6. Observations, Conclusions and Recommendations

The main objective of this research study is to investigate effects of repeated loading on shear spike strengthened, full-size timber railroad bridge stringers. The ultimate bending strength and failure mode of the shear spiked stringers are also investigated. Results of this study show that strengthening through the process of shear spiking is highly effective in restoring bending stiffness and strength of 'deteriorated' timbers. Moreover, it is shown that shear spiked timbers exhibit little detrimental effect from repeated loading at approximate service load levels. When ultimately load tested, the strengthened stringers fail primarily in flexure, i.e. a wood failure rather than failure of the shear spikes. Some notable points are:

• The flexural stiffness in the undamaged state of a stringer seems to provide an approximate upper limit of stiffness that can be regained from shear spiking of a damaged stringer.
• Attained improvements in effective flexural stiffness are comparable to those measured by Radford [49-52] (in dimension lumber), Schilling [55-57] (in railroad crossties), and Burgers [15-16] (in full scale railroad trestle bridge stringers).
• Driving shear spikes into the pre-drilled holes with a dead blow hammer proves practicable; the spikes show minimal damage from the impact of repeated dead blow hammer blows.
• By not drilling holes for the shear spikes through the full depth of a specimen, no epoxy is pushed out the bottom during installation. On the side faces of the stringers, it is visually evident that beads of epoxy penetrate from the drilled hole through the wood fibers and decay/damage voids. This migration of epoxy further most likely helped in improving the flexural strength and stiffness. From the dissected cross-sections, it is also observed that epoxy resin filled voids near the shear spikes.
  • From dissection of reinforced members, it is visually evident that the epoxy resin does, in general, bond to the entire length and circumference of shear spikes.
• In an in situ situation, shear spike reinforcement does not necessitate removal of structural members to provide access to the members to be strengthened; only portions of the top or bottom surface need to be accessed for insertion of the shear spikes. This method thus provides a highly cost effective bridge rehabilitation method when that access can be made, e.g. in timber trestle railroad bridges.

6.1 Conclusions

• Significant recovery of flexural stiffness and shear strength of full size, deteriorated/damaged timber stringers can be attained from shear spiking, when bonded-in perpendicular to the primary bending axis.
  • On average, the eight stringers in this study lost 36% of their flexural stiffness from the intentional damage. After full shear spike reinforcement, the average flexural stiffness is up to 91%, i.e. a regain of 27%.
• In general, the 'severely deteriorated' stringers experienced a dramatic recovery of flexural stiffness, with a corresponding decrease in deflection under load, when all shear spike rows had been installed.
  • The four stringers with full cut(s) inflicted on them had an average loss of flexural stiffness of 61% (i.e. to 39% of the 'undamaged' average flexural stiffness) after damage infliction. In the full shear spike reinforced state, the average flexural stiffness of these four specimens is regained to 84%, e.g. 16% below the 'undamaged' average flexural stiffness.
• The application of bonded-in, fiberglass pultruded rods, as shear spikes, in specimens with saw cut(s) through their full width produced greater increases in flexural stiffness than did shear spiking of specimens with "partial" saw cuts, i.e. not through the entire specimen width. It is thus concluded that stringers with a greater degree of deterioration show a greater potential to rejuvenate flexural stiffness and interlayer shear resistance.
• The shear spike reinforced specimens showed very little, if any, detrimental effect from the repetitive nature of the durability loading up to 25,000 load cycles; for both the cyclic levels and load levels examined.
  • The few partial failures of specimens that occurred during durability loading are instead believed to have resulted from overloading of the specimens since considerable losses of flexural stiffness generally occurred very early in the durability loading process.
  • Despite these initial partial shear failures, the specimens showed little further detrimental effect from further repeated loading.
• Dissection of specimens that had suffered partial failures during durability loading showed that debonding at the shear spike/epoxy resin interface was the primary cause of failure. The combination debonding and local crushing of wood fibers surrounding the shear spike was also evident in a handful of dissected cross-sections.
  • Vertical shear (in the plane of the shear spikes), resulting from opening and closing of gaps created within a stringer by the intentional saw cuts most likely caused the debonding at the shear spike/epoxy resin interface, which in turn led to the partial shear failures during durability loading.
  • Despite this debonding, the 'un-bonded' shear spikes still provided significant shear resistance.
• Flexural failure was the typical failure mode of shear spike reinforced specimens when ultimately load tested. In other words, the strength of wood fibers governed a composite stringers' ultimate strength, while the strengthening components remained virtually intact.
  • Even with partial shear failures and subsequent interlayer slip within specimens during ultimate load testing, additional loading was necessary to induce catastrophic failure.
  • The shear spiking thus seems to promote a ductile behavior of the strengthened stringers, when loaded to failure.

6.2 Recommendations

• The use of a primer, in conjunction with the epoxy resin, should be investigated as a means to improve the wood/FRP bond. Incorporation of a primer could be particularly beneficial in high humidity environments.
• A polyester resin adhesive should be considered in future research studies. It is believed that a polyester resin would be more compatible with the spikes, compared to epoxy resin, and therefore would provide a stronger and more durable bond.
• A continuation of the study of the shear spike/epoxy resin/wood bond that was conducted by Schilling et al. [55-57] should be considered. The bonding in shear spike should be cyclically loaded to investigate the durability characteristics of the epoxy bond.
• Related parameters to be investigated are the effects of varying humidity and/or temperature exposures on shear spiked timbers. This can be done by conditioning specimens in an environmental chamber to investigate the epoxy resin bond strength under simulated in situ exposure conditions.
• An investigation into the bond performance of shear spikes with roughened surface and/or with an irregular surface configuration, shaped much like steel rebar, is suggested. A non-smooth shear spike surface will presumably improve the FRP/epoxy resin interface and could potentially prevent FRP/epoxy resin interface failures that were observed in this study.
• The feasibility of combining the shear spiking process with pressure injection of epoxy resin should be investigated as a means of filling a greater fraction of decay voids. This would shield the interior portions of the composite member from further environmental exposure and decay. Moreover, any severe surface crack/splits/voids could be hand filled.
• The next rational step in this ongoing research program is shear spike strengthening of either a deteriorated in situ structure or a laboratory study of deteriorated members that have been salvaged from a field bridge.
• To improve the efficiency of strengthening of in situ structures through shear spiking, NDE techniques should be considered to identify locations of decay/deterioration. A combined NDE/shear spike approach might be developed; a standardized NDE based decay classification method, along with guidelines as to when shear spiking (relative to extent of decay) is effective would be highly useful.

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Endnotes

1 AREA has since added "and Maintenance of Way" to its name and is now known as the American Railway Engineering and Maintenance of Way Association (AREMA).