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Re: rotation at bearings

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The reason you get longitudinal forces in your model under transverse load
is that the bridge is bending laterally, and at the fixed bearings it
becomes effectively an encastered end, and therefore develops a net moment.
You only get this effect on single span or simply supported bridges, but I
strongly doubt that you should get high forces.

One way to get the model to show less longitudinal force is to more
accurately model the longitudinal stiffness of a fixed bearing. If you
model a rigid connection, you might get a large force, but if you model
even a tiny amount of flexibility in the bearing (1000kip/in maybe), you
may find that the longitudinal load diminishes to near zero.

Bearings always confuse people. There are these tiny little elements, and
the number of times I've wasted weeks screwing around with them is just out
of control...You have my sympathy.

Michael Ludvik, PE
Hardesty & Hanover

             "S. Gordin"                                                   
             .com>                                                      To 
             10/05/2004 11:22                                           cc 
                                       rotation at bearings                
             Please respond to                                             

Fellow engineers.

I hate long posts.  However, please hear me out.

For more than a month now I am going back and forth with an independent
reviewer on the design of two pedestrian bridges.  I managed to prove my
points on all issues, but...  On the remaining one issue I seem to have
exhausted all means of persuasion within reason and logic.  So, I would
appreciate a good advice.

Here it is...

An 80'x8' steel half-truss pedestrian bridge with four bearings: two with
two dia.1-3/8" holes each on one end of the bridge, and two with 1-3/8"x3"
slotted holes on the other.  A total of 8 anchors 1" diameter, CIP.

According to the reviewer, the 3D analysis of the bridge shows that the
application of the transverse (wind or seismic) lateral force on the bridge
produces the longitudinal reactions in the pinned bearings on one end of
the bridge.

I responded that such forces will never develop, mainly, because there is
no such thing as an ideal hinge (unless it is so purposely designed).

Indeed, the ø1” anchor bolts of the subject bridges are located in ø1-3/8”
baseplate holes (3/8” gap).  According to my own 3D analysis , the maximum
movement of the bridge at the supports due to the horizontal-plane rotation
under the transverse force of a 100 MPH wind is 0.18” << 3/8”. This means
that actual “pins” and lengthwise reactions (due to transverse forces) in
those bearings will never materialize.

The reviewer still insists on putting slotted holes in all four bearings
and to account for longitudinal seismic through actual dynamic impact on
the abutment backwall and anchors.  Alternatively, the reviewer suggested
designing the bridge abutments and anchor bolts for the couple of
longitudinal forces resulting from the application of the transverse (wind
or seismic) force…

To compare: the actual (transverse) wind reactions are about 9.3 kips per
bearing.  The aforementioned longitudinal forces resulting from the same
force are about 33 kips (acting concurrently).

The question is – am I correct here? If so, how can I convey the message to
the reviewer?  Any suggested sources?

Thank you.

V. Steve Gordin, PhD
Registered Structural Engineer
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