CREEP BEHAVIOR OF HYBRID REBAR-GLULAM TIMBER BEAM UNDER LONG-TERM LOADING

Abstract

Recently, from view－point of Global Environment, timber, i.e., one of nature-cycle materials, is being tried to be utilized as structural members of large timber buildings in Europe and North America.

A representative timber of the members is Cross-laminated timber (CLT), however, CLT structural system very often restricts planning of building because of CLT being plate member. High-stiffness-strength-timber slender beam and column are significantly desired.

This study focuses a hybrid glulam timber beam with steel deformed bar (rebar) and Epoxy resin adhesive. The aim of this study is to clarify mechanism of suppressing bending creep of the hybrid beam by rebars and to develop method of estimating its relative creep coefficient in design. This paper reported creep behavior of hybrid glulam timber beams with a small size under 4-year term loading and additional coated beams under 1.5-year term loading. The coated beam specimens were prepared in order to investigate adverse effects of moisture on shear stiffness of wood around the rebars. The result and discussion of the two experiments are summarized as follows:

(ⅰ) Creep of curvature of the hybrid beam rapidly boosted against prediction based on assumption of Navier hypothesis, during same term in every year when humidity increases in Japan, i.e., July-September, as Fig.11.

(ⅱ) The boost results from position of rebar-embedded lamina in the beam section because moisture in air around beam percolates surface of its timber and decreases shear stiffness of wood part around rebars. One of basic ways to protect the decrease is to arrange the rebar-embedded lamina as the second layer from outer layer because adhesive layers between the outer layer and the second, as seen in Fig. 20, prevent the moisture from percolating the second layer , i.e., rebar lamina.

(ⅲ) This problem may occur in the small-sized beam, however, the real full-sized hybrid beam can readily prevent it in some ways as shown in Fig.20(b),(c), by using waterproof coating, burning marginal later, or refractory coating.

(ⅳ) The protected hybrid beam is estimated to significantly suppress its bending creep by rebars, as Relative creep coefficient C_φ of curvature in 50-year for design listed in Table 8.

(ⅴ) Deflection-stiffness of timber beam and hybrid beam rarely deteriorated during creeping, as Fig.25.

(ⅵ) A prediction method for relative creep fluctuating intensely owing to seasonal influence was proposed by using two - tangent lines on logarithmic chart, as Fig.21, Fig.22, and Fig.23.