half height of wall in seismic weight computations?
half height of wall in seismic weight computations?
(OP)
In walls perpendicular to the seismic direction.. do you use the full height of the wall in computing for the seismic weight or one half of height only.. and what is the basis for half height only as some do?






RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
How would half of the seismic mass ends up at the floor above the wall when the full load of the wall is supported by the beam below it? Would your reasoning be due to your assuming the beam above has rebars extended below that carried half of the wall height?
RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Kootk is talking about lateral forces, not vertical forces.
Mike McCann, PE, SE (WA)
RE: half height of wall in seismic weight computations?
It's owing to the column connected to the walls?
If say the wall is not connected to the column.. is it due to the beam above the wall?
If the wall is neither connected to the columns nor walls (by any rebars).. do you still consider half height of it?
RE: half height of wall in seismic weight computations?
RE: half height of wall in seismic weight computations?
RE: half height of wall in seismic weight computations?
Mike McCann, PE, SE (WA)
RE: half height of wall in seismic weight computations?
RE: half height of wall in seismic weight computations?
Something's going off the rails here. A non-shear wall wall running perpendicular to the applied seismic load is not normally thought to have explicit collectors etc. It might be prudent to post a sketch showing the system that you have in mind and how you're visualizing load transfer through that system.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Also in upper floor of any building that use light plastic roofing and there is no roof diaphragm.. where will the seismic load of the upper wall go? to the existing floor that holds the roof or the upper wall simply bend without contributing to the floor?
RE: half height of wall in seismic weight computations?
Unless the top storey perimeter wall system is cantilevered from the floor below, which would be very rare, there pretty much has to be a roof diaphragm of some sort. That could include:
1) Roof deck acting as a diaphragm or;
2) Horizontal trussing in the roof plane acting as the diaphragm.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Imagine a small swimming pool on top of building.. there are walls all around it but no roof and its open to sky. So in this case, the entire wall of the top floors are taken laterally by the floor? or would the wall just bend in flexure and the lateral load the same?
And oh.. how do you construct your wall? In india.. it's infill bricks.. do you usually build your wall as solid concrete? How about others here? What is the practice in your place.
RE: half height of wall in seismic weight computations?
This is essentially a cantilevered wall system and all of the wall seismic mass would be transferred to the floor diaphragm below. You'd also wind up with seismic moments being applied to the floor diaphragm as well. In my market, a screen wall like this would probably be concrete if it were to be installed on top of an otherwise concrete building. Maybe steel post and girts if aesthetic concerns were minimal.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
If a roof diaphragm would be created.. half of the wall would have lateral load transferred to the above. But in computing for seismic story shear.. you add all the forces above and the storey or floor concerned.. so you still end up with the same story shear if you distribute the forces above or focus it on the story.. What would be the effect if there is more lateral force on the story versus when the lateral force is distributed to the floor and the roof diaphragm or another floor.. eh?
RE: half height of wall in seismic weight computations?
In short building like 2 to 3 storey, would the walls flex back and forth during seismic lateral movement? Short building is very stiff.. drift is so very low.. how would the walls behave here compare to tall buildings where it can sway back and forth? If the wall is purely none load bearing like hollow blocks which you also use in Canada.. should the reinforcement be much more in upper floor of a tall building than for purely short building like 2 to 3 storey?
RE: half height of wall in seismic weight computations?
Kootk... let me elaborate in details my inquiry. Again see:
I'm talking of the walls perpendicular to the seismic lateral forces (not parallel because here I know beam action would distort the walls in the plane and I know the effects). My question is. In short building like 2 to 3 storey, would the walls (in the perpendicular side like the picture above shows) flex back and forth during seismic lateral movement? Short building is very stiff.. drift is so very low.. how would the walls behave here compare to tall buildings where it can sway back and forth? If the wall is purely none load bearing like hollow blocks which you also use in Canada.. should the reinforcement be much more in upper floor of a tall building than for purely short building like 2 to 3 storey (again in the walls perpendicular to the seismic forces)?
RE: half height of wall in seismic weight computations?
RE: half height of wall in seismic weight computations?
According to structural engineer novice I talked to. He said walls must be flexible so it shouldn't fully contribute to seismic force. So there is basis that when walls are more flexible, there is less acceleration due to some energy being dissipated in the flexure? If true, the key to designing good non bearing walls is to make sure it won't break apart (or fall over) in a seismic movement and yet the reinforcement not so rigid to encourage flexure? is this a right attitude and do you think of it when you design your hollow block walls in Canada?
RE: half height of wall in seismic weight computations?
