Rafter Span Table Question
Rafter Span Table Question
(OP)
Has anyone noticed the rafter span tables in the IRC and AFPA seem off? I use the horizontal projection and all the factors from the NDS. Although bending seems close, deflection seems way off. For instance, the IRC shows 22'3" for 20/10 loading of 2x10@16" SPF#2 @ L/240. I calculate a deflection of L/168 at that span using 1.4 million psi modulus of elasticity. What am I missing?





RE: Rafter Span Table Question
Mike McCann
MMC Engineering
RE: Rafter Span Table Question
www.idecharlotte.com
RE: Rafter Span Table Question
Mike McCann
MMC Engineering
RE: Rafter Span Table Question
www.idecharlotte.com
RE: Rafter Span Table Question
Span = 22'-3"
L/240 = 1.11"
Uniform LL = 20 psf x 1.33 ft = 26.6 plf
E = 1,400,000 psi
Ix for a 2x10 = (1.5 x 9.25^3)/12 = 98.93 in^4
Deflection under LL only:
(5 x 26.6/12 x (22.25 x 12)^4) / (384 x 1,400,000 x 98.8)
= 1.06 inches vs. 1.11 inch limit
Seems pretty close to me. I'm not getting your L/168 = 1.59".
RE: Rafter Span Table Question
Sorry, but I beg to disagree here. See attached calculations. I will have to post twide for both pages.
Mike McCann
MMC Engineering
RE: Rafter Span Table Question
Mike McCann
MMC Engineering
RE: Rafter Span Table Question
On your equation w(perp) = 1.6/L2.....shouldn't that be 1.43/L2.
The bending moment along L2 should be calculated using the perpendicular force component. The parallel component runs down the longitudinal axis of the beam and thus adds no bending moment to the beam along L2.
This results in the same bending moment for each way of looking at it.
Just asking.
RE: Rafter Span Table Question
The deflection using w(perp) - which should also be 0.080 k/ft - is the deflection perpendicular to the beam and needs to be converted to a vertical deflection to compare with the other deflection. I don't think the deflections are equivalent but the shears and moments are.
RE: Rafter Span Table Question
However, I do not think that one should compare vertical deflections here, but deflections normal to the main axis of the rafter. I think that will give the true result, particularly if you are dealing with tile roofs where every abnormality is very observable. After all, if the rafter is laid flat, this is the deflection that it actually sees. I get a 25% difference in deflections with this method. It may be more conservative, but I think the roof performance will be much better.
Mike McCann
MMC Engineering
RE: Rafter Span Table Question
w(LL+DL)= (20+10)psf x 1.33 = 39.9psf
E= 1400000psi
I= 98.8in4
@ 22.25' defl = 1.6" or L/168
Reading through the IRC I don't see a total load deflection criteria. I personally wouldn't design this way, but at least i see where the Code is getting its Tables.
Thanks.
www.idecharlotte.com
RE: Rafter Span Table Question
FYI, where ever I have gyp on the ceiling at a roof, I use the same deflection criteria as for the floor.
Mike McCann
MMC Engineering