In the past, bridge decks
have been formed by hanging standard dimensional wood
ledgers from the bridge deck beams. These ledgers generally
are spaced 48" to 66" depending on the thickness of the deck
to be poured and the span between the deck beams. Standard 4
x 4's are then placed on these ledgers about 12" O.C.
There are several problems inherent
with this method.
First, in most cases, the bridge deck beams are not
perfectly straight which means the space between them is not
a constant. This means that in many cases, especially with
precast concrete bridge deck beams, ledgers will have to be
re-cut to fit.
Second, it is not uncommon that a bunk of 4 x 4's will
contain as many as 25% of them will be twisted and bowed,
making it difficult to lay plywood on them. Additionally,
the standard 4 x 4 can only span a maximum of 66".
FCI has developed an adjustable length
ledger to solve the first problem and rents laminated wood 4
x 4's to solve the second one.
Because of the higher strength, the 4
x 4 Tru-Beam can span up to 8'. This means that the number
of ledgers and hanging hardware can be reduced by as much as
50%. This reduces the material cost but more importantly,
reduces the labor of handling the additional material.