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strongback design

rktect 1

rktect 1

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Illinois
Just a general question to anybody who may know something about designing a strongback.

I recieved a plan for an addition. On the floor plan the addition is 27 feet wide. A interior wall was added perpendicular to this 27 foot wide house at about 8 feet in seperating the bedroom and bathroom. The ceiling joists are 2x8 @ 16" o.c. So the max. span per IRC tables was 17'-5" for the ceiling joists but what we have here is 19 feet, seperated by interior wall, and 9 feet at the other side. So at midpoint (27 feet divided by 2 = 13.5 feet) in the attic over the ceiling joists the DP added a double 2x12 strongback. The span of the strongback is about 7 feet.

Certainly not prescriptive but I am wondering where the guidelines for the design of strongbacks come from. The DP is an oldtimer and actually hand drew the set of plans.
 
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My understanding of strongbacks is that they provide support for the ceiling joists agains twisting, similar to what blocking would provide. That doesn't seem to have anything to do with the ceiling joist span.
 
The design came from builders/framers trying to please the sheetrocker/drywaller and homeowner complaining.

The drywaller complained about their drywall not hitting on the joists and having to cut a sheet just to be able to break on a joist.

The homeowner complaining about humps and dips in their ceilings. Cosmetic of course but ugly.

So builders and framers got together and figured out a way to make the joist fall on the correct centers. Add a 2 X laying flat on top of the joists in the middle of the room to keep the spans on 16 or 24" centers.

Well this made the drywallers happy but there were still humps and dips caused from the extreme crowns of the 2 X 's. One being excessively crowned then the adjacent one straight in the ceilings and the homeowners still complained.

Well the framer says..how about if we turn a 2 X 6...8 or whatever the joist size was, up on it edge and force the joists to be level. Hmmmmm sounds like a good idea. So framers started adding a 2 X nailed to the flat 2 X on top of the joists. Well this worked until the 2 X 's ended over a joist and then just butted another joist on to the end of the 2 X. Allowing an area that had a hump/dip in the ceiling.

Framer says okay....how about if we stager the joints in the 2 X's on edge and overlap them so it will be more like a beam. It came out that if the framer lapped at least two joist past the split then the ceilings became flat and no more complaints.

So there you have the history of the strong back...hog back.... Not a requiremnt one in the code. It just became standard practice to split up the rooms with a strongback.
 
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rktect 1 said:
So the max. span per IRC tables was 17'-5" for the ceiling joists but what we have here is 19 feet.
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If the span is limited by deflection rather than stress, then engineering might show that the longer span is adequate. But the strongback should not affect the span.
 
Ok, so a strongback in an attic is not used as a beam to allow for a greater span of the ceiling joists as it seems this one was intended. What is described in your post makes it seem that a single 2x6 or 2x8 could be used. In my case, he uses a double 2x12. The placement is specifically located at center due to the first floor steel beam running underneath at center where he is transferring the load.
 
If using the IRC tables; he needs to use 2" X 10"s ; R802.4(1). Unlike being able to use purlins for overspand rafters; I'm not aware of anything that can be used to "stretch" the maximum span of ceiling joists.

Uncle Bob
 
Around here, ceiling strapping was invented for just that uneven ceiling joist reason.

Using a strongback to lengthen the working span . . . as you say, not prescriptive. I have no non-prescriptive "rule of thumb" for that, either.
 
strongback.jpg
 
You see, if he installs the ceiling joists from one end to the other, those ceiling joists, either one piece or two that are laped, hold the exterior walls in place. horizontal thrust from the roof. If instead of lapping those two ceiling joists, he installs a beam, flush with the ceiling joists, I suppose we rely on the joist hanger and the nails to resist that horizontal thrust of the roof. I think this guy wants to not put in the flush beam, extend the span of the ceiling joist, maintain the lap between two ceiling joists and thereby resist the horizontal thrust of the roof. Just my guess. I'm no engineer and I didn't sleep at a holiday inn last night so.....

Also, in this case, he has used the strongback as a beam. He has spanned from an exterior wall directly over a beam to the other side of the strongback which rest on an interior partition wall, also directly over the same steel beam.
 
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Well, he could get really creative here; and using plumber's strap (you, know that metal strap with holes in it that comes in rolls); hang the ceiling joists from the ridge board to add support, and, make up for the overspaned joists.

Don't laugh too hard; I've seen attempted before.

