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Re: WOOD: Criteria Selection of Joists[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: seaint(--nospam--at)seaint.org
- Subject: Re: WOOD: Criteria Selection of Joists
- From: "Dennis S. Wish, PE" <dennis.wish(--nospam--at)verizon.net>
- Date: Wed, 02 Feb 2005 22:18:02 -0800
Bill,The biggest mistake in your comments relate to collar ties. Collar ties are not meant to prevent the horizontal spreading of the rafters or act like rafter ties - they are intended to prevent uplift from wind and really nothing more. With this said, you must understand the difference between collar ties and rafter ties. Collar ties are placed in the upper third of the height measured from the double top plate (bearing) to the peak of the roof. Rafter ties, don't have to be nailed off at the plate, they can be installed in the lower third of the height. So if your contractor decides to install ceiling joists in the shortest direction, you can, with creating a sub-diaphragm connection with blocking and straps, set your rafter ties above the ceiling joists (perpendicular to them) and face nailed to the rafters. This results in increased costs of materials and labor costs and while I don't suggest it, I have seen it done.
One of the things you can impose upon the contractor is that the rafters when placed in the shortest direction will require support of bearing walls - and foundations if the bearing walls occur inside the home. So shorter joists doesn't always equate to economy and stiffness is not an issue for ceiling rafters if used as rafter ties since the bending is compensated by the tension in the tie. If you need to add support to longer spans, you can add a strong back to the top of the rafter ties and using metal connectors add support in a number of ways - support of the strongback in walls that may require spread footings or you can use a 2x strongback that is hung from the ridgebard near the connection of the rafters in compression. This does not add much bending in the ridge and was commonly used where economy was most important.
My most stringent suggestion is not to stick frame this, but use metal side plated roof trusses designed to span the longest distance you can (I think there is a restriction in the code to 30 or 40-foot spans on plated trusses as it is a matter of transporting the rafters to the site and storing them as recommended by the WTCA. Whatever way you choose - you are the boss on the job as your stamp is on the drawings. If the framer insists on doing it his way, let him hire an engineer to relieve you of responsibility as engineer in responsible charge, write a letter to the building department to inform them of the issues that prevent you from working with the contractor OR if you represent the owner - fire the framer and find another one who will follow your design and details unless they have a valid argument. What they think they save they believe they can pocket. You need to tell them that his modifications will end up costing him more as he will have to make changes to add lumber and labor to install the ties regardless of the direction of framing that he wants. Don't forget to bill the owner or the client (if it is him) for the additional time you take to make his preference work.
Sorry to be so tough about this one, but if the framer wants to design the structural elements then let him get a license and wet seal. I am a stickler about this and will not budge when a framer chooses to ignore what I've done to do it his way. I have not problem with framers who come up with better ideas, but I once did a large custom home that was difficult to find at the framing inspection that he did not follow any of my details or references. His lame excuse was that the references were not on the page he was used to finding them so he didn't go looking for them. I charged the owner a couple thousand dollars to make it work with tearing down the work he did. The owner not only did not pay me, he threatened to take me to court and reminded me that he had more money that me and could tear me apart in court just by spending money. He would rather give it to a lawyer than to me as the General Contractor was his neighbor and the framer he picked had little or no experience in custom homes.
Be tough but be fair. In this case it sounds like you have a framer who simply ignores you to do it his way. Make him do it your way and don't listen to his complaints - if he was so good he would be a licensed engineer and would not need you.
Dennis Bill Polhemus wrote:
Not sure how to make a succinct title for this post, so away we go:Continuing the discussion with my meeting with my builder friend the other day, one of his biggest pet-peeves was "you aren't turning the ceiling joists the shortest way."He looks at each room and just turns the joist whichever way yields the shortest span.I took pains to indicate to him the requirement of the Code to have a continuous tie:[QUOTE MODE ON]IBC 2000 R802.3.1 Ceiling joist and rafter connections. Ceiling joists and rafters shall be nailed to each other in accordance [the accompanying Tables] and the assembly shall be nailed to the top wall plate... Ceiling joists shall be continuous or securely joined where they meet over interior partitions and nailed to adjacent rafters to provide a continous tie across the building when such joists are parallel to the rafters.Where ceiling joists are not parallel to rafters, subflooring or metal straps attached to the ends of the rafters shall be installed in a manner to provide a continous tie, or rafters shall be tied to 1-inch by 4-inch (nominal) minimum-size crossties...Rafter ties shall be spaced not more than 4 feet on center. [QUOTE MODE OFF]Rafters tying the ends of the rafters together across the building form what Dennis Wish calls a "Carpenter's Truss".Now, obviously we can use the rafters running the shorter direction if we like but we must provide subflooring or metal straps or other Code-approved means to carry the forces across the building. Or we can use Collar ties (what the Code calls "crossties"). The problem I have with crossties/collar-ties is that it "complicates" what is happening in the rafters. To me one cannot ignore the fact that you now have induced flexure in the rafters and therefore the design check of the rafter becomes a non-trivial thing. In short, I just don't like crossties/collar ties for that reason.And if I come back and tell him that he's got to use "huge" rafters, he's gonna give me the same old song-and-dance about how I "overengineer" everything and "it costs me money, I can't compete" and "the inspector passes it all the time."Now, in the opinion of those here who care: What is your opinion as to the relative cost of going with larger ceiling joists running parallel to the rafters where possible and installing subfloor as needed where they run perpendicular, versus LOTS of subfloor because we're running joists higgledy-piggledy to get the "shortest spans"?And how about the whole "collar tie" thing? How do YOU feel about its effect on the rafter design?Thanks for your input. I know most of you are bored by these residential discussions, but I appreciate those who do pipe up.
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- WOOD: Criteria Selection of Joists
- From: Bill Polhemus
- WOOD: Criteria Selection of Joists
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