Hi,
I'd like to get some opinions on a 24' span.

The project is a garage/vehicle shed, final outside dimensions 24'x24' (12' height). I will be using old growth SYP (most logs with DBH of ~ 24"). My plan was to use 8x8 timbers for all the short runs (10-12') and rafters ~14-15'. The building would have three bents (two ends and a middle) - 12' connectors.

The question is if I can use a 24' beam across the bent (with no support in the middle) or should I frame the bent as two 12' sections (in other words have a supporting post in the middle under the 24' beam)?

I guess a secondary question is what dimension would the 24' beam need to be if no support was used? (8x10? 8x12?)

This is my first TF project, so I may not have given all the info needed to answer the questions - please advise.

Thanks.

http://www.tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=19471&page=7

Put the post on top of the tie and still have a clear span. The tie I used was 9"x12"ish, it also had a crown in it, the span is 24'. If you have access to a 24' tie you might as well have 24' top plates as well.

Tim

you meant put a post between the tie and the ridge where the rafters meet?.. and are you referring to this picture: [img]http://tinypic.com/r/whyxq0/3[/img]

A simple truss in the middle bent. The rafters are not part of the truss. Yes, that is the picture.

Tim

ok thanks

I tried it this way;
288" clear span
full 8x10 dimension..(8x10x288)/144=160BF
160/12=13.33 cu/ft
13.33x 36 lbs/cuft=480 lbs green
Using an E value 0f 1.2 million psi and with just the self weight of the beam I'm coming up with about 3/16" deflection. Hanging 1,000 additional lbs from it, it sagged 9/16"

Figuring deflection with just self weight of the beam, with an 8x8 I came up closer to 5/16", at 8x12 it was a bit over 1/8"

Not saying not to make a king rod truss out of it, but if the bottom chord is just a tie it is in no danger with the long bottom chord alone the way I see it.

Don -

I think he's trying to figure if he is going to build a truss or drop a post to the ground.

Either way he has 24' to take care of - either a continuous ridge, or a tie that is part of a truss...

As I mention in all theoretical engineering posts in the forum - it would help to understand loading - snow, wind, etc. Use. Roofing material, etc. Open frame? Enclosed? TG on ceiling? Metal roof? Shingle? Etc. Etc.

If you decide to build a truss - you will be carrying a tributary area of 12' wide (6' each side) of the total roof load. How this spreads to the truss depends on the type of truss. Do you plan on using a ridge with common rafters (point load onto a king post) or principle rafters with common purlins (distributed load on truss rafters)?

Self weight often causes very little deflection... the good stuff happens when we throw snow on low pitches with non-slippery surfaces, then unbalance it all with wind!

24' is very doable. Timbeal points to a king post truss design with dropped rafters (or struts) that would work, assuming the loading makes sense for how the truss ties into the posts, and how the load transfers down the struts and out through the tie (or lower chord). And the joinery you design needs to be able to transfer those loads as well...

BJC could ask more questions to which we could respond. Mike points out questions which would for sure need to be answered. My example is just that. I have seen many 24' spanned buildings where the roof only loaded the walls not the floor. The king would not have to be tied into the roof system, avoiding roof loads. You then have to look at what is going to keep the building from spreading.

My suspicion is the 8x10 would be bouncy. But is there even a floor to load?

In my truss above, the struts are using a 3" tenon and plenty of relish, no issues that I can see.

Tim

Tim, not sure I follow you here. What is the point of a king post if it is not tying into the roof system? What did you mean by this?

The King is supporting the floor, not holding the roof up. Without a post under the tie, what do you do? Span 24' without a post?

Tim