CIVE10029 Concrete Structures 4

Flashcards of Key Aspects of Materials in Reinforced Concrete Design, Sectional Analysis, Shear in Concrete, More Beam Design, Concrete Framed Structures and Yield Line Analysis of Slabs

Table 3.1

What is the key source for concrete material properties according to the Eurocode?

Normal strength concretes are £C50/60, while high strength concretes are >C50/60

What is the main difference between normal strength concretes and high strength concretes, according to the notes?

Cube fck,cube, cylinder fck, and mean fcm compressive strengths

What strengths are listed in Table 3.1?

A parabolic stress-strain curve, defined by ec1 and ecu1

What type of stress-strain curve does EC2 allow for concrete in compression?

A simplified rectangular concrete stress block

For ultimate limit state design, what shape of concrete stress block is used?

Height, effective strength, and limiting strains

Name three parameters which define the simplified rectangular stress block.

It allows for the difference between bending strength and the cylinder strength of concrete, and long term effects

What is the purpose of the factor acc in determining the design compressive strength of concrete?

0.85 for flexure and 1.0 for shear.

What is the value of acc in the UK for flexure and shear?

Partial safety factor for concrete strength

What does gc represent in concrete design?

A horizontal yield plateau

What stress-strain curve should be used for reinforcing steel according to §3.2.7?

fyk=500MPa

What yield strength is most commonly used for the reinforcing steel in the UK reinforcement industry?

Partial safety factor for reinforcing steel strength

What does gs represent in steel design?

6, 8, 10, 12, 16, 20, 25, 32, 40

List common standard diameters of reinforcement bars (mm):

To ensure sufficient ductility

Why is the strain (deformation) capacity of the reinforcing steel important?

A, B, C

What are the three classes of reinforcing steel specified by EC2?

Class A is rarely used, except for welded fabric mesh; Class B and C are commonly used

Describe typical uses of the different classes of reinforcing steel in the UK.

B25 = a 25mm diameter bar of class B reinforcement

Give an example of how reinforcing steel is specified on drawings using prefixes defined in BS 8666:2005.

Transmit bond forces, provide durability, allow for construction tolerance, and provide fire resistance

What are the purposes of concrete cover?

The minimum required cover (cmin) and allowance for deviations (Dcdev)

What components make up the total nominal cover (cnom)?

Carbonation, chloride attack, or freeze-thaw action

Name three of the degradation mechanisms the cover required for durability protects against.

The concrete will split when load is applied to the reinforcement

What happens if the concrete cover is too small?

BS EN 1992-1-2:2004, “Eurocode 2: Design of concrete structures - Part 1-2: General rules - Structural fire design”

In the UK, the Eurocode that describes the fire design requirements for concrete is …

The ratio of the area of the reinforcing steel to the area of the complete cross section

What is the reinforcement ratio?

The steel area is relatively small, and hence the neutral axis (x) is towards the top of the beam.

In a singly reinforced concrete beam, what is the primary factor that defines an under-reinforced beam?

The ultimate strain of the concrete in compression

What governs the failure mode in over-reinforced beams?

The steel does not yield and the failure is brittle

Why should over-reinforced sections be avoided?

The steel yield strain and the limiting concrete strain are reached simultaneously.

For a balanced section, what conditions are met simultaneously?

Smaller values of xbal are specified to ensure that in an under-reinforced section the steel starts to yield well before the limiting concrete strain is reached.

What range of values have been specified to ensure that a section is under-reinforced.

Compression reinforcement will need to be introduced.

If the required moment cannot be achieved via increasing depth, or reinforcement in a singly reinforced section, what should be considered?

Assume that the top fibre of the concrete has reached the limit strain (ecu); then determine the neutral axis depth (x) from axial equilibrium; then check found steel strains. If an assumed layer of steel is not yielding (es < ey), start analysis again but assuming that steel is elastic. Once a valid strain distribution has been found, the neutral axis depth can be used to determine the Moment of Resistance (MRd) of the section.

Describe the general method to find the moment capacity of a beam.

Fc

Take moments about … for a simply reinforced beam.

Whether it is under reinforced (x < xbal)

What checks should be performed for a simply reinforced beam?

To determine crack widths, or for deflection purposes at SLS

State an instance when analysis is required before the ULS (Ultimate Limit State)

To be able to start with the concrete stress blocks, and not simply refer to the equations below which are derived from BS8110.

When using the old British Standards to design singly reinforced sections, what should a designer focus on with concrete?

The lever arm between the concrete force and the tensile steel, z

What is considered to find the area of required steel for a specific moment of resistance?

Tension Steel

To find the moment of resistance, take moments around which piece of steel?

The strain and stress blocks

What is examined to determine the balanced section moment and area of tension steel required for a balanced section?

Whether it is at yield or not by examining the strain diagram.

To determine the area of compression steel required, what should be checked first?

In one direction onto supporting beams

In a one-way spanning slab, in which direction does a slab transfer load?

To modify the axial equilibrium equation to take into account the axial force N¹0.

In sectional analyses when determining axial load and moment, what modification is required?

To take care about the position where we take moments

In M-N interaction diagrams, what is required when axial force is present?

If Fs1 is in compression

When does the analysis have to check whether steel is elastic or plastic?

The concrete pure compression limit

Under higher compressive force, what is the limit on the strain in the concrete?

By a computer

How can an iterative design be assisted?

Where the eccentricity, e = M / N > h/2 – d2

In a simplified design method, in what instance can the method be applied?

Weakness in tension

In concrete, what causes diagonal shear cracks form and propagate along the compressive principal stress trajectories?

Stirrups or links that wrap around the longitudinal reinforcement.

