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Last floor column design
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sriprakash_shastry
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Joined: 23 Mar 2010
Posts: 50

PostPosted: Sun Dec 02, 2012 2:16 pm    Post subject: Last floor column design Reply with quote

Dear Dr. Murthy,


We also notice in some cases in models without shear walls, that reinforcement in beams is higher in the lower floors and reduces in the upper floors. This happens also when there is no change in column sizes in the upper floors. What could this be due to. Kindly enlighten us.


Warm Regards
Sriprakash

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On 02-Dec-2012, at 6:47 PM, "cvrm" <forum@sefindia.org (forum@sefindia.org)> wrote:


Quote:
  UPPER STOREY COLUMN DESIGN

The topic of upper storey columns is recieving significant attention
during the eC. Adequate caution has been exercised by some of the
participants. Many factors can contribute to the stress resultant
demands (i.e., P, V and M) and hence the design of the upper storey
columns; this is experienced by designers as column reinforcement
being much different from those in the lower storey columns. These
factors include:

1. Column size changes::
When the column size is reduced in an upper storey, the
cross-sectional area and moment of inertia change suddently. This
increases the percentage of steel, and even the absolute amount of
steel. It could have been lesser, if the column size in the lower
storey was continued in the upper storey also.

2. Axial versus Bending Effects::
Columns in the storeys close to the bottom of the building undergo
lesser bending, but sustain higher axial load. As one goes up along
the height of the building, bending effects increase upto a certain
height and eventually reduce; the axial load reduces to the top of the
building. Thus, in the upper storeys, the design point lies in the
lower part of the P-M interaction diagram, and hence upper storey
columns are expected to undergo ductile actions. But, in the lower
storeys, the design point lies in the upper part of the P-M
interaction diagram, and hence the lower storey columns behave in the
brittle way. Columns in the intermediate storeys, have both P and M to
be reasonably high, and hence their behvaiour is mixed.

3. Higher modes effects::
Tall buildings have high fundamental T, and their higher mode T is
still high. Design earthquake spectra carry large energy corresponding
to T of higher modes rather than the fundamental modes. Hence, in
modal analysis, the share of higher modes tends to be larger. This
effectively results in the columns of middle storeys receiving higher
moment and axial demand, and the upper storey columns lesser.

4. Reduced gravity load ::
By their mere position, the upper storey columns sustain lesser
gravity load and hence the P-M interaction is diminishing to the top.

5. H/D of the building::
When the aspect ratio of the building is small, the building deforms
laterally in shear-type cantilever mode; here, the upper storey
columns do not see much inter-storey drift. The situation is the
opposite when the building is slender; the building deforms laterally
in flexure-type cantilever mode, and here the upper storey columns
undergo larger inter-storey drift. More interstorey drift implies more
bending of the upper storey columns.

6. Whip-lash effect::
Some buildings will see whip-lash effect at the upper storeys during
earthquake shaking, that has large pulses (as in near field ground
motion) and sudden reduction in column size in upper storeys. In such
cases, the upper storey columns may experience more effects. Some
codes reflect this through an additional concentrated design lateral
force at the top storey. But, this is best noticed when time history
analyses are performed.


In a tall building, some or all of the above factors may contribute to
the structural behaviour of upper storey columns, depending on the
structural configuration of the tall building and choice of locations
of column size reduction in the design process. It would be difficult
to ascribe reasons for reduced/increased longitudinal steel in upper
storey columns to just one of these factors.



C.V.R.Murty








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shahnawaz.sdk
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Joined: 24 Jan 2010
Posts: 2
Location: Thane

PostPosted: Sun Dec 02, 2012 3:02 pm    Post subject: Last floor column design Reply with quote

Hi


Even I have observed in most of the building up to 20 floors that the reinforcement requirements in beams increase from first floors to mid storey and then again reduces as we go to the last floors. Same happen with columns also. Expect columns at upper floor requires more steel then the second last floors.


I appreciate if somebody put some light on this phenomena.

Regards


Shahnawaz Siddiqui
Integrated Buildings


On 02-Dec-2012, at 20:17, "sriprakash_shastry" <forum@sefindia.org (forum@sefindia.org)> wrote:


[quote]            Dear Dr. Murthy,


We also notice in some cases in models without shear walls, that reinforcement in beams is higher in the lower floors and reduces in the upper floors. This happens also when there is no change in column sizes in the upper floors. What could this be due to. Kindly enlighten us.


Warm Regards
Sriprakash

Sent from my iPad

On 02-Dec-2012, at 6:47 PM, "cvrm" forum@sefindia.org (forum@sefindia.org))> wrote:


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