An option I try to avoid is "knee"-braced frames, which I generally treat as a
subset of moment frames. The knee is usually an angle, double angle, or tube
placed at a 45 degree angle (+/-) at the top or bottom of the beam-to-column
connection (usually bottom). This bracing system is most effective if you can
get your column grids inside the building envelope so the stud walls do not
interfere. I have used this system successfully on buiding up to 3 stories in
height, but expect it could easily accomodate 4-story structures (in low
Conceivably, knee braced connections could be modeled as partially restrained,
using the brace as the yielding member, but I haven't tried it (there's never
enough fee to be had).
As for working with architects, the key is to get in early. I try to discuss
building bracing while the architect is still working on the concept drawings,
and place extra emphasis on the lower construction cost as well as the effort
(read: fee) from my end. If the building still doesn't have the bracing bays
you need, the best approach for me has been to get reasonably close column
spacing around the exterior walls of the building. This provides you with
additional bays for "knee" braces, reduces your exterior column loads, and
permits smaller spandrel beams (which will be sized for lateral loads).
I appreciate open floor plans as much as anyone, but the tructh is that its
laziness on the part of the architect not to include bracing bays. If for no
other reason, locating the bracing in discreet bays, easily accessible from
hallways, allows for rapid inspection and repair following a disaster.
I'd appreciate comments on any of my thoughts.
--- "Hyndman, Keith" <KHyndman(--nospam--at)engineeringsi.com> wrote:
> Greetings all.
> I've been working on a lot of low rise (less then 4 stories) Steel Frame
> Architectural projects, many of them, additions to existing low rises. In
> all cases, the layouts give no consideration for vertical bracing, due to
> openings and wall alignments from floor to floor. Usually exterior walls
> are "Drivit" or E.I.F.S. Interior walls are drywall and don't line up floor
> to floor.
> Are Moment frames the only answer? They really drive the Column and beam
> sizes up. I've used Partially Restrained Connections when I have 3 or more
> column bays, however on a new project I'm looking at, I have one bay with a
> 62' span.
> Are there any good references for using interior Masonry Stair towers and
> Elevators towers to brace these buildings? I know how to design a Masonry
> Shear wall, I'm more interested on an overall building approach. How would
> you tie the towers in on each floor and the roof? Also what are the effects
> on the towers based on their location in the building plan? Can you
> Cantilever a floor Diaphragm?
> I guess I'm not used to this stuff. I came from Industrial Projects where
> Bracing in King, some of these Architects "never" heard of bracing.
Keith Fix, PE (CA)
Little Rock, AR
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