Uncle Bob
 
I have to wonder if his use of the word strongback applies here. And it just threw me off.

Lets look at this as a simple beam. He spans 7 feet. He attaches, not with plumbers strap, the 2x8 ceiling joists to the double 2x12 beam located in the attic. Reduces the span of ceiling joists to 13.5 feet, right?

So what attachment methods is he allowed at the beam and ceiling joists?
 
rktect 1 said:
You see, if he installs the ceiling joists from one end to the other, those ceiling joists, either one piece or two that are laped, hold the exterior walls in place. horizontal thrust from the roof. If instead of lapping those two ceiling joists, he installs a beam, flush with the ceiling joists, I suppose we rely on the joist hanger and the nails to resist that horizontal thrust of the roof. I think this guy wants to not put in the flush beam, extend the span of the ceiling joist, maintain the lap between two ceiling joists and thereby resist the horizontal thrust of the roof. Just my guess. I'm no engineer and I didn't sleep at a holiday inn last night so.....Also, in this case, he has used the strongback as a beam. He has spanned from an exterior wall directly over a beam to the other side of the strongback which rest on an interior partition wall, also directly over the same steel beam.
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why not design the strongback as a beam to cut the ceiling joist span in half, it is done all the time. the beam is designed to carry the ceiling joist live and dead loads with bearing at each end transferred to the foundation or beam as you describe. you get a rafter tie with no splice and a straighter ceiling to apply drywall to. the ceiling joist to strongback connection can be done with any number of simpson connectors or tor eggs structural steel strapping!( insert green smily guy here)
 
* *

pwood,

Please, let us not resurrect the "tor eggs structural steel strapping" creature again. :eek:

Mr. Randy might just be lurking in the shadows and ready to pounce again!!

* *
 
I get this explanation all the time, I ask "what is supporting the joists?" . . . answer: "the beam is supporting the joists", , ," ok, what is supporting the beam?" . . . answer: "the joists are supporting the beam" . . . uhuh . .
 
In my opinion, the weight of the beam would actually make things worse. A strongback is a replacement for sheathing; it aligns the unbraced joist edges. I'd be expecting the joists to frame into the beam.
 
TJacobs said:
In my opinion, the weight of the beam would actually make things worse. A strongback is a replacement for sheathing; it aligns the unbraced joist edges. I'd be expecting the joists to frame into the beam.
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tj,

the strongback if installed properly is a replacement for a wall! a bearing wall if supporting 100# per lineal foot or more.
 
Got to love oldtimers.

Just so I got this right this is one piece ceiling joists because the picture appears to show the joist splits at the beam. If that's the case why not a simple beam and ceiling joists tying into it instead of all the stuff he's showing.

Or use different centers or 2x10 or get some engineering or trusses?
 
Drew it wrong. I aksume it will be one piece at least until it gets to the interior partition wall about 10 feet from the exterior wall on one side.
 
Yankee said:
Around here, ceiling strapping was invented for just that uneven ceiling joist reason..
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Geez, I just thought we used strapping to run our wires and pipes so we didn't have to drill holes. The first framer I worked for used to over span all of his ceilings, he would then "strong back" the center and hang that from the ridge with 2x4's 4' OC...it wasn't until I started taking classes for my CSL that I realized that wasn't code. He never failed an inspection either, this was under the MA 5th and 6th ed.
 
I can find no exception for overspaned ceiling joists in any IRC; including "strongbacks". Perhaps strongbacks are mentioned somewhere under "sky hooks". :)

My age is showing here because now I'm trying to find the requirements for "rat runs" and I can't remember the code section because "rat run" ain't there either; and I forgot what the IRC calls the dang thing. :(

Uncle Bob
 
Maybe rat runs were the beginning of a strongback system. Except the original rat runs were 1 X 4s.

UB.... you and I are just too old if we remember stuff like that!

We called strong backs hog backs.
 
Min. code vs. engineered solution vs. acceptable engineering practice vs. ???????? How am I to know this isn't better than code? If he put this "stongback" (not certain this is what it actually is) and used the same double 2x12 as a flush or dropped beam, he'd certainly be fine, right? There is nothing in the code that says he can't put the beam in over the ceiling joists and hang the ceiling joists off the bottom of it, is there?
 
A strongback can be different things.

It can be used to make deflection more uniform by tying the joists together.

It can also be an inverted beam from which the joists are hung which will reduce their span and allow for a longer piece of lumber to be used.
 
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