Page 538 - Forensic Structural Engineering Handbook

14.28 MATERIAL-SPECIFIC FORENSIC ANALYSES

“Beam Stretcher” Moment Connections. A flexural member may be spliced providing
that the connection accounts for all the shear, axial force, and bending moment felt by the
member. Moment splice connections are used in original designs in glulam arches to shorten
the member for shipment. They are also used at knees of glulam rigid frames. Moment
connections may also be used in repair to replace a portion of a flexural member, with an
example shown in Fig. 14.6. Decayed or damaged arch legs may be repaired by removing
the damaged portion and fitting a new moment-connected base shoe. In a recent investigation
at a paper mill, a glulam beam end supported at a column had decayed in the wet environ-
ment. The decayed ends were removed and fitted with steel moment connected caps which
fit the original beam seat. The beam was salvaged without full beam replacement.
Post Tensioning. Post tensioning of beams involves application of an external compres-
sion force into the member, preferably at a location causing bending moments which
counter those due to the supported loads. Because this force is usually applied by a steel rod
or cable strand in tension, it is termed post tensioning. Numerous tensioning strand config-
urations are possible, the most common being a tension strand at or near the beam soffit to
create maximum negative moment due to the lever arm distance between the tension strand
and the beam neutral axis. Tension strands below the beam soffit may use a vertical spreader
member to increase the tension strand lever arm, creating a sort of king post truss. Since
strand tension may be quite high, anchorage of strand to beam is critical. As in post-tensioned
concrete the effects of long-term creep should be considered. Unlike post tensioned con-
crete, the tension and compression capacities of timber are similar, making concentric post
tensioning of a beam useless in a design. Also potential moisture changes can have an
effect. Post tensioning of timber is more suitable as an indoor repair than initial con-
struction. The moisture content is typically stabilized reducing changes in the volume of
timber and creep effects. The dead load effects can be countered without lifting the beam.
Eccentric post tensioning can reduce or eliminate the sag in a flexible beam, particularly
one with high dead loads.
Mild steel shapes, mild steel threaded rod, and cable have been used successfully as ten-
sion strands. Turnbuckles may be used to apply tension to steel rod tension strands. Cable of
the type used for prestressed concrete has advantages in that long-term losses due to creep and
moisture change are reduced with the high strength steel. Also, inexpensive “button” type of
end anchors may be fastened to the cable and a predetermined amount of tension applied
using a calibrated hydraulic jack. The amount of force applied may be accurately controlled
without risk of overtensioning. (See Case Study 3, Tinora High School.)
Post tensioning of trusses may be utilized to counteract overloading or calculated over-
stress due to changes of criteria. Post tensioning of trusses usually consists of a tension
strand at middepth of the lower chord applying an axial force to the chord without induc-
ing moment. Truss post tensioning will reduce lower chord tension and reduce the possi-
bility of lower chord distress.

Radial Tension. Wood separations due to radial tension induced by bending of curved
glulam members are rarely seen today as radial reinforcing has become a typical design
procedure installed during manufacture. Field repairs of un-reinforced beams containing
radial tension separations may be engineered using a lag bolt field installed to carry the ten-
sion perpendicular to grain force. The beam section must be adequate to sustain section loss
from the repair dowel. To close the separation, the lag bolt may be installed from below in
holes / 16'' oversize below the separation and above the separation in undersize lead holes
1
as prescribed in the NDS for lag bolts in withdrawal for the wood specie present.
However, the installation of full thread lag bolts or the placement of epoxy-embedded
deformed reinforcing bars perpendicular to grain can also resist the design radial torsion forces
in the unsplit portion. Data sufficient for an engineer to design either a lag or epoxy-embedded