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half height of wall in seismic weight computations?

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?

You use the full height of the wall for the seismic mass and apply that at mid height of the wall which means that, by statics, half of the seismic mass ends up at the floor above the wall and half of the seismic mass ends up at the floor below the wall.

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?

(OP)

Quote:

You use the full height of the wall for the seismic mass and apply that at mid height of the wall which means that, by statics, half of the seismic mass ends up at the floor above the wall and half of the seismic mass ends up at the floor below the wall.

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?

The full gravity is carried by the beam below. Seismic load, like wind load, is laterally oriented and therefore distributed to the high and low floor plates via wall flexure.

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?

Hocho:

Kootk is talking about lateral forces, not vertical forces.

Mike McCann, PE, SE (WA)


RE: half height of wall in seismic weight computations?

(OP)

Quote:

The full gravity is carried by the beam below. Seismic load, like wind load, is laterally oriented and therefore distributed to the high and low floor plates via wall flexure.

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?

Where did the column come into it? I'm very confused now.

RE: half height of wall in seismic weight computations?

(OP)
How else would wall flexure be distributed to the high and low floor plates (as Kootk put it) without any columns? Just would like to confirm if column is the collector itself. Diaphragms are connected continuously as in floor slabs.. but the walls are below and above the diaphragms.

RE: half height of wall in seismic weight computations?

I too am confused about the column issue here as it should be a non issue. Usually walls span vertically floor to floor because that is the shorter span direction and the strength direction of ant wall reinforcement, be it studs, rebar or otherwise. Hence, the distribution of any lateral force to the floor diaphragm structure, not the columns.

Mike McCann, PE, SE (WA)


RE: half height of wall in seismic weight computations?

Hocho, are you referring to infill walls that only connect at the bottom and maybe at columns? Those would be different (and not recommended in seismic areas).

RE: half height of wall in seismic weight computations?

Quote (hocho)

Just would like to confirm if column is the collector itself.

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?

(OP)
I got it. You connects walls up and down. But I also connect column to column and up to down too.. so thinking the columns have contributions too. I use hollow infills (but filled with concrete as paste) connecting with rebars extending horizontally to columns and vertically to top and bottom beams for strength. In this case. It's not half of wall taken by beam above during lateral oriented force movement. But perhaps like 2 way slabs where the column also has share to get the load of the wall? But still it's the same seismic storey force.. the wall load are connected to beam and columns.

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?

Yeah, I think that we're on the same page now. Where columns are involved, it would be akin to a two way slab with respect to wall flexural design. As you mentioned, however, the same loads ultimately make it to the floor diaphragms above and below whether it's via the walls or the columns.

Quote (hocho)

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?

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?

(OP)

Quote:

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.

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?

Quote (OP)

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?

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?

(OP)

Quote:

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.

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?

(OP)
Kootk,

Quote:

The full gravity is carried by the beam below. Seismic load, like wind load, is laterally oriented and therefore distributed to the high and low floor plates via wall flexure.

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?

(OP)

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?

When it comes to the seismic demand on the walls, the demand is a function of acceleration, not displacement. As such, all else being equal, I would expect the seismic demand on a wall at the top floor of a three story building to be greater than the seismic demand on a wall at the top of a 90 story building. Of course, there are all kinds of exceptions and things get murky when considering medium height buildings. The only reliable answer is "it depends on the anticipated floor accelerations generated by your structural analysis effort."

RE: half height of wall in seismic weight computations?

(OP)

Quote:

When it comes to the seismic demand on the walls, the demand is a function of acceleration, not displacement. As such, all else being equal, I would expect the seismic demand on a wall at the top floor of a three story building to be greater than the seismic demand on a wall at the top of a 90 story building. Of course, there are all kinds of exceptions and things get murky when considering medium height buildings. The only reliable answer is "it depends on the anticipated floor accelerations generated by your structural analysis effort."

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?

Quote (Hocho)

So there is basis that when walls are more flexible, there is less acceleration due to some energy being dissipated in the flexure?

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?

(OP)

Quote:

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.

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?

Quote (hocho)

You make sense as always.

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?

(OP)

Quote:

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.

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?

The most common block wall here seems to be 8" with 15M at 48" o/c. Each wall is custom designed to suit it's span and loading, however, so there is considerable variation.

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?

(OP)

Quote:

The most common block wall here seems to be 8" with 15M at 48" o/c. Each wall is custom designed to suit it's span and loading, however, so there is considerable variation.

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?

15M is metric rebar about 16 mm in diameter. Our block looks like yours but is generally fabricated in 16 in modules.

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?

(OP)

Quote:

15M is metric rebar about 16 mm in diameter. Our block looks like yours but is generally fabricated in 16 in modules.

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?

That's exactly how most of our block walls go together. ACI 530 gives formulae for strength design of masonry. It is very similar in concept to concrete design.

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?

(OP)

Quote:

That's exactly how most of our block walls go together. ACI 530 gives formulae for strength design of masonry. It is very similar in concept to concrete design.

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?

Precast walls are structurally the same as cast in place walls. They can have benefits with respect to economy, durability, and aesthetics when employed judiciously. As with the cast in place wall, the connection to the existing structure will be the critical bit.

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?

(OP)

Quote:

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.

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?

I'm still practicing Hocho. All day, every day.

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?

(OP)

Quote:

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.

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?

Quote (hocho)

therefore roof diaphragm would be also for the benefit the lateral force resisting system?

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.

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