Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

Re: Open Web Stl Joist Bridging

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Often it is difficult, if not impossible, to pull citation for some item
from reference books.  So, as engineers, we should understand structural
behavior well enough that we can explain to others when the situation
demands it.  What you have narrated is one such situation.  

The cross bridging member, which is an angle, is generally very slender
(in fact, in metal building industry everything is slender). The amount
of axial force seen by a typical bridging will be small and that is the
reason for its slenderness.  Any weight suspended from such a member
will produce bending moments and, a thin angle member does not have much
resistance for flexural stresses. One alternative for you is to run a
quick calculation for the angle member as a beam column.  If I remember
correctly, the axial load in each cross bridging is about 2.5% of the
total vertical load on the truss divided by the number of cross
bridgings.  Calculate bending moments and check the member as a
beam-column.

Second alternative is for you to put in writing your objection to load
the bridging because of the principle involved in its design etc. Also,
suggest to the owner that instead of adding any loads to the bridging,
you can design a member that will safely carry the additional loads.  If
the owner refuses your recommendation, courteously tell him, in writing,
that you will not be held liable for any structural problems that may
occur.(In real world, structures are abused more than one can imagine. 
Structural steel, in particular, is very forgiving.  I have seen some
weird pipe supports, some pipe support column members which has very
little meat left in them etc.  In many cases accidents are waiting to
happen, but are not obvious to "engineers" in the field. I have been in
many such situations where I have held my ground. I have had some foes
because of my stand, but I have also earned respect from some others)

Rajendran

CanitzCF(--nospam--at)aol.com wrote:
> 
> A co-worker asked for my assistance regarding the following. Her project is
> the complete architectural, mechanical, sprinkler and electrical upgrade of an
> existing one story 30 year old seismic zone 1 building. The roof framing
> includes open web bar joists which are laterally braced with diagonal(single
> angle) cross bridging. The problem is the mechanical subcontractor has
> supported HVAC ductwork, via sheet metal straps, from the cross bridging. I
> also noticed a few sprinkler drops also supported by the cross bridging. I
> indicated that this was unacceptable since joist bridging is not intended as a
> support for mechanical equipment. Naturally, the field response was "show me
> where this prohibition is indicated within the CD's".
> 
>   Normally, a project's steel joist specification includes this requirement.
> However, no new steel joist installation is required for this project and an
> associated spec section was therefore not included within the CD's. Nor does
> the project's general mechanical specification does not address this issue.
> Our office's mechanical engineer stated that SMACNA does prohibit this type of
> support. I also reviewed the SJI spec's and again found no definitive
> direction. Can anyone refer to other guidance for this type of situation?
> 
> Thanks,
> 
> Charlie Canitz, PE
> Bel Air, MD
>