Chapter 6 - SLS Deflection and Crack Control Flashcards
Why should compressive stresses in concrete be limited?
To avoid longitudinal cracking, microcracks and excessive creep. All which accelerate material deteriation.
Why should tensile stresses in steel be limited?
To avoid inelastic strains, unacceptable cracking and unacceptable deformation.
Why is the maximum crack width limited?
So that the cracking doesn’t impair the proper functioning or durability of a structure.
How is minimum area of reinforcement used to control cracking?
To control cracking after the the first crack. It’s used to try and have lots of small cracks rather than allowing cracks to propogate.
It ensures that when concrete cracks, it can smoothly transfer its stresses to steel without further propogation.
What is the equation for deflection at midspan of a simply-supported beam? (State what each term means)
v = (5qL^2)/(384EI)
Where:
q = UDL on the beam.
L = Span of the beam.
E = Elastic modulus of material.
I = Second moment of area.
What is the equation for total deflection?
δ = ζδc + (1-ζ)δu
Where:
ζ = 1 - β*(Mcr/M)^2
What are the 2 possible β values and in which scenarios should you use each one?
β = 0.5
Used for sustained or cyclical loading.
β=1.0
Used for a single short term loading.
What is tension stiffening and how does it impact the shape of the moment-curvature diagram?
Tension stiffening is when the stiffness of a concrete section increases due to the transmittion of stresses from reinforcement bars to the concrete between adjacent cracks.
Causes the gradient of the EIc section of the graph to be shallower due to inceased stiffness. This line starts to the left of the normal diagram but both finish at the yield point.
What is creep and how is it’s effects modelled in analysis?
Creep is permanent deformation of concrete due to long-term loading.
It is modelled as a reduction in the elastic modulus of concrete.
What is shrinkage, how is it’s effects modelled in analysis and how does it impact the shape of the moment-curvature diagram?
Shrinkage is additional deflections due to drying or chemical reactions.
It is modelled as an additional curvature without an increase in the loading.
It’s impact on the moment-curvature diagram is that it shift the entire diagram to the right (increases curvature).
