Civil Engineering Reference
In-Depth Information
when marking the bearer to go on concrete stumps it is best to place the bearer upside
down on the stumps so that the markings for drilling the holes is relative to the pin where
it comes out of the stump. Marking this way negates the problems associated with drilling
off square to the bearer. If when drilling the hole in the bearer it is drilled off square, the
hole at the other side of the bearer could be as much as 20 mm out of position relative to
the stump. The stump when fitted correctly will not move sideways to accommodate this
variance. If the bearer was forced onto the stump regardless it will almost certainly crack
the stump, which would then need to be replaced.
Straightening bowed bearers
On many occasions during installation there is a large bow in the bearer that cannot be
straightened to enable correct positioning onto the stumps. In this instance you can
cripple the bearer. To cripple the bearer refers to cutting the top of the bearer to allow it
to bend down into the correct position more easily (see Figure 3.6).
Continuous
span
Single
span
Figure 3.6
Crippling the bearer reduces it to a single span and reduces its span capacity
When a bearer is crippled it may result in a reduction of the carrying capacity of the
bearer so you should check with your building consultant or engineer to ensure that the
bearer has the excess carrying capacity to allow for this. The carrying capacity of timber
changes if the timber is supported in at least three places rather than just two. When the
timber is supported in three places the timber is deemed to be continuous and thereby
often able to carry a greater load than a single span that is supported in two places only.
You will note in Figure 3.6 that the bearer is positioned over four stumps. The bearer is
then crippled over one of the stumps, which will result in part of the bearer being
continuous span and the balance being single span.
Fixing bearers to timber stumps
To fix bearers to timber stumps, you will insert a skew nail through each side of the
bearer into the top of the stump. If you prefer, you can use long heavy gauge screws
through the top of the bearer down into the stump, however I find this unnecessary.
Drive the nails through the moisture barrier including the ant cap, if ant caps are to
be installed.
