The name of our firm is misleading!
We are one of the major supliers of bridges with steel beams
As bridges get longer the use of timber beams becomes an issue
The stiffness is achived as the span increses, by increseing the thickness of the beams and/or incresing the depth of the beams.
The beams get huge and are difficult for our kit bridge useres to manage on site.
our prefered installers mutter darkly when given bridges with huge beams.
In softwood we run into 3 issues.
The availability of large sections. Trees are usualy felled in a small diameter than we need to be converted into sutible beams in the right species of timber.
The suitability of the timber fro stress grading
It is possible to get longer lengths in Hardwood but the costs are off the clock.
It is possible to use laminated beams, these are thin layers of wood bonded together by glue and dowels to "make" glue lam beams.
These dont last well in the UK for reassons we do not understand, they seem to last better in countries with less harsher climates.
Some years ago we took out one rotten glu lam beams with rotten ends about once every other month.
Water had migrated through the deck fixing screw holes onto the core of the beams and rotted from the inside out.
In our view using a bridge with steel beams become viable at or about 9 meter span.
Our steel fabricators offer high quality supply of steel beams with a galvanised finnish ready to receive out wooden parts.
These can be supplied as assembled bridges or in kit form.
Our client Sensible Access have adopted the same policy.