Rafter load sharing 4

[vonlueke]

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.

 

[ishvaaag]

In the past I would have looked to some note which indicated such load sharing (...was for RC concrete joist, where load sharing is no doubt more complete).

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.

 

[Zulak]

The repetative member factor in the wood code is appropriate for this case. If the load is in between the rafters, then the load is shared by the adjacent rafters (P/2 to each rafter), and the repetative member factor also applies.

 

[Ron]

If load is directly on the rafter, apply all of load to rafter. If on sheathing between two rafters, distribute according to location (i.e, if half way, distribute 50/50, etc.). Even though some load sharing does occur, its quantity varies with several conditions such as whether there is a sheathing joint under the load, or where the sheathing joint occurs relative to the load.

 

[vonlueke]

What would NDS Sect. 15.1.1 mean, then, for my first question (paragraph 1 of original post), when P is directly above one rafter? (And assume for the question no sheathing joint occurs at nor adjacent to rafter.) Thanks.

 

[ishvaaag]

The position of the load along the span also has influence. Amidst the span, if there is continuity, you will have adjacent joists' collaboration, Contrarily, very small you will have if your load is quite close to the support of the joist. For uniform loads, any closed form proposition answering your question can't be but a statistical reduction (and simplification).

 

[redhead]

If you have an actual situation where this condition occurs, why not introduce solid bridging between adjacent rafters at the point of the load. Then you can be assured that all three rafters will share the load more or less equally.

 

[Trussdoc]

NDS 1997 Section 15.1.1 appears to be oriented more to beams supporting heavy timber plank floors rather than the flexible roof covering mentioned in the original question. My recommendation is to use only the repetitive member factor (as posted by Zulak)and place all the load on the one truss, or a proportional division for two.

 

[vonlueke]

Thanks for your comments. Admittedly Sect. 15.1.1 is too cryptic, in my opinion. I had gotten the impression "2 inch plank" meant "2 inch wide (i.e., 1x2) planks"; and then when I saw "laminated," I got the impression they were referring to "plywood." You're interpreting "2 inch plank" as "2 inch thick plank." I assume you're also referring to 2x4 lumber (actually only 1.5 inch thick) nailed flatwise...included in the "2 inch plank" group? Any other interpretations or comments out there on 15.1.1 are greatly appreciated.

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.

 

[engcomp]

One Australian standard that has a bearing on this question is AS 1684.1 - 1999 - Residential Timber Framed Construction: Design Criteria

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/

 

[redhead]

Vonlueke:
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.