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Re: Knee Brace Literature

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Dear Jeremy White,

As mentioned earlier, if knee bracings are provided to replace the
moment connections, overall deflection of the structure may increase
under the effect of lateral loads. Consider the case of beam-column
junction at an intermediate floor ( columns at top & bottom, beams at
left & right) :


************************************
Moment Connection :
************************************

At the beam column junction of an intermediate floor, end moments will
be generated at both the ends of beams (from left & right ) as well as
at ends of columns (top & bottom).

***********************************
Knee Brace :
***********************************
In case of knee brace, all knee brace members will be pinned at both
the ends and end moments of the columns and beams may be released at
the junction.

To generate the same stiffness by knee brace as that of the moment
connection, ideally, knee bracings are to be provided below the floor
as well as above the floor. However, due to functional requirements,
knee brace may be provided at the bottom of the floor only. As,
moments at the ends of the columns will be released, frame above the
floor will be hinged frame as compared to continuous frame in case of
moment connection. Hence, stiffness of the joint will be reduced in
case of knee bracings, provided only at bottom of the floor.

Hence, up to two story buildings lateral deflection will be almost
identical in both the cases. However, as the number of story
increases, stiffness of the frame will reduce for knee brace structure
as compared to moment connections, resulting in increase of
deflection.


Regards,
Bhavin Shah






On 1/18/06, Jeremy White <jwhite(--nospam--at)holbertapple.com > wrote:
>
>
>
> Bhavin,
>
>
>
> Thanks for the tips.  Currently I have the building modeled in RAM Structural.  This should give similar results as a space frame model (using special software?), correct?  With regard to item #6, I thought that the knee braced frame would have at most the same drift as the moment frame or maybe smaller if the braced and moment frames used the same beam and column sizes.  This would be because the rigid joint is larger making the effective length of the beam and column shorter.  My RAM model is giving me results that lead me to believe this is happening.  What could be a reason that the deflection would be greater?
>
>
>
> Thanks,
>
> Jeremy White
>
>
>
>
> -----Original Message-----
> From: Bhavin Shah [mailto: bhavin.design(--nospam--at)gmail.com]
> Sent: Wednesday, January 18, 2006 12:08 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Knee Brace Literature
>
>
>
> Dear Jeremy White,
>
>
>
>             To avoid moment connection at the beam-column junction, knee bracing may be provided, if space is available for the same. Some of the basic points regarding knee bracing are listed below which may be helpful to you :
>
>
>
> 1)        Knee bracings should be modeled in the Space Frame Model.
>
> 2)        Moment at the beam-column junction may be released partially or fully. Knee bracings are to be designed accordingly.
>
> 3)        Due to vertical loads, knee bracing will be under Compression. However, due to lateral loads, knee bracing may be under compression or tension, depending upon the direction of the lateral load. Hence, knee bracing and its connection with the beam-column are to be designed for Compressive load as well as for tensile loads.
>
> 4)        In the beam and column, at the junction of knee bracing, force in the knee bracing may be resolved in two components (horizontal & vertical). These components will cause bending in the beam & column. Hence, beam and column are to be designed for bending at the junction of knee bracing.
>
> 5)        As mentioned above, local concentration of force will be there at the junction of knee bracing and beam, column. Hence, local strengthening may be needed at the junction of knee bracing and beam & column.
>
> 6)        If  knee bracings are provided to replace the moment connections, overall deflection of the structure may increase under the effect of lateral loads. To decrease the deflection, partially restrained moment connection may be provided along with the knee bracings.
>
> 7)        Beam-column junction may be designed as a shear connection or partially restrained moment connection, depending upon the assumptions considered in Space Frame Model.
>
>
>
> Regards,
>
>
>
> Bhavin Shah
>
>
>
>
> On 1/16/06, Jeremy White < jwhite(--nospam--at)holbertapple.com > wrote:
>
>
> I'm working on a two-story steel office building and I'm investigating moment frames vs. knee bracing on our wind frames.  We have plenty of ceiling space so we believe knee braces may be a good alternative to moment connections.  I intuitively understand how knee bracing works, but I want to know if there are any nuances that might be overlooked.  I've checked in some of our steel books around the office and can't find anything that discusses knee bracing (just x and k bracing).  Does anyone know of a good resource for info about knee brace design and/or theory?
>
>
>
> Thanks,
>
> Jeremy White
>
>

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