structure’ for Building Regulations so I only need the calculations for my own re-assurance.
I’m designing a lean-to pergola in SHS/RHS with a lightweight twin wall polycarbonate roof.
It’s now 45 years since I last consulted Timoshenko & Young so I’m just a tad rusty.
Firstly I’m seeking re-assurance that my Design Load basis is sensible.
Dead load - Polycarbonate neglible so self weight of the steel rafter only.
Live loads – various websites suggest around 1 kN/m2 snow load. Span is 3m and rafters are on 1m centres so snow load is 3mx1mx1kN/m2= 3kN uniformly distributed.
Alternatively point load of 100kg at midspan to represent a lardy maintenance worker and tools (me).
Wind load is an uplift issue, but as it is located at the sheltered rear of the house I’m unconcerned (until it lands in the neighbours garden in small pieces).
I found an Australia publication giving load capacity for all the common sections as simply supported beams with uniformly distributed load:
http://www.austubemills.com.au/publications-and-resources/technical
Design Shear Capacity
40x40x2 (2.31kg/m) = 53.8kN
Design Moment Capacity
40x40x2 (2.31kg/m) = 3.47kN
L/250 Deflection
40x40x2 (2.31kg/m) = 0.474kN
So I will be deflection limited only? Seems sensible given the very modest loads.
If so Baldrick has a cunning plan shamelessly copied from all the pergolas I saw recently in Dalmatia – more later.
The rafters will be supported by a wall plate at one end and a lintel or beam at the other. This will also be 3m long. So a rafter will bear directly on the columns at each end and deliver point loads at the 1 and 2 metre points on the beam. Can I just treat that as a uniformly distributed load instead?
The total snow load would be 3mx3mx1kN/m2 = 9kN but only half this would bear on the beam so 4.5kN.
But half of the load of the 2 outer panels would bear directly on the columns so I can subtract the load equivalent to one full panel of3mx1mx1kN/m2 = 3 kN leaving just 1.5 kN in total.
This seems very low, so where have I gone wrong?
The point load of 100kg at midspan would also need checking.
That cunning plan? All the ones I saw use rebar in tension to add stiffness as in the sketch below:
That’s 40x40x2 SHS 3m long. 8mm rebar at 5 deg angle. Calculating deflection for the two loads is beyond me.
Anyone got access to programs to do that?
A pergola is Permitted Development and an ‘exempted structure’ for Building Regulations so I only need the calculations for my own re-assurance.