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The study guide shall focus on the analysis and design of shear of rectangular beams. The topics include understanding shear stress and shear strength of concrete, calculating stirrups requirement, and understanding the importance of spacing requirements/limitations.
what is a shear failure?
what is a shear failure?
The failures of reinforced concrete beams in shear are quite different from their failures in bending. Shear failures occur suddenly with little or no advance warning. Therefore, beams are designed to fail in bending under loads that are appreciably smaller than those that would cause shear failures. As a result, those members will fail ductilely. They may crack and sag a great deal if overloaded, but they will not fall apart, as they might if shear failures were possible.
TYPES OF SHEAR FAILURE
TYPES OF SHEAR FAILURE
- diagonal tension failure
Diagonal tension failure occur when the shear span is greater than 3d or 4d
2.shear-compression failure
2.shear-compression failure
Shear compression failure occurs when shear span is from d to 2.5 d
3.splitting or true shear failure
3.splitting or true shear failure
Splitting failure occurs when the shear span is less than the effective depth d
BASIC CODE REQUIREMENTS
BASIC CODE REQUIREMENTS
The basic code requirement (Sec 411.2) on shear strength is that the factored shear force Vu shall be equal or less than the design (phi) Vn.
For beams with no reinforcement, the shearing force causes the first diaognal cracking to be taken as the shear capacity of the beam. For beams that does contain web reinforcement, the concrete is designed to carry a constant amount of shear force Vc and the shear reinforcement need only to be designed for the shear force Vs in excess of that carried by the concrete.
LIGHTWEIGHT CONCRETE AND MODIFICATION FACTOR
LIGHTWEIGHT CONCRETE AND MODIFICATION FACTOR
Provisions for shear strength Vc in the chapter apply to normal weight concrete. When lightweight concrete are used, one of the following modifications shall apply: When fct is not specified, all values of sqrt(f'c) affecting Vc, Tc, and Mcr shall be multiplied by modification factor lambda, where lambda is equal to 0.85 for sand-lightweight and 0.75 for all-lightweight concrete.
For normal weight concrete, lambda=1.0
SHEAR STRENGTH PROVIDED BY CONCRETE
SHEAR STRENGTH PROVIDED BY CONCRETE
SHEAR STRENGTH PROVIDED BY REINFORCEMENT
SHEAR STRENGTH PROVIDED BY REINFORCEMENT
spacing limits of shear reinforcement
spacing limits of shear reinforcement
The spacing of shear reinforcement placed perpendicular to axis of members shall not exceed d/2 in non pre-stressed members and 0.75 h in prestressed members, nor 600mm. When Vs exceeds 0.33 sqrt(f'c)bwd, maximum spacing given by the above limits shall be reduced by one half.
STEPS IN VERTICAL STIRRUP DESIGN
STEPS IN VERTICAL STIRRUP DESIGN
SUPPLEMENTARY MATERIALS
SUPPLEMENTARY MATERIALS
20221121 - CENG132 Module 4.pdf20221126 - CENG132 Module 4b.pdfACTIVITY/COURSEWORK
ACTIVITY/COURSEWORK
Activity 1 (CENG132)_BARTOLOME (1).pdfCoursework #4-5 (CENG132_BARTOLOME).pdfREFLECTION:
REFLECTION:
This module entailed NSCP's provisions on design shear values in order to prevent sudden shear failures in reinforced concrete beams, aside from bending failures. After answering the courseworks and activity, I learned that shear failures occur suddenly with little or no advance warning. This is because beams are typically designed to fail in bending under loads that are appreciably smaller than those that would cause shear failures. As a result, those members will fail ductilely. And since tension stresses in beams are a serious matter, diagonal principal tensile stresses may occur in different places in concrete beams. Therefore, additional reinforcing such as web reinforcing is highly important in such scenario. Also, I found out that there are inclined cracks which can develop in the webs of reinforced concrete beams, that exist either as extensions of flexural cracks or occasionally as independent cracks. Additionally, as a crack moves up to the neutral axis, the result will be a reduced amount of concrete left to resist shear—meaning that shear stresses will increase on the concrete above the crack. In such case, the member will fail otherwise the cracked concrete section can resist the applied forces. That is the reason why web reinforcement is very important in designing of beams based on shear.
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