Bridge caps

[rhohf@eagle.idcnet.com]

I found the discussion of bridge caps in the archives good reading.  It was
both interesting and informative.  I would like to add a couple of things
which I did not see mentioned.

It has always seemed to me that one of the functions the cap serves is  to
cover the  multiple glue joints between the bent laminations and present just
one joint to the atmosphere. Bridge capping became practice at a time when the
only glue available was hot hide glue, a glue where failure is directly
related to exposure to the atmosphere.  A laminated bridge glued with hide
glue would be many times more resistant to failure with a cap.  This practice
of encasing glue joints and exposing the minimum number of joints is
demonstrated throughout piano construction (consider  veneering patterns).  Of
course, with modern glues, it is debatable whether this reasoning still
applies capping.

Another reason bent-wood bridges need caps is to resist the spring-back which
occurs over time after bent wood is released from its mold.  It is clear from
observing patterns of soundboard cracks that some cracks form to relieve
tension between the bridge and the  soundboard.  Bridge sringback is often the
source of this tension. The soundboard, being softwood, does not have the
compressive strength to resist a bridge which is trying to straighten out.  A
hardwood cap on top can provide considerable resistance to springback.

I have great respect for solid-body bridges with their with their sinuosities
unattainable by bending.  Their scarf joints are often masterpieces of
planning;  imagine the dimensions of the lumber necessary for such
constructions. I can gaze for hours at the exquisite notching, hoping to glean
the secrets of its method.   I do this, right before I plane it all off.  I am
very heartless in this:  every exposed bridge gets a new cap in my shop.

Bob Hohf