Question 1. Determine which best describes the type of of crack in different reg

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Question 1.Determine which best describes the type of of crack in different regions of a prestressed concrete beam.

1. Moment is large and shear is small:

A.Vertical flexural cracks B.Web shear cracks C.Inclined cracks

2.Moment is small and shear is large:

A.Vertical flexural cracks B.Web shear cracks C.Inclined cracks

3. Both the moment and shear force are relatively large:

A.Vertical flexural cracks B.Web shear cracks C.Inclined cracks

Question 2. Which of the following contributes positively to shear strength of prestressesd concrete structures?

select one or more:

a. vertical stirrups

b. prestressing force

c.longitudinal tensile steel

d.aggregate interlock

e.uncracked concrete

f. tensile force

Question 3. In the truss analogy use to analyse the behaviour of concrete, which represents the members of the analogous truss:

1. vertical member:

A. stirrups/shear reinforcement   B. concrete compressive zone+ longitudinal compressive reinforcement C. longitudinal prestressing steel+ non-prestressed reinforcement in the tensile zone

2. top compressive chord:

A. stirrups/shear reinforcement   B. concrete compressive zone+ longitudinal compressive reinforcement C. longitudinal prestressing steel+ non-prestressed reinforcement in the tensile zone

3. bottom chord:

A. stirrups/shear reinforcement   B. concrete compressive zone+ longitudinal compressive reinforcement C. longitudinal prestressing steel+ non-prestressed reinforcement in the tensile zone

Explanation / Answer

1. When the tensile strength of concrete is exceeded, cracks will be formed.

Shear stress caused by the shear forces can result in principle tensile stress at the critical section which can exceed the tensile strength of the concrete.Cracking in prestresses concrete beams at ultimate load depends on the local magnitudes of moment and shear.

1) In regions where the moment is large and shear is small, vertical flexural cracks appear after the normal tensile stress in extreme concrete fibres exceeds the tensile strength of the concrete.

3) When the moment and shear are relatively large, flexural cracks which are vertical at extreme fibres become inclined as they extend deeper into the beam owing to the presence of shear stresses in the beam web.

2) Another type of inclined cracks in the mid- depth region ( where the moment is small and shear is large) of the beam occurs due to high principle tensile stresses before flexural cracking occurs in the extreme fibres. Such cracks are generally called web-shear cracks, most often in beams with relatively thin webs.

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