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Glulam Beams Strengthened by FRP

  • Friday, 3rd September 2004
  • Reading time: about 3 minutes

A recent Strengthening project by Quakewrap received the 2004 Award for Excellence in Structural Engineering by the Structural Engineers Association of Arizona.

A US school gymnasium required strengthening in order to open up space between two floors, and to accommodate the additional strains that were placed on the structure due to an increase in traffic on the above gym floor.

A collaboration between project engineer (Mark Larsen) and the FRP materials supplier technical staff (Mo Ehsani) led to the development an innovative technique that utilized both high-strength carbon plates and fabrics with a laboratory investigation at the University of Arizona proving the viability of this new concept.

The carbon plate has a tensile strength of about 310 ksi and is 0.05-in. thick x 3-in. wide. The flexural capacity strengthening of the glulam beams required the bonding of one carbon plate to the bottom of the beam. To balance this force with an equal compressive force, a plate of the same area had to be added to the top of the beam. In order to prevent buckling of the plate in compression under large strains, two 1 5/8 in. deep x 1/8 in wide groves were cut horizontally near the top of the beam along the span. The 3-in. wide carbon plate was divided into two 1.5- in. wide strips; these narrower plates were saturated with resin and inserted (i.e. hidden) inside the aforementioned grooves. This creative installation allowed the carbon plates on the compressive side of the beam to contribute significantly to the increased moment capacity of the retrofitted beams.

Tests indicated that if only the carbon plates were added, the capacity of the beam would be limited by a horizontal shear failure near the bottom of the beam (between two laminates). To prevent this mode of failure, the beams were wrapped in a bi-axial carbon fabric with the fibres along the axis of the glulam beam contributing to flexural strength, whilst the fibres in the vertical direction increased the shear capacity of the beam (similar to stirrups in a R/C beam).

According to QuakeWrap, tests demonstrated that the flexural strength of such a beam could be increased by 67% and that the carbon fabric eliminated the undesirable shear failure that was observed in previous test when only the carbon plates were used.

QuakeWrap provided sealed design calculations, the materials and installation for the retrofit System, with twenty one beams being strengthened using this approach. The construction took less than two weeks with the costs of the retrofit being approximately $1300 per beam, or $8.50 per square foot of floor area.

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