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shearwall-multiple hold downs

shearwall-multiple hold downs

(OP)
hi,
I'm trying to spread uplift (5kips) with 2 hold downs per side (this is just an example).
Is there a simple way to determine what would be an uplift if I use 2 hold downs?
thanks alex

RE: shearwall-multiple hold downs

IMO the uplift is approximately the same in each hold down

I would be careful to check rebar in stem wall, and make sure that uplift is resolved all the way to footing.

RE: shearwall-multiple hold downs

You meant 2 hd per end - not side, right?

Each hd gets 2.5 K

RE: shearwall-multiple hold downs

Is that an FEA printout? Throw it in the trash. Use common sense.

RE: shearwall-multiple hold downs

If there was one hd per end, no dead load resisting,

Uplift = lateral shear x wall ht divided by wall length.

Are you a degreed engineer?

RE: shearwall-multiple hold downs

Why would you want to spread the uplift?

I would be concerned with not utilizing the full capacity of either hold down if they are placed too close together. And if they are spread further apart, some of the leverage is lost.

As for the difference in the results, perhaps Josh Plum could comment on that. You might reconsider posting this in the RISA Technologies forum if you do not get the answer you seek. Josh is in there more and is the resident expert. You would have to red flag this string first before doing that though.

Mike McCann
MMC Engineering

RE: shearwall-multiple hold downs

I know RISA is an excellent software...I have said so before.

IMO it is being mis-applied for a simple shear wall.

RE: shearwall-multiple hold downs

I use multiple hd all the time. I don't worry about the theoretical loss of leverage.

Look at what happens when you have HDU in a short shear wall, now that is major loss, not in an 8' wall such as the OP>

RE: shearwall-multiple hold downs

I don't trust the numbers shown on that print out very much. If there were to be a difference between the two hold downs, I would expect the outer hold down to take the greater load. Sigma = Mc/I after all....

It's always a little problematic putting in two perfectly rigid supports so close to each other. Therefore, you might either do a hand calc (like AELLC suggests), or replace the perfectly rigid reaction with a more flexible spring support.

The real behavior of a shear wall is actually a lot different than that model. You have continuous shear resistance along the length of the wall. You have continuous compressive vertical resistance along the wall. Heck, you probably have something close to contiunous tensile resistance as well. The NDS stiffness and deflection equations assume this type of continous resistance as well....we just don't rely on it for design purposes. Instead, we create a pretend force couple in the chords to resist the total moment. At least that's the current "standard of care" in the industry. If you do something else (however rational) then you probably want it to be more conservative than what the hand calc method would give.

RE: shearwall-multiple hold downs

Interestingly, the results are on the left are similar to (½ total shear)(height)/(distance between holddowns).
½(10000 lbs)(10)/10=5000 lbs
½(10000 lbs)(10)/8=6250 lbs
What are the horizontal reaction?

If the holddowns are on the same chord member, taking 1/2 the load to each holddown is reasonable. If each holddown is on a separate chord, more analysis may be warranted. I would be interested to see the result if the tension holddowns are modeled as springs. For a spring constant, I would start with k= hold down capacity/ holddown slip.

RE: shearwall-multiple hold downs

at times I think we are being generally over-conservative on the hold down design. When you've got standard anchor bolts holding your bottom plate to the stem wall at some regular spacing you have more of a continuous tensile resistance, and definitely continuous compressive resistance.

RE: shearwall-multiple hold downs

That is true but the plans checkers and the lawyers don't see it that way.

RE: shearwall-multiple hold downs

It is too much of a gray calc I think- hard to prove rationally and exactly.

RE: shearwall-multiple hold downs

(OP)
thanks for comments and feedback everyone
I have about 11 kips uplift in 3 story condo shearwall. Capacity of 7/8" hilti HY200 in 8" conc wall about 6kips
that's the reason I'm trying to have 2 hold downs on separate chords(16" apart).

RE: shearwall-multiple hold downs

won't the failure cones overlap with only 16" difference in location. especially if you're going deep enough to activate 6kips?

RE: shearwall-multiple hold downs

Don't forget to specify special nailing to the interior stud - the one 16" inboard. Otherwise it will get only 12"oc field, and that will get you into big trouble.

RE: shearwall-multiple hold downs

failure cones - yes. Design special rebar

RE: shearwall-multiple hold downs

Many times the loss of leverage is pretty small compared to what you gain in uplift capacity. If you had a second holdown 16" away your moment arm changes by 8" but you double your holdown resistance. What I have found is like others have said, you have to be careful that you pay attention to what is happening at the foundation.

RE: shearwall-multiple hold downs

There is a difference - Just look at the solution of a moment about the out of plane axis. In practice this is frequently discarded. Meet the minimum separating distance and ensure you can actually engage sufficient dead load.

RE: shearwall-multiple hold downs

11 kips of uplift? Heck, that's easy. Simpson makes hold-downs that go quite a bit beyond that and you can specify the PAB anchor bolts that go directly into the footing. Not terribly familiar with Hilti products.

Are you hooking into a typical spread footing or turn down slab?

RE: shearwall-multiple hold downs

Also, if you are trying to reduce the hold-down size, try a perforated shear wall and use more wall.

RE: shearwall-multiple hold downs

Or, put a continuous glulam beam over the two shear walls, and use hold downs only at the top and bottom of the end columns under the glulam. Hold downs will not be needed anywhere else.

Mike McCann
MMC Engineering

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