cve60069
Civil/Environmental
- May 1, 2010
- 84
Hello
I am in the process of 'converting' to the eurocodes, notably eurocode 5: Timber; and I am having trouble understanding the principles of deflection of a beam. I attended a seminar and have two books explaining the principles and each source describes the method differently with differing results.
Before eurocode, I would calculate the deflection for a udl as [bending = 5/384*(W.L^4)/E.I] + [creep = M.Kform/A.G] where Kform = 1.2 and G = E/16. The creep factor was for long-term effects.
The eurocode states simply that the instantaneous deflection is found for unfactored permanent and variable loads and each result multiplied by (1+kdef) and (1+'quasi-permanent factor'*kdef) respectfully to obtain the final deflection. The code makes no mention of what the instantaneous deflection is other than a mention of using mean values for elasticity and shear moduli.
I attended a seminar at TRADA and the lecturer calculated the instantaneous deflection for a udl as [5/384*(W.L^4)/E.I + M.Kform/A.G] for each of the unfactored permanent and variable actions; and then calculated the final deflections by multiplying the permanent action by (1+kdef) and the variable action by (1+'quasi-permanent factor'*kdef) and summing to get the final deflection. Please note that the lecturer included the creep factor [M.Kform/A.G] in his analysis.
I have two books at my disposal: 'Structural Timber Guide to Eurocode 5: Porteous and Kermani' and 'Manual for the design of timber building structures to Eurocode 5: Institute of Structural Engineers' and both books offer different advice.
The Structural Timber Guide to Eurocode 5 explains that kdef is a factor that takes into account creep and the factor increases the instantaneous deflection by 60%. The quasi-permanent-factor is normally 0.3 so the variable deflection is increased 18%. When I consider the book examples for a joists (page 150), the authors get bogged down in mathcad and it is very difficult to follow the logic, particulary when amplification factors are discussed which I associate with vibration. The authors do, however, include creep in the instantaneous deflection calculation.
The Manual for the design of timber building structures to Eurocode 5 gives less information but rewrites the formula in the eurocode so that creep is treated separately from the instantaneous deflection (page 22).
I am totally confused. To me, the instantaneous deflection is [5/384*(W.L^4)/E.I] and the final deflection includes the creep factor, whether it be [M.Kform/A.G] or (1+kdef) and (1+'quasi-permanent factor'*kdef); The creep factor should not be assessed at the instantaneous stage as it would be assessed twice.
Any advice or references would be very much appreciated.
I am in the process of 'converting' to the eurocodes, notably eurocode 5: Timber; and I am having trouble understanding the principles of deflection of a beam. I attended a seminar and have two books explaining the principles and each source describes the method differently with differing results.
Before eurocode, I would calculate the deflection for a udl as [bending = 5/384*(W.L^4)/E.I] + [creep = M.Kform/A.G] where Kform = 1.2 and G = E/16. The creep factor was for long-term effects.
The eurocode states simply that the instantaneous deflection is found for unfactored permanent and variable loads and each result multiplied by (1+kdef) and (1+'quasi-permanent factor'*kdef) respectfully to obtain the final deflection. The code makes no mention of what the instantaneous deflection is other than a mention of using mean values for elasticity and shear moduli.
I attended a seminar at TRADA and the lecturer calculated the instantaneous deflection for a udl as [5/384*(W.L^4)/E.I + M.Kform/A.G] for each of the unfactored permanent and variable actions; and then calculated the final deflections by multiplying the permanent action by (1+kdef) and the variable action by (1+'quasi-permanent factor'*kdef) and summing to get the final deflection. Please note that the lecturer included the creep factor [M.Kform/A.G] in his analysis.
I have two books at my disposal: 'Structural Timber Guide to Eurocode 5: Porteous and Kermani' and 'Manual for the design of timber building structures to Eurocode 5: Institute of Structural Engineers' and both books offer different advice.
The Structural Timber Guide to Eurocode 5 explains that kdef is a factor that takes into account creep and the factor increases the instantaneous deflection by 60%. The quasi-permanent-factor is normally 0.3 so the variable deflection is increased 18%. When I consider the book examples for a joists (page 150), the authors get bogged down in mathcad and it is very difficult to follow the logic, particulary when amplification factors are discussed which I associate with vibration. The authors do, however, include creep in the instantaneous deflection calculation.
The Manual for the design of timber building structures to Eurocode 5 gives less information but rewrites the formula in the eurocode so that creep is treated separately from the instantaneous deflection (page 22).
I am totally confused. To me, the instantaneous deflection is [5/384*(W.L^4)/E.I] and the final deflection includes the creep factor, whether it be [M.Kform/A.G] or (1+kdef) and (1+'quasi-permanent factor'*kdef); The creep factor should not be assessed at the instantaneous stage as it would be assessed twice.
Any advice or references would be very much appreciated.
