Rafter load sharing
Rafter load sharing
(OP)
Let's say a single, concentrated load P is applied to one rafter (upper chord) of a wood truss roof (near that rafter's midspan), rafter spacing is 16 inch on centers, rafters are 2x6 No. 2 southern pine, roof is typical asphalt shingle, and assume typical residential roof sheathing (panels) underneath shingles. What percentage of P would one typically apply to that rafter for analysis, and what percentage would be shared by the two adjacent rafters?
And, secondly, same question except, what if P is applied not exactly above one rafter but instead somewhere in between two rafters (and sheathing is assumed adequate to transmit the load)? Any help would be appreciated. Thanks.
And, secondly, same question except, what if P is applied not exactly above one rafter but instead somewhere in between two rafters (and sheathing is assumed adequate to transmit the load)? Any help would be appreciated. Thanks.






RE: Rafter load sharing
In any case as of now you can solve this easily with a 3D model in RISA 3D or any 3D FEM structural analysis package. If you make several cases you may even complete a parametric study about the question you ask.
RE: Rafter load sharing
RE: Rafter load sharing
RE: Rafter load sharing
RE: Rafter load sharing
RE: Rafter load sharing
RE: Rafter load sharing
RE: Rafter load sharing
And, redhead, or anyone else: What might that solid bridging you mentioned look like and how might it be configured at lowest cost (minimum change) to an already completed building? Thanks.
RE: Rafter load sharing
For RAFTERS Live loads this code states that concentrated loads and partial area loads may be factored in accordance with AS 1720.1, assuming the crossing members are battens with rigidity and spacing as follows:
1. Sheet roofs: EcIc = 2.7*10^9 Nmm2 and spacing = 1200 mm;
2. Tile roofs: EcIc = 380*10^6 Nmm2 and spacing = 330 mm.
The factors are g42 for concentrated and g43 for partial area loads respectively.
AS 1720.1 - 1997 - Timber Structures Part 1: Design methods gives the following procedure for finding g42. The procedure is set out here so that it may be processed in any word processor document, using MATHSERV, which may be downloaded from:
http://www.engs-comp.com/mathserv/
Set magnitude of point load in kN, P = 1.1
Select only one flexural rigidity from the following two:
1. tile roof batten in Nmm2, ECIC = 380*10^6 := 380000000.
2. sheet roof batten in Nmm2, ECIC = 2.7*10^9 := 2700000000.
Select only one spacing from the following two:
1. tile batten spacing in mm, S = 330
2. sheet roof batten spacing in mm, S = 1200
Rafter modulus of elasticity in MPa, EB = 7900
Rafter moment of inertia in mm4, IB = 35*170^3/12 := 14329583.3333
Rafter span in mm, L = 3600
Rafter spacing in mm, RS = 600
Number of crossing members, NC = floor(L/S) := 10
Then HB = EB*IB/L^3 := 2.4263
HC = ECIC/RS^3 := 1.7593
G42 = 0.2*log10(HB/(NC*HC))+0.95 := 0.7779
Finally effective load in kN, PEFF = G42*P := 0.8557
Regards, Helmut
http://www.engs-comp.com/
RE: Rafter load sharing
The solid bridging would be a 2x of the same depth as the rafters and can be attached to them with joist hangers at each end. This detail can apply to new or existing construction.