huangyhg

timber rafter ceiling joist connection
i am using method of joints to calculate the tension force in ceiling joist but the result is 2 times the computer run or any roof framing book table, anyone has any suggestions, for example if uniform load is 54 lb/ft for ld=1 on the rafter(horz) and the span is 17 ft with 1 ft overhang each side, the reaction at the wall = 459 lbs, the ceiling joist tension force=t=1377 lbs now if you use any framing book, they recommend 6-16d nail which capacity is 1/2 around 650 lbs which is close to computer run result, am i missing something here?
based on your numbers and the information given, i am guessing the roof pitch is 8v:12h...is this correct?
if so, the tension force in the tie for the loading given is 689 lbs, half of what you are calculating.
when you use the method of joints the result that you get at that joint is the force in the member, you do not need to add in the force from the joint at the other end of the
no the pitch is 4v:12h, and no i just took the joint by itself, as t=c cos(teta) and c=r/sin(teta), teta pitch angle=18.434,
well then at 4v:12h the force would be what you calculated, and i would check your software if it is giving you half this value.
with this flat roof you will get significant connection forces that aren't really accounted for in the "standard nailing details".
here's a link to a pretty good study that looked at traditional nailing patterns for various roof loads.
thanks ttk,
the result is 1/2 of joint method.
do the roof framing books have the # nails required for each roof pitch or do they just list 6-16d for this "standard" connection?
i did look at the online program in the link you provided, but is too "black box" for my peace of mind. i sent the website manager an e-mail notifying them of a potential bust in their program...let's see if they respond.
bottom line though, is you need to specify more nails in this connection than the "standard detail". you could use bolts if you are concerned with the density of the nailing pattern.
the correct tie tension force is;
(459/2) lbs * (12/4) = 688.5 lbs
only half the reaction is used to calculate truss axial forces as the other half is the rafter reaction at the support (ie. only the rafter reaction at the apex is used).
(trust my explanation is understandable)
but how could the joint be in equilibrium, if using only 1/2 of the reaction?, if you look at any truss analysis, the reaction is used to calculate the bottom cord tension force.
apsix, please expand on your explanation....it seems to defy logic.

calc1,
see pdf in link below from trus joist. they have nailing patterns for heel connections for various loadings and roof picthes. note the high number of nails required for the 4v:12h roof pitch.
allowable load per 16d @ ldf=1.0 is about 112# (depending on wood species), so you are looking at about 12 nails. that seeems impractical...no wonder light metal-plate connected trusses are so common. that roof pitch of only 4:12 is not good, especially since i assume this is snow loading.

__________________
借用达朗贝尔的名言：前进吧，你会得到信心!
[url="http://www.dimcax.com"]几何尺寸与公差标准[/url]