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RE: Steel Bar Joists as Bridge Beams

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David,

One joist that might be of interest is the KCS series of bar joists.  These
joists are designed to allow a specific joist to handle a specific shear
value at any point along the length of the joist.  This could be handy in
dealing with moving loads such as on a bridge.  The KCS series should be
available from any supplier that supplies SJI Joists.  Vulcraft, SMI Joist,
and Canam Steel are 3 examples of suppliers.  These are available in lengths
up to 60 ft long.  Typically the span should not exceed 24x the depth of the
joist.  If you need longer than 60 ft you should look at LH or DLH joists.
If you are unconfortable designing the joists from a table then most
suppliers will design the joists based upon loading diagrams supplied by
yourself.

As far as cross-bridging I would use the SJI recommendations as minimums and
possibly increase as desired since the bridge will have moving loads.

I would provide some sort of lateral bracing, such as rods, or diaphragm
deck (using the bar joists as struts) and not count on the bar joists for
lateral loads through weak-axis bending.

One thing you might consider is serviceability to keep the bridge from
bouncing as the truck moves over it.  The KCS joist table provides a gross
moment of inertia that could be used for the deflection calcs.  Not sure
what deflection and vibration limits apply to bridges though.

Not sure where you are located but here are a few contacts if you need them.
If the locations are not close you could contact the manufacturer of your
choice and obtain an address/phone for a closer location.

Vulcraft
PO Box 637
Brigham City, Utah 84302
(801) 734-9433

SMI Joist Company
Highway 32 North
PO Box 2000
Hope, Arkansas 71802
(501) 777-8777
(800) 643-1577

Canam Steel Corporation
2000 W. Main St.
Washington, MO 63090
(314) 239-6716
if for some reason the area code has changed on this one it might be 473.

Hope this helps,
Greg Effland, P.E.


-----Original Message-----
From: PEC - Lake City [mailto:pec(--nospam--at)isgroup.net]
Sent: Monday, December 04, 2000 2:06 PM
To: Seaint
Subject: Steel Bar Joists as Bridge Beams


I've been contacted by a local builder who wants to build a bridge
connecting two pastures separated by a stream.  This will be a private
bridge - no government controlled loading criteria, but almost certainly the
water management district has jurisdiction.

The span is 40', single lane to accommodate a loaded pickup truck (builder
says use 7000# total live load).  Structure is 2 bar joists @ 7'-0" with 4 x
4 decking - total width of about 12 feet.

I have designed a number of bridges, but none using bar joists.  In fact,
I've never used bar joists for anything since I don't normally do buildings.

With that ignorance in mind, can bar joists be successfully used in such an
application?  I'm concerned about the wheel loads that occur directly over
the bar joist - there would be pretty good concentrated loads between panel
points.  Also, without a rigid diaphragm deck (concrete, for example), I
don't see any load sharing for lateral loads.  I can see the need for some
pretty significant cross-bridging.

I've already tried to talk the guy into using precast double tees or even a
proprietary arch culvert, but he is stuck on the bar joist idea.

Any comments, suggestions, ideas?

David Finley, P.E.