half height of wall in seismic weight computations? 1

[hocho]

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?

 

[KootK]

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.

 

[hocho]

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?

 

[KootK]

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.

 

[hocho]

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?

 

[KootK]

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.

 

[hocho]

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.

 

[KootK]

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.

 

[hocho]

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)?

 

[KootK]

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.

 

[hocho]

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/fastwall-lightweight-concrete-panels/

 

[KootK]

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.

 

[hocho]

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?)

 

[KootK]

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.

 

[hocho]

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?

 

[KootK]

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.