Creep Of Lvl And Its Effect On The Stability Of Structures

Laminated veneer lumber (LVL), one of the engineered wood products, is widely used in building structures due to its excellent mechanical performance. Creep is the prominent behaviour of wood, long-term deformation of wood members and wood structures due to creep is of key importance. As far as large span wood structures are concerned, buckling analysis is another key important issue for design of the structures. Creep of wood makes the structures exhibit different behaviour from those made with steel, buckling of wood structures becomes more complicated. Yet in-depth and systematic study of long-term load effect and long-term stiffness of wood structures, so far, has not been conducted, and analysis of buckling of wood structures taking creep into consideration remains untouched. It is, therefore, very useful both theoretically and in a sense of engineering practice to investigate creep of LVL and the effect on the structural stability.In this study, tension and compression tests of creep of LVL were conducted under normal in-door service conditions in Harbin. The LVL specimens were divided into three groups in both tension and compression, with each group subjected to a stress level of 0.2, 0.4 and 0.6 time the instantaneous mean strength of LVL, respectively. The test lasted for one year, and the creep behaviour of LVL under normal service conditions was obtained. It was found that creep is proportional to stress in a the range of stress levels applied. The creep constitutive model of LVL was developed, and a user-defined subroutine UMAT in ABAQUS was encoded to deal with the creep behaviour in the analysis. Then, the numerical simulations on the long-term derformation and the effect of creep on the stability of LVL structures were conducted.Long-term deflection and stiffness of elementary wood members were comprehensively studied, and the effect of the time coefficient and period of load on the long-term behaviour of wood members was obtained. By comparison, treatments of long-term deformation of wood structures in the design codes of different countries were investigated, and suggestions for China national code for design of wood structures to improve calculations of the long-term deflection of wood structures proposed.The creep buckling and the creep effect on long-term stiffness of LVL arches and single layer reticulated LVL shells were investigated. The load level, a dimensionless load parameter, was introduced to the analysis of the structures. The relationships between the load level and the critical creep buckling time of arches and single layer reticulated shells were revealed via parametric studies. It was found that the critical creep buckling time is only related with the load level of the structures, while it has nothing to do with the structural parameters, such as the span, ratio of rise to span and the cross-sectional dimensions of member. The buckling load level decreases sharply with the critical buckling time at first, then the decrease becomes slow and reaches nearly a constant when buckling time is long enough. The long-term safety load against buckling is about 35% of the instantaneous elastic buckling load in relation to the basic design period of 50 years. The equations in the form of exponential polynomials of load level-critical buckling time of the arches and the reticulated shells were established by regression of the analytical results. The equations can be used to evaluate the long-term buckling load or critical buckling time of the structures.The effect of creep on the residual buckling load of arches and the shells having underwent a certain period of normal service was also investigated. The relative residual buckling load level, also a dimensionless parameter, was introduced into the analysis. It was found that the residual buckling load level of the structures has nothing to do with the structural parameters either. A linear relationship between the relative residual buckling load level and the logarithmic service time under a certain load level was disclosed. Then the logarithmic equations were established to evaluate the relative residual buckling load level at an arbitrary service time.