Hip Rafter Tributary loads

[sapoton]

I am trying to build an irregular hip roof and need to utilize hip beams. I had a program figure the point load at the ridge. My question is, when I design the support for the hip beam do I need to double the load for the other hip?

24’6” span, floor joists 2x8 @ 12” O.C. The hip beams will need to be supported by 4 2x8’s. if I am accounting for the single load, one of the supports will need to be cantilevered 1ft and then supported by 4 more 2xs if I have to design for double the load. 463lb LL and 271 DL at the hip / ridge connection (I am assuming this is only one of the hips). Single 2x8 hip and 2x6 rafters @ 24 O.C.

Any help would be appreciated.

 

[Ron]

It would help if you would provide a sketch. I'm not sure about your point load comment.

 

[HeavyCivil]

Also agree that a sketch would help. On another note just remember that larger tributary areas for roof beams mean big wind loads just like they mean big gravity loads.

You probably know this - but I see ridge beams with huge tributary areas for uplift simply toe-nailed into a stud-stack all the time.

 

[sapoton]

Here is a pic hope you all have powerpoint. I determined the load via the strucalc program and was wondering if it calculated on hip or both. Since the rafters are ofset I need to design the hips as beams. Problem is I am spanning the whole distance with 2x8 ceiling joists with 2x6 rafters. I cannot put lager beam members in the ceiling to support the hip posts. If it is one load i can support it with a built up beam of 4 2x8s is its two loads the 2x8s wont carry it.

 

[hokie66]

Both hips have to be supported, but why not use trusses?

 

[rowingengineer]

Hokies advice is sound (as always), if you were to use trusses your layout would be better if it was something like attached.

Arguing with an engineer is like wrestling with a pig in mud. After a while you realize that they like it

 

[sapoton]

I initally planned to use trusses but the truss manufacturer gave me the run around for 2 weeks and I eventually ran out of time to get the building blackened in before I go on vacation. This is the root of the problem. I had it half framed before I realized I had to have a hip beam and not just a hip rafter.

Thanks for everyones help

 

[rday]

StruCalc only does one side when calculating the size requirement of a hip beam.

My question from your sketch is how are you supporting the top end of this beam (these beams)?

StruCalc assumes a pin connection at each end. From you sketch, I can't tell how you're addressing this issue.

 

[sapoton]

I wasnt planning on pinning it. Im assuming by pin we are talking about a dowel of sorts from the hip to the ridge. If I support the hips at the top with a post and in the middle with another post StruCalc says a single 2x8 is sufficient. I wouldnt think there isnt enough room to get two pins into the 2x ridge. Wow all of this is getting extremley complicated for a hip rafter spanning 12 feet. If my jacks were not offset I would be able to frame it as a rafter and not a beam.

 

[BAretired]

I would prefer to place the trusses as shown in the attached sketch. If the truss supplier is giving you the runaround, you can use a pair of tied arches instead.

Whether it's a truss or tied arch, the midspan concentrated load is the sum of the reactions of two hip beams and a short ridge beam. It also carries approximately two feet of uniform load.

Piece of cake. You don't need a computer to solve this baby.

BA

 

[KootK]

Your hip beam needed to be designed as a "beam" regardless of whether or not your jacks lined up. With wood framing, there are always other factors at work such at diaphragm action etc. They generally shouldn't be relied upon however.

Are you going to support the hip beams on a post at the top? Is that post itself going to be supported by a beam? If so, take care to ensure that the top of that beam is braced somehow. Presumably, it will not be braced by the roof sheathing.

 

[sapoton]

Kootenaykid... Which beam are you referring to? you said to make sure the top of the beam is braced... did you mean the post? If your talking about the hip beam don't the rafters keep the top of the beam stable?

 

[sapoton]

Again thank you all for your input and advice. Trusses are no longer an option the framing is almost done. The beams will be supported by posts which will rest on built up 2x8 beams underneath. Each hip beam will have its own support post (offset from eachother) one hip beam on each side will be cantilevered 1FT. so in esentially I have 4 beams in the center supporting the hips by posts. Each beam will be 4 built up 2x8s. There will also be intermediate supports roughly half way down each hip ratfter, again each hip supported by a beam and a post. I think this will work, it just seems excessive to me but I am not an engineer and that why I am here. One other question; What keeps the hip walls from spreading? I know gables use ceiling joists. Do I need to run rafter ties above the ceiling joists tying the hip jacks together?

 

[KootK]

Sapton: I was referring to those 4-2x8 beams that will be supporting the posts. At those dimensions, however, lateral torsional buckling may not be an issue.

If the rafters are properly supported vertically, there should be no tendency for the walls to spread under gravity load. In a manufactured corner set, there would be bottom chords to the jacks which tie the rafters together somewhat at the ceiling level. If it's still an option, you might try to mimic that arrangement.

I agree with you, what you've proposed sounds excessive. I used to work as a roof truss designer back in the 90's. I don't think I ever came across that standard hip rafter that an end supported double 2x12 couldn't handle. And I used that at a ski lodge in Whistler, BC. Of course, you typical manufactured hip set is in the 8-10' range.

I'm sure that engaging the services of a local structural engineer would result in a more efficient system. Of course, the $500+ you will spend on an engineering solution may well offset the material/labor savings. Time is probably an issue as well.

 

[sapoton]

Again thanks for everyones input on this thread. I have a stub joist arangement 2ft on center attached to the jack rafters on the hip side. I just dont see how these which are face nailed to the first ceiling joist help the wall from pushing out especially since the main side calls for the joists and the rafters to be connected by 8 (cant remember the d size) nails pre connection. Either way again thanks you all what I have planned will work though it seems overkill, but im not an engineer. Now we move on to attic ventilation with a 4 ft ridge.