Civil Engineering Reference
In-Depth Information
b. Improving the current situation
When there is insucient spatial stiffness or bearing capacity of structure or members due
to overload, change of service condition, design or construction errors, structural member
defects, etc., it is necessary to conduct retrofitting according to the main hazards, or even
rebuild the whole structure.
This level of retrofitting focuses not only on wood structure itself, but also on its loads,
bearing states and even the surroundings as well. For example:
a) If the timber surfaces at roof trusses or column bearings are eroded by humidity and
enclosed, the rotten parts should be removed and antiseptic chemicals should be applied. It
is more important to modify the configuration at the bearings to eliminate moisture.
If the bearing part has been eroded severely, measures should be taken to replace the rotten
members with new ones. If moisture proofing of the supporting parts is hard to attain, the
rotten parts should be replaced with rolled steel components or precast reinforced concrete
joints.
b) Wooden roofs are vulnerable to rot in an environment of high temperature, humidity
and poor ventilation. This may cause corrosion and additional deflection of the wood struc-
ture. Besides retrofitting the structure, it is preferable to improve the surrounding condition
and keep the structure in a dry and normal environment.
c) If the members are overburdened because of excessive load or service alteration, the
original members should be replaced by new ones with much lighter materials. One or two
columns can be added to the structure, if possible, to shorten the span and reduce the
stress of members. Then more attention should be paid to the changing of stress in original
structural members caused by the added columns.
d) Rebuild, strengthen or extend spatial bracing system that is deficient in the original
structure.
e) Strengthen and modify the original bearing components such as walls and columns to
ensure the normal serviceability of wood structure.
(5) Main points of retrofitting designs of wood structures
a. Economic effects should be taken into comprehensive consideration when conducting
retrofitting designs. The original structure should be damaged as slightly as possible, while
the serviceable structural members should be reserved.
b. Structural calculating sketches should be in accordance with actual load and stress on
the structure.
c. After long-term service, the material strength of the lumber will decrease to some
extent. Therefore the material strength should be able to meet the real condition.
d. The actual state of structural deformation, cross section changes, increase or decrease
of members and nodal displacements should be accurately determined as the basis of the
calculation.
e. The favorable conditions of original structure should be used to improve the irrational
structures.
f. Steel is preferable to timber. It is better to make tension rods and clamps with steel.
(6) Main points of retrofitting constructions of wood structures
Construction schemes should be formulated before retrofitting constructions. Retrofitting
should be executed in the sequence of supporting first and strengthening second.
Make full-scale mould boards according to actual dimensions of designs and members, and
number them. Manufacture the strengthening members according to the mould boards.
When wooden clamps are used, the materials for retrofitting and the diameters, quantity,
and position of bolts should conform to design demands. When the holes for splicing are
drilled, related components should be positioned, fastened temporarily and drilled through
at one time, to ensure that the positions of holes on each member are consistent.
The
