ceilinlg joist as collar tie
ceilinlg joist as collar tie
(OP)
refer to thread507-105386: How to calculate rafter thrust? -Help!
Similar situation, but simpler. 13'-4" between rafter supports. 27" vertical dimension (based on 4:12 roof pitch) from ceiling joist to ridge. 40 psf total S+D loads. Rafters @ 4' o.c.
Rafter reaction = 1066 lb.
Summing moments about the ridge I get the tension in the collar tie to be 3158 lb. Sounds really high. Anyone see my error, or maybe it is just because it is such a low roof pitch.
Thanks.
Similar situation, but simpler. 13'-4" between rafter supports. 27" vertical dimension (based on 4:12 roof pitch) from ceiling joist to ridge. 40 psf total S+D loads. Rafters @ 4' o.c.
Rafter reaction = 1066 lb.
Summing moments about the ridge I get the tension in the collar tie to be 3158 lb. Sounds really high. Anyone see my error, or maybe it is just because it is such a low roof pitch.
Thanks.






RE: ceilinlg joist as collar tie
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: ceilinlg joist as collar tie
W = 2*R = 2132#
M = WL/8 = 3553'#
h = 27" = 2.25'
T = M/h = 1579# (tension in ceiling joist)
You are out by a factor of 2.0
BA
RE: ceilinlg joist as collar tie
I was looking at a free body diagram, summing moments about the ridge, solving for T = (1066 x (13.33'/2))/(27/12) = 3158. What moment are you calculating with WL^2/8? That is for a "uniformly loaded beam".
W = 2xR ?
RE: ceilinlg joist as collar tie
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: ceilinlg joist as collar tie
RE: ceilinlg joist as collar tie
Hokie66, that is almost exactly what i got originally by my method. Do you not agree with the simple logic of the 2 gentlemen above?
Is one of our methods just simplified too much?
RE: ceilinlg joist as collar tie
BA
RE: ceilinlg joist as collar tie
The value calculated by Mike and BA for the bottom chord force is correct. These guys are rarely wrong, at least both at the same time. You and I both made different errors and got the same wrong answer.
I think you neglected the uniform load on half the truss when summing moments about the ridge. When you deduct that moment from the reaction x L/2, you get the correct answer.
In my case, in using the method of joints, I neglected the fact that half of the 1066 reaction goes directly into the support through top chord bending, thus does not affect the axial forces. For axial force at the joint, the bottom chord force is 1066/2 x 12/4.
RE: ceilinlg joist as collar tie
It all hinges on equating the moments (uniform and point force applications) and the force freebody.
Sorry for the quick sketches. I am obviously not an art major.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: ceilinlg joist as collar tie
BA
RE: ceilinlg joist as collar tie
RE: ceilinlg joist as collar tie
I greatly appreciate the guidance, gentlemen.