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Where Goes the Load?
 Can you answer these exciting questions?
A. If a 12-foot window header spans a pair of side-by-side windows, will the mullion in between the windows take any load, or must the header be designed to span the entire 12 feet?
B. If a stair jack is supported by a number of posts or studs along its length, must the stair jack be as large as if there were no intermediate supports?
C. If you were adding a beam to help support sagging, bouncy floor joists, how much load would that beam take? Should you design it as if the floor joists weren’t taking any load?
D. If your house was built using masonry or ICF (Insulated Concrete Forms) in combination with stick-frame construction, how much lateral load would go to each part during an earthquake?
The fundamental answer to each question is:
When load is shared by two or more structural members, load goes to the stiffest one(s) first.
That simple explanation is the basis for all structural design. Every builder, designer, code official and engineer should understand it thoroughly.
To illustrate, let’s go through each of the four questions:
Answer to Question A: Many times a header spans two or more side-by-side windows with the window panes being separated by mullions. Whether you like it or not, those mullions, if they touch the bottom of the header and are framed continuously to the floor, will support the header. In this case, mullions, serving as columns, are stiffer than headers, being beams, and will take load first.
As an example, say there’s a house with a header across a three-pane window that supports roof trusses, and the house is hit by big snowstorm. All that weight will tend to make the header sag. But before the header can sag, it will have to push the mullions down.
The mullions are supported solidly by the floor and can’t go down. So unless the floor fails or the mullions buckle, the header can’t sag and it won’t take the load.
Your bottom line:
- Make sure mullions are stout enough to take the load.
- With proper load-bearing mullions, headers don’t have to be designed to span an entire bank of windows.
A note of caution: If the mullions in question are an integral part of a factory window assembly and are not wood framed, they probably won’t take much, if any, load. This is because window assemblies have a little give built into them in order to allow adjacent wood to move without loading up the glass. In this case the header beam, receiving no intermediate support, must be designed to span the entire assembly.
Answer to Question B: Wood framed stair jacks (a.k.a. “stringers”) are notoriously bouncy. This is because they are commonly over spanned, and many times, they are hacked from 2x12 material.
Common sense should tell you that you can’t hack that much out of a 2x12 and expect it to maintain its original strength. Smart builders know this and nail stair jacks to walls, build walls under them or add support posts. Every connection point or post reduces the effective span of the jack.
Say a stair jack is nailed to every stud in a framed wall. The span of the jack then becomes the stud spacing, probably 16 inches or 24 inches. Even a hacked up 2x12 can span 16 inches.
The support studs, which serve as columns, are stiffer than the stair jack or beam, so load is soaked up by the studs first, and the stair jack won’t sag.
So, in answer to this question, a stair jack that is supported along its length does not need to be as large as one that has no intermediate support along its length.
Answer to Question C: My brother recently faced the issue of how to support the floor joists under the dining room of his 50-year-old home. The floor joists are 2x6s at 16-inch spacing, spanning about 12 feet.
A quick check with your handy ConstructionCalc ProBeam will tell you that 2x10s should have been used, and that the 2x6s are about 65% under designed. To make matters worse, my brother’s joists are riddled with holes for plumbing and electrical — further weakening them.
I advised him to add a beam at about mid-span of the joists. Unfortunately, the basement below the dining room has a low ceiling. A large, new beam protruding even a little into that cramped basement space would probably result in some serious head banging, so a non-protruding option was called for.
My first recommendation was to install a 4x6 steel tube beam oriented flat-wise. My brother, every bit the cheapskate that I am, didn’t want to mess with the expensive and obstinate steel. So, we went with a second option — a 5-inch-tall by 5-inch-wide PSL (parallel strand lumber) beam.
Unlike the steel option, the PSL was not structurally adequate on its own. However, because it was sharing load with the still viable joists, it was okay for the task.
So, to answer the original question: When bolstering inadequate joists or rafters, if you install a really big, stiff beam under them, the beam will soak up most of the load and relieve the sagging members completely.
However, this generally is not necessary if the sagging joists are still structurally sound. A lesser beam will deflect some under heavy load, allowing the joists/rafters to do some work, too.
Answer to Question D: The concept of “load going to the stiffest member first” isn’t restricted to gravity — downward acting — loads. It applies to lateral — wind and earthquake — loads, too, and comes into play when wood framing is combined with masonry or ICF.
Masonry and ICF shear walls are far stiffer than stick-framed ones and will soak up lateral loads first. In fact, wood shear walls won’t even feel the earthquake or wind until the masonry or ICF fails or deflects enough to spread the load around.
Of course, for any shear wall to absorb lateral load, it must be well connected to horizontal diaphragms at the top and bottom (roofs and floors).
So, to answer the original question about lateral loads, you can’t tell exactly how much lateral load will go to each shear wall element unless you do complicated calculations. But, in general, lateral loads go to stiff concrete and masonry elements first, and only when they fail or deflect will loads be transferred to flexible elements like plywood shear walls.
The point here is, why spend a money engineering and constructing wood shear walls capable of absorbing lateral loads if you’ve got masonry ones competing for the load? The masonry will win every time.
Whether intended or not, shared loads go to the stiffest structural elements first. It behooves builders and designers to understand this so that they can predict, at least globally, what needs to be strong and what can get away with being weak.
Tim Garrison of ConstructionCalc.com, is a professional engineer, author, and software producer for the building industry. Check out his new book, "Cracks, Sags, and Dimwits — Lessons To Build On," available at www.lulu.com, Amazon and Barnes and Noble.
Send e-mail to buildersengineer@constructioncalc.com. Tim reads every one.
This column cannot be reprinted without permission from the author.
The views expressed in this article represent the personal views, statements and opinions of the author and do not necessarily represent the views, statements, opinions or policies of the National Association of Home Builders. NAHB does not necessarily endorse any of the views expressed by the author and NAHB is not responsible for any direct or indirect consequences arising out of the views expressed in this article.
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