I just finished the structural analysis for a 3-story, stick framed apartment building. It’s an addition to an existing complex with a footprint of about 10,000 square feet. I did not engineer the original buildings, which were constructed some ten years ago, but I did look at those plans because I wanted to see how the previous engineer approached the lateral (wind and seismic) design.
What I saw shocked and horrified me.
I didn’t count them all, mostly because life is too short, but there had to have been several hundred holdowns in each of those previous buildings. And many of them were herking, bolted-types that cost a bundle both to purchase and to install. I said to the project architect, “Mark, I bet I can do this design using only 10% of the holdowns that the first buildings used.”
“No way!” he exclaimed, nearly losing his toupee. “Holdowns at every corner, door, and window are the status quo.. the fashion… the rage… the American way. You… you can’t buck that kind of tradition. Can you?”
“I can, and I will. We’re in an era of build-green. These days, less is more.”
“But there must be some kind of trick,” he gasped, “some sort of warping of the fabric of the building code. Are you sure you can handle it? I mean, the stress. It just isn’t natural.”
Concerned for my rattled colleague’s blood pressure I tried to sooth him. “Baloney,” I said. “It’s just a matter of simple statics. If after taking into account all the applied and resistive forces there is no uplift, there is no need for a holdown. Plain and simple.”
I left Mark to his mutterings and strode confidently from the meeting. I felt like a magician after having just sawed the pretty girl in half.
It is now a week later and the lateral design is done. I used a grand total of 12, light-duty, strap-type holdowns. Not bad for a 30,000 square foot building. But will this structure blow away in the first wind storm? Will it get knocked off its foundation in the first earthquake? No way. Does my design meet code? You bet.
Before telling you how I did this let me give you my opinion of how we became so enamored with holdowns in the first place. I remember when holdowns hit the scene, in the 1980s or so – around the same time as big-hair bands and spandex. More stringent building codes upped wind and seismic forces. Suddenly, when you analyzed a shear panel you found that at each corner, under full load, an uplift force occurred, which if not mitigated would theoretically tilt the panel right of the foundation. And in those days, puny 1/2” anchor bolts at 6-feet spacing, usually poorly installed, were the norm. The only way to alleviate this huge “new” uplift was to install a holdown. Simpson Strongtie company couldn’t have been happier.
After the initial furor over having to install something besides bubble gum anchor bolts wore off, builders came to accept holdowns as necessary. Of course building codes became more and more stringent and thus have driven more and more, larger and larger holdowns.
Engineers have no incentive to minimize holdowns. To them, more equals less liability. They don’t pay for holdowns so don’t care how much cost they’re heaping on the owner. Also, to really analyze ALL the forces at a shear panel edge takes quite a bit of time. Engineers usually bid their projects fixed fee, so if they can shave minutes here and there, but still project an image of having done the work, they make more profit. And, of course plans examiners aren’t going to make an issue of too many holdowns. They only care if there are too few.
ENGINEER EGGHEAD: (in a puffed up, biggety voice) “Well, Mr. Owner, I’m done with the structural analysis of your project. Turns out your building needs seven hundred and twelve, HD14A holdowns!”
MISTER OWNER: “Wow, Mr. Egghead, you must have really calculated up a storm.”
ENGINEER EGGHEAD: “Yes I did, sir. Nearly got calculator cramp in the process. But that’s okay, heh, heh, I’ll be fine. And don’t you worry, sir, your new building will be the last one standing when the big one hits.”
MISTER OWNER: “That’s great! Um, by the way, what’s the installed cost of an HD14A?”
ENGINEER EGGHEAD: “Er, well, you need to take that up with your contractor. Say, I’ve gotta run – gotta meet Buffy at the Lexus dealership.”
So that’s how we got to our current love affair with the holdown. Now I’ll share some tips for minimizing them.
