Shear Strength of Joints in Precast Concrete Segmental Bridges

Shear Strength of Joints in Precast Concrete Segmental Bridges. Paper by Xiangming Zhou, Neil Mickleborough, and Zongjin Li

In their paper, the authors showed some interesting results of load tests of specimens with single and multiple shear keys. It may be surprising that the design for shear in segmental bridges is still not clarified whereas many such bridges have already been built.

The discussers would like to add some aspects that are not fully taken into account in this paper. First, our design formula is not based on numerical analyses only. Tests on single and multiple keys, both dry as well as glued, were done to calibrate the finite element model. The behavior of the joint and the ultimate load were well predicted. For this reason, the model is used to develop a design formula.

The problem of design formulas is to consider the situation on site that always results in imperfections. This is done by safety coefficients. While developing the design formula effects of imperfections are investigated numerically, it can be shown that imperfections have a great importance on the shear capacity of joints. Especially for dry loads, the contact between the surfaces is important to transfer the loads. For this reason, a relatively high value for the safety coefficient should be used. The discussers suggest a coefficient of γ = 2.0.

The authors confirm the importance of imperfections because the measured shear capacity of keys in three-keyed joints was always lower than those in single-keyed joints. This is true for dry joints. Using epoxy, the influence of imperfections is reduced because of a more uniform distribution of stresses among the keys. However, epoxy joints have many other problems, such as an improper use on site or very brittle failure behavior. The authors show that shear capacity decreases when the epoxy thickness amounts to more than 2 mm. This cannot be controlled on site. For this reason, the discussers' design formula is not valid for epoxy joints. To consider the problems, it is necessary to reduce the ultimate shear capacity of epoxy joints. Also, the formula used in AASHTO is valid for dry joints only. Therefore, for epoxy joints, the discussers' "dry" formula should underestimate the ultimate shear strength because of the improved load distribution by using epoxy joints as opposed to dry joints. As it can be seen, this is true until the thickness exceeds a value of 2 mm, but these improvements cannot be considered in calculations because of the problems that may occur on site.

For dry joints, the authors show that the shear capacity of keys in three-keyed joints is lower than for single-keyed joints. The discussers' formula and the AASHTO formula are based on a linear correlation between the number of keys and the maximum shear capacity. For this reason, it is possible that both our formula as well as the AASHTO formula underestimate the shear capacity of single-keyed dry joints but overestimate the shear capacity of multiplekeyed dry joints. Because our formula is more conservative than the AASHTO formula, the overestimation of our formula (3.2 to 53.7%) is much less than that of the AASHTO formula (19.7 to 61.8%). The decrease in shear capacity of keys in three-keyed joints can be due to the fact that the number of imperfections is higher when using multiple keys because the probability of defects is higher. In the discussers' formula, imperfections are considered by using a safety coefficient. In future work, it would be interesting to specifically investigate the effects of imperfections to define an economic safety coefficient. By using a value of γ = 2.0, the discussers' formula is able to calculate the shear capacity sufficiently.

Discussion by Guenter Rombach and Angelika Specker

Professor, Technical University of Hamburg-Harburg, Germany; Doctorate, Boeger + Jaeckle Consultant Engineers, Germany

Copyright American Concrete Institute Nov/Dec 2005
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