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]

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.

 

[hocho]

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?

 

[KootK]

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.

 

[msquared48]

Hocho:

Kootk is talking about lateral forces, not vertical forces.

Mike McCann, PE, SE (WA)

 

[hocho]

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?

 

[Trenno]

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

 

[hocho]

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.

 

[msquared48]

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)

 

[Gumpmaster]

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).

 

[KootK]

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.

 

[hocho]

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?

 

[KootK]

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.

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.

 

[hocho]

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.

 

[KootK]

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.

 

[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.

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?

 

[hocho]

Kootk,

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?

 

[hocho]

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)?

 

[KootK]

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."

 

[hocho]

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?