HOLD DOWNS
Supposing you're at Thanksgiving this year with your entire large family and everyone is having such a great time… until you switch on the evening news and learn of an impending thunderstorm.
Everyone is scared and tense, especially after electricity is cut out in the wake of the thunderstorm. The wind is banging every window, rattling the door hinges, setting everyone on edge. Then, all of a sudden, one of the walls in the house starts to overturn!
Alright, let's cut out the fearsome imagery now, but I hope you get the point now, right? To avoid such unfortunate circumstances, hold downs are the knight-in-shining-armour coming to the rescue of your walls.
What Are Holdowns?
Simpson Strong-tie HTT Holddown
Holdowns are key components in a continuous load path. They help prevent uplifts that could result when a shear wall overturns. Holdowns, or hold-downs, are steel devices installed at the base of a shear wall in a wooden structure.
Why Are Holdowns Necessary? (Uses Of Holdowns)
When a lateral load(in plain language, just imagine pushing something sideways, yeah that’s basically a lateral load you’re applying) is applied at the top of the wall in a timber structure and the force reaches a certain amount, then the overturning moment will match the resisting moment, such that any other additional force applied, no matter how little, would cause the wall to topple.
This is exactly what would have caused the kind of scenario we created at the introduction of this article. To prevent such disaster, hold downs need to be installed to hold down the walls to the structural elements below. Their purpose is to prevent the end of s shear wall from lifting off the foundation and toppling over.
Holdowns are also required even where the over turning moment does not equal the resisting moment. They could also refer to a clamping device used to fasten a pipe to a structural steel element or a concrete floor. It could also be useful in aiding movement of the pipe in an axial direction.
In roofing and roof construction, holdowns are useful for anchoring concrete or masoning walls to the roof framing.
Categories Of Holdowns
There are two major categories holdowns could be grouped into. The first is cast-in-place while the other is post-installed.
Cast-in-place or strap holdowns are those holdowns that look like straps that emerge from the foundation and nail to the edge or face of a stud. They are installed during the placement of concrete by using nails to attach them to wood framing or by using screws to attach them to CFS framing.
On the other hand, post-installed or anchor bolt holdowns connect foundation bolts to threaded rods to the studs with the aid of bolts or nails. Such holdowns are attached to anchor bolts after the placement of concrete. The holdown is connected to the concrete foundation or structural slab by an embedded anchor bolt.
More about Holdowns
Prescriptive designs usually require a cast-in-place or strap holdown whereas for engineered designs, taking a look at design loads could provide more clarity and eliminate certain choices of holdowns.
For instance, for cast-in-place holdowns, the maximum load it can bear is 5,300 pounds. For nailed holdowns, the maximum load is 5,090 pounds; for SDS screws, 14,445 pound, and for bolted holdowns, 19,070 pounds.
When selecting holdowns, versatility should be considered. Embedded strap holdowns are economical uplift solutions but they should be accurately located so as to properly align with the framing of the wood.
Where the anchor bolt is not properly located for a post-installed holdown, you could simply raise the holdown up the post and eliminate many problems.
Where there are missing anchor bolts, anchors could be glued in place of them.
Where holdowns are installed on opposite sides of the wood member, tabulated loads for bolted holdowns could be doubled, however the capacity of the wood member and the anchorage should first be evaluated.
For nailed or screwed holdowns, tabulated loads could also be doubled when holdowns are installed on opposite sides of the wood member. The wood member, however, should be thick enough to prevent interference from the opposing holdown fastener, else the holdowns should be offset to eliminate fastener interference. Again the capacity of the wood member and the anchorage should be evaluated.
Elements Required For Holdowns To Function Properly
Even if you have chosen the perfect holdown for the construction you're working on, there are certain things you must ensure you do not miss while installing holdowns.
The anchor bolt must be of the correct size, else it won’t function properly.
The holdown too must also be of the correct size if it must function correctly.
The holdown must be correctly attached to the end stud with sufficient nailing or bolting.
The nut as well must be properly tightened on the anchor bolt.
The strap or bolt must be correctly embedded into the foundation.
The bolt holes should be drilled to the proper size.
The holdown should be properly positioned at the end of the wall such that it is adjacent to the end post.
The holdown should also be positioned at the right distance from the edge of the foundation.
Nailed or screwed holdowns should be installed without fasteners interfering with each other.
Once you select the right holdown for your design, you should also select the correct anchor for the demand loads.
Got a wooden project you need professionals for? Let's help you get started on it! You can speak with our professionals at JPC Design Consortium to get you started on your project today. We also give consultation services just in case you need some more information for your project. Contact us right away.