INTRODUCTIONIt was interesting to read this paper and together with the previous work published in 1998 it is very useful in highlighting the importance of restraint stiffness. However, I think the conclusion that the restraint force should be increased from 1%-2·5% is too conservative and not justified by a closer examination of the numbers. Providing insufficient stiffness could also lead to bracing systems that are not stiff enough to restrain the member.The use of 2·5% is similar to that in the Australian code AS4100-1998 that specifies a 2·5% restraint force and assumes that any system capable of taking this load will have sufficient stiffness. The older version (AS1250) did specify a stiffness required. However, 1% is also used by the AISC LRFD of 1999 which specifies a restraint stiffness required. The British Standard is perhaps unusual in not specifying the stiffness required although guidance is given in the SCI publication 093.The purpose of the restraint is to reduce the effective length of the strut to a length based on the distance between the braces. If it is not stiff enough to do this, the strut may be underdesigned. Both the AS1250 and the LRFD require factors on the minimum stiffness from an elastic critical analysis. The LRFD is based on the assumption that the strut is pin-ended between flexible retraints and the factor required is 2. The SCI also mentions the need for a stiffness significantly higher than the minimum required, with a criterion based on the strut stiffness that equates to a factor greater than 7.Concerning the example given in appendix 1 of the paper. I have looked at the derivation of the strut values using both a 'hand' method and second-order analysis and I do not get the same asnswers as the design example. Further explanation is given below.The example does, however, show the need to have adequate stiffness. If the stiffness is limited to the value given as being the minimum consistent with the 2·5% restraint force, the strut will not be restrained at mid-span. I carried out a buckling analysis that gives a critical buckling load for the strut of about 8600 kN. This equates to an effective length of 6·9 m and a strut capacity of about 5300 kN-that is, less than the applied load. The actual stiffness applied (5 : 45 kN=mm) gives a critical load of about 16 000 kN. The mode still includes a displacement at mid-span but because of the end stiffness the equivalent effective length is close to 5 m. In practice this stiffness is lower than that which should typically be applied.For a pin-ended strut, Timoshenko and Gere show that the minimum mid-span restraint stiffness required to give buckling in the two halves is such that k 3 L=P e is 16. With this stiffness, the L=500 initial imperfection and P=P e ¼ 2, the restraint force required is only 1·3%. Putting a factor of 2 on the stiffness reduces the restraint force to 1·1%. The 1% factor in BS 5950 8 could be considered to be slightly low but if a slightly tighter imperfection limit, say 1 in 600, were assumed, it would be a...
This study presents a detailed analysis of the lateral forces generated as a result of vertically applied loads to recycled plastic drainage kerbs. These kerbs are a relatively new addition to road infrastructure projects. When concrete is used to form road drainage kerbs, its deformation is minimum when stressed under heavy axle loads. Although recycled plastic kerbs are more environmentally friendly as a construction product, they are less stiff than concrete and tend to deform more under loading leading to a bursting type, lateral force being applied to the haunch materials, the magnitude of which is unknown. A method is proposed for establishing the distribution of these lateral forces resulting from deformation under laboratory test conditions. A load of 400 kN is applied onto a total of six typical kerbs in the laboratory in accordance with the test standard. The drainage kerbs are surrounded with 150 mm of concrete to the front and rear haunch and underneath as is normal during installation. The lateral forces exerted on the concrete surround as a result of deformation of the plastic kerbs are determined via a strain measuring device. Analysis of the test data allows the magnitude of the lateral forces to the surrounding media to be determined and, thereby, ensuring the haunch materials are not over-stressed as a result. The proposed test methodology and subsequent analysis allows for an important laboratory-based assessment of any typical recycled plastic drainage kerbs to be conducted to ensure they are fit-for-purpose in the field.
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