An explanation for the rate-of-loading and the duration-of-load effects in wood in terms of fracture mechanics

1982 ◽  
Vol 17 (10) ◽  
pp. 2831-2840 ◽  
Author(s):  
J. S. Nadeau ◽  
R. Bennett ◽  
E. R. Fuller
Keyword(s):  
Fracture Mechanics ◽  
Load Effects ◽  
Duration Of Load ◽  
Rate Of Loading

Duration of load effects in lumber. Part I: A fracture mechanics approach

1982 ◽  
Vol 9 (3) ◽  
pp. 502-514 ◽  
Author(s):  
Kenneth Johns ◽  
Borg Madsen
Keyword(s):  
Fracture Mechanics ◽  
Stress Ratio ◽  
Failure Time ◽  
Creep Function ◽  
Load Effect ◽  
Mechanics Model ◽  
Current Design ◽  
Load Effects ◽  
Duration Of Load

In Part I of this paper, the deterioration of the strength of lumber with continued application of constant stress, called the duration-of-load effect, is treated using a viscoelastic, limited ductility fracture mechanics model. The model is explained and developed in a general way, then modified for use with commercial lumber. The problems of assigning correct creep function parameters and values of stress ratio for use in calculations involving the model are discussed. The evident weakening of boards surviving a long-term test can be used to project a failure time that is longer than the test period. Numerical results are shown and compared with the Madison curve, the basis for current design codes. Parts II and III of this paper demonstrate experimental verification and discuss design implications.

Duration of load effects in lumber. Part II: Experimental data

1982 ◽  
Vol 9 (3) ◽  
pp. 515-525 ◽  
Author(s):  
Borg Madsen ◽  
Kenneth Johns
Keyword(s):  
Experimental Data ◽  
Fracture Mechanics ◽  
Load Test ◽  
Design Practice ◽  
Coefficient Of Correlation ◽  
Similar Test ◽  
Mechanics Model ◽  
Viscoelastic Fracture ◽  
Load Effects ◽  
Duration Of Load

The viscoelastic fracture mechanics model developed in Part I is calibrated using a 70-day duration of load test on commerical lumber. The model is then verified using another similar test on a different population of boards, and on a unique 6-year test just terminated. All three test programs are described. The model is used to tentatively project results beyond the period of testing. The strength predictions of the model compare well with those obtained experimentally during the tests, the coefficient of correlation being 0.95. Implications for design practice are outlined in Part III of this paper.

Duration of load effects in lumber. Part III: Code considerations

1982 ◽  
Vol 9 (3) ◽  
pp. 526-536 ◽  
Author(s):  
Borg Madsen ◽  
Kenneth Johns
Keyword(s):  
Fracture Mechanics ◽  
Structural Design ◽  
Design Codes ◽  
Specific Design ◽  
Damage Models ◽  
Mechanics Model ◽  
Complicated Model ◽  
Load Effects ◽  
Elastic Fracture ◽  
Duration Of Load

The implications for practical structural design with wood of a visco-elastic fracture mechanics model, recently developed and verified experimentally by the authors, are presented. These are compared with those of the presently used Madison concept, and with those of the cumulative damage models. Following discussion of present practice and the consequences of the fracture mechanics model, specific design recommendations are made as to how this more complicated model might be incorporated into design codes.

Duration of Load Effects in LRFD for Wood Construction

1991 ◽  
Vol 117 (2) ◽  
pp. 584-599 ◽  
Author(s):  
Bruce Ellingwood ◽  
David Rosowsky
Keyword(s):  
Wood Construction ◽  
Load Effects ◽  
Duration Of Load

Duration of load tests for shear strength

1986 ◽  
Vol 13 (2) ◽  
pp. 188-195 ◽  
Author(s):  
R. A. Spencer ◽  
Borg Madsen
Keyword(s):  
Shear Strength ◽  
Stress Ratio ◽  
Control Sample ◽  
Term Strength ◽  
Short Term ◽  
Current Design ◽  
Short Term Strength ◽  
Load Effects ◽  
Duration Of Load

The strength of wood falls with time under load, and in current design codes the short-term strength of wood is reduced by about 40% to account for duration of load effects. This figure is based on tests made on small bending specimens. In this paper are described tests made on wooden torque tubes to investigate the effect of duration of load on shear strength. A control sample was tested to establish a curve for short-term strength, and four groups of 80 specimens each were then tested under various levels of constant long-term load. Stress ratio at failure was estimated by assuming that the short-term strength of each group could be represented by the control curve, and that under long-term loading specimens would fail in the order of their short-term strength. In each group the stress ratio at failure fell with time under load, and this reduction appears to be related to that predicted by a viscoelastic plastic model. It is concluded that the Madison curve presently used to predict duration of load effects may be conservative at normal levels of applied stress. Key words: wood, shear, long-term loading, duration of load.

Rate-of-Load and Duration-of-Load Effects for Wood Fasteners

1999 ◽  
Vol 125 (7) ◽  
pp. 719-724 ◽  
Author(s):  
David V. Rosowsky ◽  
Timothy A. Reinhold
Keyword(s):  
Load Effects ◽  
Duration Of Load

scholarly journals Duration of load effects on development of deformations in bolted moment connection

2020 ◽  
Author(s):  
Janis Fabriciuss ◽  
◽  
Lilita Ozola ◽  
Keyword(s):  
Knee Joint ◽  
Experimental Test ◽  
Numerical Test ◽  
Moment Connection ◽  
Test Models ◽  
Load Effects ◽  
Joint Connection ◽  
External Section ◽  
Duration Of Load

Experience saved in the construction industry shows that the timber portal frames with semi-rigid connections at knee joint exhibit permanently increase displacements at the knee and apex point. Normally, timber portal frame with semi-rigid knee joint connection is made with mechanical fasteners located in double circles and cannot be designed without relevant rotation at connection during structures’ exploitation time. The only way to increase connection rigidity is to rise distance from fasteners location at connection, but at the same time, the tension and shear stress become significant at the external section of members. The previous experience is obtained by a model testing showing that deformations at semi-rigid connections are non-linear. These were tests under short-term load and did not disclose creep effects, which can be significant. This study is aimed at the examination of increasing deformations with time under constant static load (creep effects) in semi-rigid dowelled connection. Experimental test models were made and set under long-term load in controlled environmental conditions (heated laboratory room). Results show a significant creep influence to decrease connection stiffness. Corresponding numerical test of orthotropic 3d model by Dlubal RFEM software tools was performed analyzing the value of expected deformations. Results of the numerical test showed that friction between timber elements and extra nuts on bolts can increase connection stiffness. Research results in this stage show that the creep can affect connection stiffness more than expected. Also, experimental test results showed lower deformation values comparing with the ones obtained by the numerical test.

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