Chapter 9 Serviceability Limit States Cracking
Chapter 9 Serviceability Limit States ——Cracking
10.1 Concepts of Serviceability Limit States Design 10.1.1 Serviceability limit states In general,the most important serviceability limit states include Deflection-the appearance of a structure or any part of a structure must not be adversely affected by deflections. Cracking-the appearance or durability of a structure or any part of a structure must not be adversely affected by any cracking of the concrete. Durability -this must be considered in terms of the proposed life of a structure and its conditions of exposure
10.1 Concepts of Serviceability Limit States Design 10.1.1 Serviceability limit states In general, the most important serviceability limit states include Deflection – the appearance of a structure or any part of a structure must not be adversely affected by deflections. Cracking – the appearance or durability of a structure or any part of a structure must not be adversely affected by any cracking of the concrete. Durability – this must be considered in terms of the proposed life of a structure and its conditions of exposure
Currently,excessive flexural deflection and excessive flexural cracking must normally be considered in design
• Currently, excessive flexural deflection and excessive flexural cracking must normally be considered in design
Other serviceability limit states that may be reached include Fire resistance-this must be considered to flame penetration and heat transfer Excessive vibration-causing discomfort or alarm as well as damage. Fatigue-this may be considered if cyclic loading is likely. Special circumstances-any special requirement, such as earthquake,explosion (nuclear ~thermal )impact,etc
Other serviceability limit states that may be reached include Fire resistance – this must be considered to flame penetration and heat transfer Excessive vibration – causing discomfort or alarm as well as damage. Fatigue – this may be considered if cyclic loading is likely. Special circumstances – any special requirement, such as earthquake, explosion (nuclear ~, thermal ~), impact, etc
10.1.2 Load combination Load combination for serviceability limit states =1.0Gk+1.0Qk In most cases,design of reinforced concrete will be based on ultimate limit state requirements,and serviceability behaviour will be considered as secondary check to ensure satisfactory performance under working conditions Short term loads combination 荷载效应的标准组合 Gk+Ok Long term loads combination 1荷载效应的准永久组合 Gk+Vqi Ok
10.1.2 Load combination Load combination for serviceability limit states = 1.0Gk + 1.0Qk In most cases, design of reinforced concrete will be based on ultimate limit state requirements, and serviceability behaviour will be considered as secondary check to ensure satisfactory performance under working conditions Short term loads combination Gk+Qk Long term loads combination Gk+ Qk 荷载效应的标准组合 荷载效应的准永久组合 qi
10.2 Reasons to cause cracking Loading
10.2 Reasons to cause cracking Loading
Shrinkage Temperature heat Uneven deformation of ground base Corrosion of reinforcement Rebar
Shrinkage Corrosion of reinforcement Temperature heat Uneven deformation of ground base Rebar
10.2 Unfavorable effects of concrete cracking Corrosion of reinforcement Leakage Decrease stiffness and increase deflection Insecurity feeling of human being etc
10. 2 Unfavorable effects of concrete cracking Corrosion of reinforcement Leakage Decrease stiffness and increase deflection Insecurity feeling of human being etc
10.3 Acceptable limits in cracking control There are three grades in cracking control The first grade-Cracking is forbidden Osc≤0 The second grade Short term loading oc≤f C,=0.31.0 Long term loading Oc≤0 y-the section modulus plastic coefficient 截面抵抗矩塑性发展系数 The third grade≤[ol +0.20.4mm
10. 3 Acceptable limits in cracking control There are three grades in cracking control The first grade——Cracking is forbidden 0 SC SC ct t f 0 LC 0.3～1.0 ct The second grade Short term loading Long term loading －the section modulus plastic coefficient 截面抵抗矩塑性发展系数 The third grade max 0.2～0.4mm
10.4 Cracking inner force f For uniaxial tension: N,=(1+贷AM=fA 89
10. 4 Cracking inner force For uniaxial tension: 0 ( A ) f f A n N cr A c s t t t , p t f