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Re: 8" Columns

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Dear Gerard,
 
    I agree with your reply but can one not use a smaller diameter rebar say 3/8" for main and 1/4" for ties but a higher fy say 60 or even 75 ksi although a composite section would probably be more economical. I am asking this because I have considered using pre-cast concrete structural members for residential houses.
 
   Thanks for your reply.
 
Alex
 
  
 
  
----- Original Message -----
Sent: Saturday, October 06, 2007 2:21 PM
Subject: Re: 8" Columns

But it's higher E doesn't offset the geometric reductions in I, S, Z, r, J, etc...and would not be as strong as a 3000psi 12"x12" for most failure modes except possible pure axial compression (depending on the unbraced length) and shear.

f`c has very little to do with capacity for flexure or beam-column elements when even a slight eccentric axial load is applied. Slenderness governs most capacity checks in column design.

Why not use a TS filled or encased in concrete instead?

Also, but the time you radius the ties and clear cover requirements, the bars are so close together your d is virtually nothing.

-g

On 10/5/07, Alex C. Nacionales <anacionales(--nospam--at)insightts.net> wrote:
David,

    8"x8" RC columns with Fc'=10000psi has more
compressive strength than a 3000 psi 12"x12" column.
The 8" column would be more expensive i am sure but Architects sometimes
limits dimesions for aesthetic reasons.

Alex C. Nacionales


----- Original Message -----
From: "David Fisher" <dfisher(--nospam--at)fpse.com>
To: < seaint(--nospam--at)seaint.org>
Sent: Saturday, October 06, 2007 1:22 AM
Subject: RE: 8" Columns


> 8" - 2*1.5" (cover) - 2* 0.375" (ties) = 4 1/4"
>
> Not much section left!
>
> I've seen 8" sonotubes under a porch, but thats it.
>
>
>
> David L. Fisher SE PE
> Senior Director
>
> Cape Cod Grand Cayman Holdings Ltd. - Cayman
> Fisher+Partners Structural Engineers Ltd. - Cayman
> 372 West Ontario Chicago 60610
> 75 Fort Street Georgetown Grand Cayman BWI
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>
> -----Original Message-----
> From: refugio rochin [mailto: fugeeo(--nospam--at)gmail.com]
> Sent: Friday, October 05, 2007 12:21 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: 8" Columns
>
> That is not true.  ACI 21.3.1.4 allows for a width of 10" for Flexural
> members of special moment frames.
> ACI 21.11 for Frame Members not proportioned to resist forces induced
> by earthquake motions, does not place any restrictions on the minimum
> dimensions.
>
> I am mainly wondering how most people approach the design of such a
> member that has to accept seismic displacements?  Is there any
> research in the realm of 8" concrete members undergoing design
> displacements, and their strength capacities?  I do not want to design
> such a member and have it bust due to seismic loads and lose vertical
> strength capacity...
>
> -RR
>
> 2007/10/5, Gerard Madden, SE <gmse4603(--nospam--at)gmail.com>:
>> 12" is the minimum dimension for a concrete structural member other than
>> walls and slabs.
>>
>> -g
>>
>>
>> On 10/5/07, refugio rochin < fugeeo(--nospam--at)gmail.com> wrote:
>> >
>> > I think I should explain a bit more...  8" Concrete Column.  The
>> > column should be able to take the lateral deformation imposed by the
>> > overall structure though, which is generally attached to a rigid
>> > diaphragm ...
>> >
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>> --
>> -gm
>
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-gm