There's some merit to this logically. If your wall was a bowling ball suspended from rubber bands above and below it probably wouldn't draw any seismic load at all. As a practical design strategy, however, I feel that this "flexible wall" business is fundamentally flawed. No common wall assembly would be anywhere Near flexible enough for this strategy to bear fruit. In essence, all walls accelerate the same amount as the floors they're attached to.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
You make sense as always. Anyway. We have a unique construction problem. We made a one meter wall (of about 7 inches thickness).. but 2 years ago the rebars protruding outside at top is cut (to make way for waterproofing net). Now any idea how to add new walls above it (about 2 meters)? One idea is to chisel the wall and bring out the rebars (which is distance 0.6 meter apart horizontal).. but splicing it would be a problem because of not enough development length.. mechanical couplers would be expensive and we fear chiseling the wall may weaken the bars. Have you heard or seen any angle bars or metal plates being bolted to the walls below and the rebars (10mm) welded to it (or angle bars used instead as the vertical reinforcement?) to add new wall? This is actually suggested to me instead of chiseling. Is this a common scenario or a rare one?
RE: half height of wall in seismic weight computations?
That's very kind of you to say. I should have you talk to my kids.
Extending existing concrete is a common problem. Depending on available funds and labor, there are a number options (you've already mentioned most):
1) Chip and splice with mechanical couplers.
2) Chip and splice with bar to bar welds.
3) Chip and splice bar to plate to bar with welds.
4) External steel plating as you've suggested.
5) Sister a new wall beside the existing wall and connect the two.
6) Drill and epoxy new dowels into place and either rely on concrete in tension breakout or effectively lap the new bars with the old.
7) Demo the existing wall and start anew.
As a first step, I'd evaluate the existing wall/slab joint for the new wind and seismic moments. A 7", 3m high cantilever wall with one curtain of light rebar might be a problem if you're in a high seismic or hurricane prone region.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
In Canada.. what kind of hollow block walls do you use? How many inches is the thickness of the block and the total finished surfaces? And what is the diameter of the vertical reinforcement you use (connecting to upper and lower beam) and what's the spacing? Just curious how it compares to ours? All of us here use spacing of 600 mm horizontally and 10mm bars connecting to upper and lower beam.. and also 600mm spacing and 10mm connecting column to column. Imagine a net of 10mm bars with spacing of 0.6 meter vertically and horizontally. Distance between stiffener column are 3 meters. How about yours?
RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
What do you mean by "15M" above?
Is your Hollow Block in Canada like the following?
If not.. any site with sample of how Hollow block look like in Canada? What is it made of? Just curious. Thanks. :)
RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Ok. Is the following also how your hollow blocks are layered together?
Do you know what formula is the basis for the spacing of the vertical reinforcement? I couldn't find it in ACI. How do you compute for it? It's not like slabs where you have formulas. I want to know the flexural basis for spacing it at 0.6 meter O.C. using 10mm.. it's practically used by everyone here. I wonder what is the computation justification for it and how it performs under acceleration.. do you have formulas in your Canadian code or elsewhere for the reinforcement spacing (any references)?
RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Kootk.. what are you experiences with precast walls.. this seems to be easier to install than typical hollow blocks.. the disadvantage in the latter is you have to apply the mortar finishes outside the building.. then polish it.. and paint it, waterproof it outside which can be expensive... imagine workers hanging by ropes outside.. so precast walls with smooth outside would be a big factor. I learnt PIR insulated walls can burn and has flashover and only last 30 minutes.. while concrete precast walls has 2 hour fire resistance and stronger.. which one have you actually work on.. are there kinds of them that are good in resisting acceleration during seismic movement? See precast for example in
http://ibuildph.com/cms/index.php/ibuildph/fastwal...
RE: half height of wall in seismic weight computations?
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Last question for this thread.. just wanna follow up what you mentioned above earlier. Do you mean by *seismic moments* applied to the floor diaphragm from the cantilevered wall from *flexure* of the floor? But all floor diaphragms have flexure (seismic moments?) too since the walls lateral movement of each storey (1/2 above and below) goes to the diaphragm.. Unless you mean the extra mass of the cantilevered wall would produce extra flexure or seismic moments in the floor diaphragm?
Many thanks for all the enlightening answers. You would make a very good professor (do you still practice or retired already?) :)
RE: half height of wall in seismic weight computations?
A wall that spans from a diaphragm above to a diaphragm below can be, and usually is, modelled as pinned top and bottom for the purpose of diaphragm design. A cantilevered wall, on the other hand, must generally deliver its base moment as:
1) torsion in the supporting beam,
2) flexure in the slab below or;
3) flexure in the wall below if such a wall is present.
That's all that I was getting at with my previous comment.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: half height of wall in seismic weight computations?
Just to clarify.. you mentioned the following facts earlier:
"Recognize, however, that the wall load delivered to the columns still eventually ends up in the diaphragms that laterally restrain those columns."
"The roof diaphragm would be for the benefit of the walls rather than the lateral force resisting system."
But a roof diaphragm would prevent the diaphragm below from getting more stressed (torsion, flexure, moments from cantilever column & wall).. therefore roof diaphragm would be also for the benefit the lateral force resisting system? Aint it?
RE: half height of wall in seismic weight computations?
You bet. In continuous concrete construction, everything affects everything else. The primary benefit is still the difference between a cantilevered wall and one supported at both the top and bottom of course.
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.