Shear walls are those that are identified by the engineer to resist wind and seismic (lateral) loads. Actually, all walls resist lateral loads but they’re not all counted in the design. The trick is knowing which walls to count. Here is my strategy:
- First, use exterior walls only (no interior walls) up to the point where the load in them becomes relatively large, say greater than 350 pounds per lineal foot (plf). Beyond that, then start to consider interior shear walls (see next bullet point.) When shear forces exceed 350 plf, holdowns become more likely, and also 3x boundary members (at sheathing edges) are required. It’s usually cheaper to spread load around to many walls, thereby keeping exterior shear walls less than 350 plf. And, by the way, spreading load around also produces a stronger, safer building.
- Second, use interior shear walls only when they’re easy to connect at top and bottom, and the shear force in them is low. Interior shear walls can be sheathed with drywall, OSB, or plywood. If drywall, the allowable shear is in the 120 – 200 plf range; quite a bit lower than a plywood or OSB wall. Still, if there is enough length of wall to keep the shear below 200 plf, and you’re going to apply drywall anyway, and if connecting to horizontal diaphragms at top and bottom is easy, why not use the wall?
- Third, count all the dead load on a shear wall that will truly be there. Light framed walls are generally bearing walls. Which means there is a lot of load, both live and dead, directed through them. The dead load can be used to resist overturning, a.k.a. uplift. Many engineers don’t count dead load from attached perpendicular walls at corners and partitions, but they could. Many engineers also don’t count the concentrated dead load at the ends of door and window headers but they could. It becomes an accounting chore to tally loads from roofs, floors, connected walls, and the panel itself, but the loads are really there so they should be counted.
- Fourth, anchor bolts resist uplift. Many engineers don’t count anchor bolts as having any resistance to uplift. Why not? The code doesn’t preclude it. I have seen lots of photos of walls ripped off sill plates from an earthquake but the sill plate is still there, it’s scantily-spaced 1/2” anchor bolts holding it fast. As long as edge nailing to the sill plate is good, anchor bolts will resist uplift – somewhere around 750 lb. each. (Note: that 750 number depends on several things and should be calc’d. It may be higher or lower but it sure as heck is not zero.)
- Fifth, a shear panel likely has more than one anchor bolt per end holding it down. A shear panel or “pier” many times is connected to a perpendicular wall which also has an anchor bolt less than a foot away. Also, if a shear panel is long and the spacing between anchor bolts is short, more than one bolt will resist uplift.
- Sixth, floor-to-floor holdowns may not be necessary if the horizontal wall sheathing splice is in the right place. If wall sheathing is spliced at the wall bottom plate, the only connections holding the wall down, if there is net uplift, are the bottom plate nails. In this case, a floor-to-floor holdown is necessary. If, however, sheathing is spliced a couple feet above or below that, then uplift at the bottom plate is resisted by the continuous sheathing over it. Putting a floor-to-floor holdown there does nothing.
- Seventh, use the most advantageous panel / pier height in the uplift calculation. Uplift depends on the height of the shear panel – the taller the panel the greater the uplift force. The IBC allows panel height to be calculated two ways. If the ‘short’ way is used, uplift loads are less, but the code requires detailing around the opening adjacent to the panel. Interestingly, the code doesn’t say what this detailing should be. In my opinion, either a simple king stud or a header that extends 16” minimum beyond the opening suffices.
Where I live, north of Seattle, wind and seismic forces are high but not extreme, thus with a little creativity and knowledge of how buildings are really built, numbers of holdowns can be kept at a minimum. In hurricane areas and where seismic forces are very high, more holdowns will, of course, be needed. Regardless, in this era of build-green, engineers should sharpen their pencils, stop being so lazy, and produce the most efficient designs possible.
Copyright July, 2009. All rights reserved by Tim Garrison, P.E., The Builder’s Engineer™, Author of “Cracks, Sags, and Dimwits – Lessons to Build On” and “Structural Concepts for the Non-Engineer” available at http://www.ConstructionCalc.com.