shear wall constructed later. 1

[zaes73]

A two story building existing on ground for more than 11 years was working fine until seismic activity produced some serious cracks in columns. When this buiding was analyzed for gravity loads it was just fine, but in case of Seismic load (zone 3) the columns were failing in shear. So we decided that we would incorporate shearwalls in the building to resist lateral loads. Now when shear walls are incorporated in the computer model of this building, the structure proves to be safe against lateral loads, so we decided to make shear walls in the building, but! one of our engineer insists that non-monolithic construction of these shear walls now with already existing columns is useless! and the shear wall would just be a vertical plate and would practically not resist the lateral loads. ? is it true ? what would be a proper way to now make these shear walls.

 

[apsix]

The walls are a lot stiffer than the columns against horizontal forces, and therefore should attract a greater proportion of the force as I see it. Obviously there must be a positive connection along the bottom and top of the walls eg. by dowelling to the existing concrete.

 

[csd72]

zaes73,

I dont see what the problem is as long as you use non-shrink grout to the underside of the beam.

It would beahave more like a brick infill shear wall (see page 7 of the following reference).

http://www.eng.upm.edu.my/hrc/pc/handouts/precast frame - 6 stability.pdf

Of course supporting the beam with this wall would result in a redistribution of moments from bvertical loads that needs to be checked.

csd

 

[whyun]

The means of transmitting the seismic force to the added shear wall must be verified to be adequate. If not, the weakest link in the load path will fail before the shear wall "sees" the force it is designed for.

 

[msquared48]

Being in a Seismic Zone 3 (1994 UBC probably the code of record), was this originally a reinforced concrete frame structure with no shear walls?

What happened that the seismic forces were not applied to the model when it was originally designed? Something doesn't seem right here.

Mike McCann
McCann Engineering

 

[csd72]

whyun,

I am not a seismic engineer, but...

If you infill between two columns then the frame cannot sway without exerting force on the shear wall. That is the means by which the force is exerted, no other anchorage required to make this happen(except that to stop it falling sideways).

Refer my reference.

csd

 

[msquared48]

csd72:

Your statement that "no other anchorage required to make this happen" is just not true. Remember that this is a SHEAR wall.

There must be a shear transfer from the building to the wall and vice versa to make the mechanics of the wall work. These shears transfer also into overturning forces to be resisted by the existing columns. Confinement and the use of friction to transfer the shear is just not good enough here. There must be a dedicated and designed physical connection.



Mike McCann
McCann Engineering

 

[whyun]

When you add any additional lateral force resisting system (LFRS) to an existing structure that already has existing LFRS's as laid out by the original engineer or record, the design seismic force is re-distributed some taken by the new LFRS and the remainder by the existing LFRS. If the new work triggers the seismic force to be per current code, you could have a higher design force than the original design.

In the op's situation, where new shear walls are introduced, diaphragm, collectors and chords shall be checked using the new layout in addition to shear walls' top and bottom connections.

 

[csd72]

msquared48,

You need to read the reference that I have given above. Infill shear walls do not rely on friction of any kind, They basically have diagonal compression struts with vertical and horizontal reactions into the beam and column (respectively).

the shear load on the column and beam needs to be checked though.

Cross bracing does not work in shear, but it still does the same job as a shear wall.

whyun,

You made some good points with that last post.

csd

 

[msquared48]

csd72:

Your link will not load up for me.

However, I can see the logic of the application here in that it is not a true shear wall, but merely a mass placement infill to allow the diagonal compression struts to form as the seismic or lateral event proceeds. I assume that diagonal "column" or strut reinforcing with ties is utilized within the wall structure?

Mike McCann
McCann Engineering

 

[zaes73]

Mike McCann

this building was build in Islamabad, Pakistan a city which had never seen much seismic activity until october 08th 2005 an earhtquake shook this city and killed 88,000 people in this region; it was a huge disaster. and after this earthquake my consultancy firm based in dubai did a lot of work here in pakistan. This specific building was designed orignaly for some mild zone as per UBC 97, i did a litle research here through the documents and some simple design calculations but could not figure out the seismic zone it has been designed for but looking at the amount of reinforcement and the column splices etc. and lack of detailing according to seismic provision i think the building is for zone 1 ( keep in mind Pakistan does not have its own design code, norm or manual of any type), however now the "pakistan engineering council" has declared this city in seismic zone 3 as per UBC 97

csd72 thanks for the publication! it simply makes sense

 

[zaes73]

how about i used a Wide flange section ! and anchor it using expansion or epoxy anchor bolts to the concrete structure and brace the bay like i would brace a steel structure ? that would act as strut and tie bracing ! right ?
after reading this article " design of precast concrete skeletal structures" provided by "csd72" i feel that if a preacast wall can be used for bracing then why cant we use a steel beam ?

 

[csd72]

zaes,

yes there are probably a few options for this. A single strut though would require anchors that were adequate for tension in cracked concrete.

I would suggest you google 'seismic retrofit' for some other options.

csd