Need a book? Engineering books recommendations...

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

RE: Cold-Formed Joist Lap Splice Question

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Good question!  First of all don't forget your bearing stiffener over
the support.  

Now about your question, The "Prescriptive Method for Residential Cold
Formed Steel Framing" recommends that splices be avoided unless
specifically engineered with a few exceptions.  First of all if your
beam is part of a plywood resistant diaphram, be careful to maintain
your continuos connection from chord to chord of the diaphram.
Secondly, you would want to use the same section as the beam for your
splice.  Calculate the number of screws required for any axial tension
(shear).  These will be installed in the web of the section (use a
minimum of four screws minimum on each end for bracing against web
buckling).  If you want to be fairly accurate, a shear flow calculation
based upon the two connecting sections will tell you how many screws to
use in your connection (based upon gage and dia of screw).  Fa=(Ma x A x
ybar)/I  

Fa is the total force transferred.  ybar is the distance between the
centroid and the neutral axis (assume this is one quarter the depth of
the joist.  This means that you are designing the compression and
tension fasteners separately although they will almost always be the
same. A= 1/2 of the section area of the beam (half goes to compression
and half goes to the tension), I = 1/2 of the section property listed
for the moment of inertia.  

The reason you divide everything in two to calculate the connection is
so that the tension and compression forces of a symmetric section do not
cancel each other out. 

Once you calculate Fa, determine the number of screws and install an
equal amount near the top and bottom flanges of the beam. 
Lets say you calculate 8 #10 screws.  install 4 in the top flange, 4 in
the web within an inch of the top flange, 4 within an inch of the bottom
flange and four in the bottom flange.  If you want to be conservative,
you can install 5 screws at each location instead of 4.  This would give
you a 25% safety factor but each screw value already contains a safety
factor of 4 so you should be fine.
 
-----Original Message-----
From: Ritter, Mike [mailto:mritter(--nospam--at)lgt.lg.com]
Sent: Tuesday, August 17, 1999 2:33 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: Cold-Formed Joist Lap Splice Question


	I have a general question for the group regarding approach for
lap splice design using light-gage, cold-formed steel joists.  I have
heard two views on this matter and wish to get the group's concensus if
possible.  Here is the situation...

	Suppose we have a two span condition with a 3' long lap splice
located over the center support.  At each end of the lap, there is a
cluster of self-drilling screws connecting the two members.  A couple
more screws are used directly over the center support to hold the
members together.  Due to the typical gravity loading, there is negative
moment over the center support.  The clusters of screws at each end of
the lap are not at inflection points, so there will still be negative
moment in the continuous members at those locations.  Now my question...

	Do we design the cluster of screws for :

	a)  the magnitude of negative moment at that exact location; or

	b)  only vertical shear forces due to the resolution of the
negative moment into a couple about the center support?  ie, each
cluster of bolts would have only shear and no moment.  The moment at
these locations would be resisted by the capacity of the single
continuous member at that point.

	Thanks in advance for any help or insight,

	Michael Ritter, PE