# Deflection Control

Deflection control is done automatically.

**ICONS**

**E _{c}**

**Modulus of elasticity of concrete**

_{ }=

**f**_{ctd}**Design tensile strength of concrete**

_{ }=

**I**_{cr}**Cracked sectional moment of inertia relative to the neutral axis**

_{ }=

**I**_{ef}**Effective moment of inertia**

_{ }=

**I**_{c}**Gross sectional moment of inertia**

_{ }=

**l**_{n}**Clear span of the member**

_{ }=

**M**_{cr}**Cracking moment of member under bending**

_{ }=

**M**_{max }**Maximum bending moment for member**

_{ }=**y**Distance between extreme tension fiber and neutral axis

_{ }=*= Total deflection*

**δ**_{t}*= Instantaneous deflection*

**δ**_{i}*= Instantaneous deflection due to permanent loads*

**δ**_{ig}*= Time-dependent factor for permanent load*

**γ**_{t}*= Permanent deflection multiplier*

**λ***= Ratio of compression reinforcement*

**ρ'****13.2 - DEFLECTION CONTROL**

**13.2.1 - General Regulations**

In members subjected to flexures, such as slabs and beams, deflections that would impair their function,

affect their appearance, and cause cracking or crushing in adjacent non-structural connected members

should not be allowed to occur. Immediate deflections due to permanent and live loads and deflections due

to shrinkage and creep effects for these members should be computed by considering their cracked state.

In the case of beams and especially slabs, which do not support or are not attached to nonstructural

members sensitive to deflections and whose depth to span length ratios are within the limits given in **Table****13.1**, deflection calculations need not be carried out.

If more reliable methods based on the principles of structural mechanics, which also take reinforced concrete

behavior into account is not used, the immediate deflections can be calculated by using the approximate

the method given in **Clause 13.2.2** and the time-dependent deflections by the method presented in **Clause****13.2.3**.

**13.2.2 - Approximate Calculation for Instantaneous Deflection**

Instantaneous deflection values of reinforced concrete flexural members under permanent and live loads

with uncracked sections in the span **(M _{max} ≤ M_{cr})** should be calculated in accordance with structural

mechanics principles by using the gross sectional moment of inertia.

For cracked sections

**(M**, the instantaneous deflection should be calculated as given above using

_{max}> M_{cr})the effective moment of inertia values obtained from

**Equation 13.1**and by considering support conditions.

The modulus of elasticity

*to be used in this calculation should be taken from*

**E**_{c}**Section 3.3**.

The cracking moment for the section should be calculated in accordance with **Section 13.2**. For continuous

beams and slabs, the two-moment of inertia values of the span section and the support section (average of

two supports) should be calculated separately by using** Equation13.1** and then the average of these two

values should be used as the effective moment of inertia. In the case of cantilevers, the moment of inertia of

the support section should be used as the effective moment of inertia.

**13.2.3 - Time-Dependent Deflection Calculation**

Calculation of additional long-term deflections due to shrinkage and creep in reinforced concrete structures

should be made in accordance with **Clause 3.3.4**. If more precise calculations are not required, the total

deflection, including the time-dependent deflection can be obtained by using **Equation 13.3.**

The coefficient **γt** in the equation should be taken from **Table 13.2 **according to the effective time of the permanent loads. * ρ'* is the ratio of compression reinforcement in the section.

The coefficient, **γt**, in the above equation, which depends on the duration of the permanent loading, can be

obtained from **Table 13.2**. * ρ'* is the ratio of compression reinforcement in the section.

**13.3 - Deflection Limits**

Allowable maximum deflections for flexural members are given in **Table 13.3**: ln is the clear span.

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