Chapter 5 - Design of RC Beams in Shear Flashcards
What type of failure is shear failure?
Shear failure is a brittle failure.
What is a shear span and what is its symbol?
Shear span (av) is the distance between a point load and a support.
What is shear span ratio and what is its symbol?
Shear span ratio (av/d) is the ratio between the shear span and the beam’s depth.
What are the 4 beam behaviours in shear and what are their shear span ratio values?
0 < av/d < 1 = very short
1 < av/d < 2.5 = short
2.5 < av/d < 6 = slender
6 < av/d = very slender
What does the ‘trench’ represent a moment-shear span ratio diagram represent?
The ‘trench’ represents the difference between flexural capacity and shear capacity (without reinforcement) of a section against the shear span ratio.
The bottom of the trench aka. when the difference in flexural and shear capacity is highest is when av/d is roughly 2.5.
What are the forces that need to be transfered by a crack?
The dowel action of longitudinal reinforcement due to double curvature of rebar.
Aggregate interlock.
Shear in the compression zone.
Describe the design process to work out if shear rebar is required in a section?
- Calculate the shear resistance of concrete (VRd,c).
- Calculate the minimum shear resistance of concrete (VRd,cmin).
- If VRd,cmin > VRd,c then take this as the new VRd,c value.
- If VRd,c > VEd, no shear reinforcement is required.
If VRd,c < VEd, shear reinforcement is required.
Describe the shear reinforcement design process.
- Calculate VRd,max for cotθ = 2.5. If VRd,max < VEd, the section needs redesigning.
- Calculate VRd,max for cotθ = 1. If VRd,max > VEd, use cotθ =1 for further calculations. If VRd,max < VEd, calculate the value of θ using equations given.
- Use the angle calculated in step 2 to calculate Asw/s.
- Assume a 150mm or 200mm spacing to calculate Asw.
Describe the failure mode for ‘very short’ beams in shear.
Inclined cracking between the point load and support (across the shear span).
Failure of the compressive strut.
Describe the 2 failure modes for ‘short’ beams in shear.
- Shear-tension failure
Final failure caused by bond failure along the tension rebar. Inclined cracks are developed. - Shear compression failure
Caused by crushing of compression zone under the point load. Big half-rainbow style cracks.
Describe the failure mode for ‘slender’ beams in shear.
Shear-flexural cracks become more flattened as you more towards the top of the section (due to meeting more resistance) and the closer to the point load you get.
Describe the failure mode for ‘very slender’ beams in shear.
The beam will fail in flexure.
