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Re: Braced Steel Frame System Question

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To Efren Allan Yango,

	I have used flat bars as bracing with mixed results.  If you do decide
to use them keep the following in mind.

1.)	They do not meet the requirement for L/r ratios < 300.  This is a
code technicality; some European codes (I'm told) don't even have this
requirement.

2.)	They will only work if they're tightly installed and this is the
most critical part.  Rod bracing overcomes this by being adjustable.

3.)	If you're going to use them in spite of the code they work best when
you provide some form of intermediate support such as connecting them to
metal studs within a wall.

4.)	If practical, and architectural considerations permit, you are
better off using something else other than flat bars as bracing.

	I hope this is helpful.

				Regards,

				H. Daryl Richardson

Efren Allan Yango wrote:
> 
> To Engineers with braced frame experience :
> 
> I am doing a design check on a mall-type structure with some portions
> 3-storey high and others 2-storey high. All beams and girders were
> assumed simply supported by the original designers (no moment frame
> construction). The columns were designed for simple axial load and
> minimal moment usually resulting from the beam connection eccentricity.
> The lateral force resisting system adopted was an X-braced frame system.
> The size of the whole structure is around 15,000 sq. m with beam spans
> as long as 12m.
> 
> I have checked the path for the gravity loads and it met all the
> criteria of our codes here. My questions are now related to the braced
> frames.
> 
>     1)    Can the braced frame be located at the 2nd floor to 3rd floor
> only? (One end of the digonal at the 2nd floor and the other at the
> third floor). No braces
>             at the first floor were located. I believe that this would
> create a soft storey. In terms of member forces, this would pass but I
> have doubt on this type of
>             design. That's like terminating your shearwall at the second
> floor instead at the foundation level.
>     2)    Can flat bars (designed as tension-only members) be used as a
> bracing system? I believe that tension-only design is discouraged by the
> governing codes.
>             How about for low-rise structures? Is the slenderness limit
> of 720/sqrt(fy) strictly followed?
> 
> For now, I don't want to conclude that the building has been improperly
> designed. I would like to know first if I'm missing out on something. I
> have other questions but I would like to limit them to these 2 for the
> meantime. I hope engineers with experience in this type of structures
> could help.
> Thanks
> 
> AllanYango
>