Page 537 - Forensic Structural Engineering Handbook

TIMBER STRUCTURES 14.27

TEMPORARY AND PERMANENT
REMEDIAL REPAIRS

Simply stated, any structural repair must satisfy the results of the structural analysis. The repair
of members and connections must satisfy the shear, bending moment, tension compression,
etc., from the analysis plus any condition such as eccentricity introduced by the repair. As such,
the repair engineer must have analysis data available prior to final repair design.
Shoring. When a condition of structural distress is first identified, the situation is usually
a state of emergency. Shoring may be required to prevent collapse before there is time for
detailed analysis. Emergency shoring usually consists of what is readily available. Wood
members have an advantage in that they may be easily field-trimmed to length. Round
wood poles and square or rectangular timbers work well. If only dimension lumber is avail-
able, it may be nailed into T, H, or box-shaped members to resist buckling. Wood or steel
wedges may be used to bring shoring tight under a distressed member. Engineered tubular
steel shoring with known load capacity is ideal, if available.
When trusses from the lower chord are shored, it is usually necessary to shore all panel
points. This is particularly true at bowstring trusses where web-to-chord connections are
relatively small due to axial web forces which are small compared to axial chord forces.
The shore becomes a truss reaction, and webs fastening to the lower chord close to the shore
tend to act as the top chord of a reconfigured truss. Partial shoring of a bowstring truss will
cause substantial increase in web compression adjacent to the shore and may cause web-to-
chord failure and possible truss collapse. Shoring under the top chord of a distressed truss
is preferable to lower chord shoring but more difficult to accomplish. An analysis of a
recent truss failure showed that placing a cross-wall tight under a bowstring truss lower
chord increased compression at one web from 3 to 78 kips and caused progressive web fail-
ure and subsequently truss chord failure.
In the case of overloaded beams, placing additional columns to shorten the span may be
the most favorable solution. (See Case Study 11, William Shore Memorial Pool.)

Replacement. Replacement of a distressed member inside a structure is usually not fea-
sible owing to adjacent members and interference with other building elements. In those
situations where replacement is possible, it is usually more economical to replace a mem-
ber than to design and install a repair, as most repairs are labor-intensive. Replacement of
deteriorated truss members is feasible where the full truss may be stress-relieved by top
chord shoring.
Sistering. Addition of a new structural member where the original member is physically
left in place is often a solution for reinforcing a distressed member in an industrial struc-
ture where appearance is not crucial. The new sistering member should be designed to carry
all the load and should be fastened so as to transfer forces from the original members if
required. Sistering members of like materials are preferred for ease of connection, but steel
channel or plate may be used for sistering timber members.
Bolt Tightening. Sawn timber structures fastened with bolts and shear plates or split rings
should have bolts tightened after moisture equilibrium is achieved, and periodically if bolts
loosen. Large sawn timbers are too large to be kiln-dried and are usually installed wet with
shrinkage occurring after installation. Shrinkage at connections fastened with shear plates
or split-ring timber connectors may cause a separation between timbers at the load transfer
point, which reduces the bearing area of wood against the plate or ring, thereby reducing
connector capacity. Glulam is kiln-dried and more dimensionally stable, so it is less in need
of periodic bolt tightening.