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Table of Contents
2. Considering the rebar confinement effect
3. Application of rebar confinement in midas Civil
1. Introduction
Concrete shrinks as the cement paste harden. The reinforcements in the section will also serve as a kind of restraint source to prevent concrete from shrinking. For a cantilever rectangular concrete beam, we do not expect a secondary effect. However, if we consider that shrinkage is just happening in concrete and not in reinforcement, then the reinforcement would restrain (confine) the concrete from freely deforming. we can consider the effects restrained by reinforcement for the creep and shrinkage in midas civil
The influence of rebar can be considered in Midas civil for linear and nonlinear analysis. We can define the reinforcement in the section manager for linear analysis and verify the results. We can use fiber material properties to analyse the inelastic time history. In this blog, we are concentrating on linear analysis.
2. Considering the rebar confinement effect
Shrinkage depends on many other factors related to the quality of the concrete and conditions of exposure such as the maximum size of aggregate; type of cement; the amount of cement paste; the notional size of concrete members; the amount of steel reinforcement; and curing conditions.
To consider the effect of restraint on creep and shrinkage in the PSC section we need to follow two steps.
a. Click on the Check box on the consider reinforcement for section stiffness calculation in Analysis tab> Main control data as shown in fig 1.
Fig1. Main control data dialog boxb. Click on the check box for Consider Re-bar Confinement Effect in Construction stage analysis control data as shown in Figure 2.
3. Application of rebar confinement in midas civil
To explain the rebar confinement effect, I have considered an example of a cantilever beam along with the construction stage analysis. We can reinforce the section to analyze the restraining effect of the rebar. I have compared the models with and without the rebar confinement effect.
The dimensions considered are
Depth = 1000 mm
Width = 1000 mm
Span length = 10m
The eccentricity of the reinforcement = 400 mm
Manual Calculation for the shrinkage secondary effect
Fx = - shrinkage strain x Es x As
Es = Modulus of elasticity of steel = 200000 MPa
As = Area of reinforcing steel =4908.7 mm2
The cumulative shrinkage strain is shown in the table below
Shrinkage strain = -3.0743e-04
Axial force (Fx)
Fx = 3.0743e-04 x 200000 x 4908.7
= 301816 N
= 301.8 kN
Bending moment (My)
My = Fx * eccentricity
= 301.8 x 0.4
= 120.7 KN-m
After performing the analysis in the midas Civil, the values of axial force for shrinkage secondary in construction stage 2 for the same model are compared without and with the rebar confinement effect, as shown in fig 4 & 5.
The value of bending moment [My] in the construction stage 2 without and with rebar confinement effect for the shrinkage secondary is shown in figs 6 & 7 respectively.
4. Conclusion
We have seen significant forces and moments in the concrete member due to the restraining effect of reinforcing bars. Therefore, for considering the restraining effect due to the reinforcement, we can use the function of the rebar confinement effect and consider the reinforcement for section stiffness calculation available in the midas civil.
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