A thought on infilled frames 2

[Bannu]

I have a question lingering in my mind for long. The traditionally concrete frame structures are designed and constructed to resist gravity and lateral loads by only framing action.

After the frame is finished we fill the frame with brick walls (India). To my mind these walls neighter help to resist gravity loads nor lateral loads. Beacause of heavy weight of this wall the beam-column junction may damage in the event of an earthquake.

Is there any way to make sure that these infilled walls act as shear walls with proper connection to the frame? This is possible with CMU but we dont use CMU here?

Any thoughts on this topic are welcome.

Thanks

 

[ishvaaag]

Really the infill of entire panels with masonries, ore evn indirectly through masonries outside the panel points through bond to floors distroys the initial assumption of the frames acting against lateral loads as moment resisting frames.

Most of the bonded infilled levels act as a rigid body whilst integrity allows.

This conflict between the assumptions in design and factual properties of the structure is of course cause of damages for structures subject to earthquakes where no proper consideration of this and other earthquake related issues is being made, or even where if so the earthquake exceeds what by the code expected. The more common failure is the formation of a soft story at the low level, that kneels down under the rigid body push of the levels above.

This kind of construction is also common in Spain.

However modern codes include to the level of the expected earthquake some care for this, in that the lateral displacements for the building are enforced be limited to some amount compatible with accepted limits of damage. To this purpose -and evaluation and prevention of the possibility of pounding- is that maximum drifts are fixed for the earthquake loadcases.

To prevent substantial damage under the earthquake force in buildings with attached masonries, maximums distortions in each panel are set for the earthquake loadcases at about 1/400 (horizontal displacement/interstory height) at any level, and so in the wole height of the building as well, and with a proper assesment of the lateral displacements, that will include, if with the same intially derived from forces reduced on account of extant ductility some amplification of the displacements gained from such reduced forces.

Of course you may enter also the effects of the masonries by direct modelization, then checking the whole for strength and serviceability.

Other concern is infill at partial height that uses to cause som problems in low buildings by unaccountedly kicking in the column.

 

[100islands]

Bannu

I have personally seen structures (designed as frame without the infill walls) survived earthquakes here in my country, Philippines, and in Taiwan. They survived (i.e., did not collapse) because the building's infill walls are arranged almost symmetrically, there is no soft story, and the frames were detailed properly to fail in ductile manner.

 

[IJR]

My understanding is that

1) there will be a lot of lateral stiffness initially and codes will punish your column detailing if you have a brick wall and a short column, to account for this.

2)This stiffness dies out quickly and frame action moves in, and in addition the building softens and there will be larger period and less earthquake forces.

3)Whatever you do, failures occur more because of short columns and soft story than they do from stiff walls.

4) why trust brittle bricks to resist earthquakes, while ductile concrete frames are known to have good seismic response? Anything brittle should not be encouraged in seismic resistant systems.

my two cents worth.

IJR

 

[Bannu]

IJR,

You are right, but that was not my question. My concern is that the ductile frame damage (Atleast at corners) by the brittle brick walls.

As ishvaag said we can limit the lateral drift so that the infill wont effct much on frame damage, but that would boost up my member sizes.

 

[IJR]

Thanks Bannu

Yours is a useful post and should be enhanced for as much as our knowledge and experience can let go, not necessarily a direct answer to your question. In this forum I believe in discussion rather than answering a question.

Designs utilizing wall stiffness may not be close to reality, walls are live loads, they may be there today and not tomorrow.

Some book I read suggested use of soft material between columns and walls (a gap of some 10cm and the like) to let frames sway freely, without feeling the stiffness of walls.

I personally have never employed this gap method and would like my more experienced friends here to comment on that.

Two cents worth,
IJR

 

[Bannu]

Thanks, but I too have never seen this gap method employed in field.

Looking forward for our fellow engineers response on this.

Bannu