Slabs

Question 1

What is a slab?

Answer 1

A structural element that is used to create flat horizontal surfaces such as floors, roof decks and ceilings

Question 2

From one way slabs, what must emax / emin be?

Answer 2

Greater than 2

Question 3

For 2 way slabs, what must emax / emin be?

Answer 3

Less than or equal to 2

Question 4

What are the advantages of one way slabs?

Answer 4

Primary reinforcement in one direction, and simpler construction

Question 5

How do slabs at the top surface fail?

Answer 5

The slabs crack over the continuous supports where the hogging moments are the largest

Question 6

How do slabs at the bottom surface fail?

Answer 6

The slabs crack at mid-span where the sagging moments are the greatest

Question 7

What are the advanantages of 2 way slabs?

Answer 7

More efficient structure, thinner slabs, lighter structure, longer spans

Question 8

How do 2 way slabs work?

Answer 8

Primary reinforcement in 2 directions
Find reinforcement per metre run in each directions

Question 9

What is the main failure mode of 2 way slabs?

Answer 9

The main failure mode is bending, when the bending stesses exceed the slab’s capacity

Question 10

In two way slabs, where does cracking typically occur?

Answer 10

Near the mid-span or the regions near supports, depending on the loading patter and the slab aspect ratio.

Question 11

Why are 2 way slabs more susseptible to shear failure?

Answer 11

The shear force is distributed in both direction, diagonal cracks can form, typically around the column supports.

Question 12

Where is punching shear failure common in a two way slab?

Answer 12

Around column connections

Question 13

The most common form of elastic analysis is usng computer software, e.,g. finite element anaylysis. What are the advanatages and disadvantages of this?

Answer 13

Easy to use, quicl and detailed results.

The disadvantages involve, boundary condtions influence solution forindeterminate structures. Unrealistic localised high stresses close to fixed boundaries. Solution can be dependant on if material’s mon-linearity is included to predict failure modes.

Question 14

What is yield line analysis?

Answer 14

A plastic method (upper bound) of analysis for slab and plate like structures.

Yield lines are the equivalent for slabs of pastic hinges in beams

Question 15

In terms of yield line analysis, when do collapse mechanisms form?

Answer 15

When there are sufficient yield lines in an appropriate pattern for the slab to collapse.

Question 16

What are the advanatages of yield line analysis?

Answer 16

-Straight forward
-Takes account of collapse mechanism#
-Understanding of ultimate behaviour
-Can be applied to non-regular shapes

Question 17

What are the disadvantages of yield line analysis?

Answer 17

-Requires more understanding of slab behaviour
-More challenging to implement in computer mode
-Provides an upper bound, so potentially unsafe and dangerous designs are possible.

Question 18

In yield line analysis, what does a single hatch mean?

Answer 18

It is a simple support.

Question 19

In Yield line analysis, what does a double hatch mean?

Answer 19

It is a fixed support

Question 19

In yield line analysis, what is mx?

Answer 19

Bending moment about the y-axis

Question 20

In yield line analysis, what is my?

Answer 20

Bending moment about the x - axis.

Question 21

What is punching shear failure?

Answer 21

Local shear failure around a concentrated load on a slab and it involves a truncated cone or pyramid-shaped surface around the column.

Question 22

When can punching shearoccur?

Answer 22

When a large point force is applied to a slab or where a slab surrounding the column is unable to resist the shear force and the column punches through.

Question 23

When will felxural failure occur in a column slab section.

Answer 23

Through a combination of ‘hogging’ cracks on the surface between supports and the ‘sagging’ cracks growing diagonally through the supports.

Question 24

What is the difference between the flexural fail of a column/slab section and punching shear?

Answer 24

Punching shear is likely a sudden brittle fail mechanism, it is highly undesirable.

The nature of flexural failure depends on the reinforcement arrangement. If appropriately under-reinforced, failure can be ductile, which is heavily preferred.

Question 25

What is the mechanism of punching shear?

Answer 25

The shear failures rotated around the loaded area give it a failure surface

Question 26

Where should the shear resistance be checked?

Answer 26

At the face of the column and at the basic control perimeter u1

Question 27

What are the 2 checks involved with punching shear calculation?

Answer 27

Check 1 : At the face of the column, if vED < V,Rd,max no changes in the section of the slab. If VEd > vrD,MAX, change the height of the slab.

Check 2 : At the basic p[erimeter (2d from the column face) If VEd < Vrd,c no shear reinforcement needed. If Ved > Vrd,c shear reinforcement is needed.