What is one form of shear reinforcement?

Shear within the compression zone of the concrete; “aggregate interlock” between the surfaces of a crack; “dowel action” (shear) of the flexural reinforcement; and the shear stirrups.

What are the load transfer mechanisms within a cracked beam?

The shear strength of a concrete beam without shear reinforcement can be calculated.

According to EC2 §6.2.2, what check can determine if a beam requires shear stirrups?

Different

In calculating fcd, is the design compressive strength acc the same or different for flexure and shear?

EC2 §6.2.3 allows the stirrups to be angled (defined by a)

Does the design method account for angled stirrups?

sin q = V/Fq

Per vertical equilibrium, the force in the compressive strut ( Fq ) is related to the shear force ( V) by…?

A shear crack forming between two stirrups.

What must the spacing of stirrups avoid?

Slabs Where transverse distribution reinforcement is provided

Where are shear stirrups not needed in a beam?

Direct strut action

What type of action allows the shear capacity of a short beam or corbel to be assessed?

The Strut and Tie Method

What allows the capacity of a corbel or short beam to be checked and the necessary reinforcement to be determined?

Bent up bars

Other than shear stirrups, what alternative is available?

Within their walls

In what circumstance must beams subjected to torsion carry shear stresses in EC2?

To distribute the moments away from areas of maximum moment

Why use moment redistribution?

The sectional ductility and the ratio of xbal /d

What does moment redistribution depend upon?

The support

To redistribute moment away from the central hogging support, apply a moment hinge at what location?

Moment Redistribution can occur between diagrams.

After construction of the plastic moment diagram, what can allow for a greater load capacity?

EC2 allows up to 30% redistribution (d=0.7)

The text states a limit that must be recognized when applying greater rotation capacities as a result of moment redistribution?

At the end of §5.5 in EC2

When redistributing, what part of EC2 holds the notes?

In sagging

Aside from end supports and hogging, when does analysis of One Way Beams require determination?

As T-beams (or an L-beam at the edge of the slab).

When designing the beams beneath a one-way spanning slab, how are the "embedded" beams be modeled?

The surrounding concrete

Anchorage of steel relates to which other element?

Lap joints and transfer of force

If a single piece of reinforcement cannot be used for a lapped joint, what is required as a result?

The tensile force that acts in the longitudinal reinforcement (Fs)

In relation to tension force , what helps curtailment of reinforcement?

Crack widths and deflection

The deformation of a structure depends primarily on meeting design code checks for these two main factors:

Load-induced cracks

In a reinforcement what is required to meet the maximum bar spacing requirements in EC2 §7.3.3

It can be assumed that the deflection criteria are met if the beam or slab complies with the span/depth ratios given in §7.4.2

How are deflection and crack widths managed to avoid the design of a slab by the rigorous method?

Detailing requirements

From an engineers perspective, what checks and requirements involve adding reinforcement to the concrete slab per section 9?

Unbraced structure which uses shear and Braced structure which uses shear wall

Describe briefly in each of the two types of framed structures.

Non-rectangular slabs

Analysis of slabs what will require that a 4th year is achieved in Plastic Frames?

The UK national annex (case 3) allows slabs to be designed for the single case of all spans loaded.

Are slabs restricted by national annex.

Panel designs require that longer spans cannot be more then 2 times shorter spans.

Are spaning limitations for slab designs restricted?

IStructE §5.2.3.3 Bending moments per unit width

Are Bending moments needed for Restrained Spanning Slabs?

Prevent uplift at the corners

Regarding the torsion effects on Corner Supports, what must torsion accomplish?

Design moments of the slab panels middle strips and column strips.

In Flat Slabs designs the slab panels can be divided into what?

Large shear stresses through the slab.

A Punching Shear failure can cause?

By adding a drop (local thickening of the slab near the column), increase the diameter of the column head.

How can a punch shear loading be resolved?

A narrow strip, of width of be

At the edges of a flat slab, the moments must be transferred into the edge column over what?

A ‘critical control perimeter’

Shear Stresses in Flat Slabs must be checked across what?

The axial load (N) and moment (M)

Why are Columns are defined about mid-height of the section?

Limited moments in two directions

In Biaxial Bending, internal and edge columns are subjected to what?

EC2 are satisfied: §5.8.9(3) that eccentricity and dimensions are held.

Per rectangularity, when can a column be designed distinctly?

Which causes additional moment due to its lateral deflection.

When does the curvature in a slender column result?

Nominal Curvature value or Method

Which value should be used to find design moment in Slender Columns?

A piece of slab rotates as part of the collapse mechanism.

To prepare for work using complex geometries, it is important to establish the concept of the axis of rotation along which?

For sections of slab that are plastically deforming

When does Slab deformation create very large tensile crack widths.

It is almost always the load scenario

What determines if the loading of the slab is taken to be loaded by a uniformly distributed load q?

Parts of a Slab's deformation cause different Axis of Rotation.

What happens about Axis's of Rotation, as parts Slab rotate and

In Orthogonal layers to yield reinforcement moments

For each direction of force how must forces to transfer across slab layers?

Axis projected from both sides with forces in relation to said lines.

To determine the forces with yield lines how must be the lengths of the lines be aligned, in relation to projected Axis, for every part.

Double, 2X value of Strength

Using Yield Line Theory, Built ins are what relation of Strength?

Moment/Force, especially due displacements.

Isotropically what is common in almost all Slab Sections, so that the effects become?

Equalibrium

Aside from Load, Moments forces are always?

Forcfully verticaly curved due compression or tension, in different stages.

What is a characteristic of loading forces in relation to slabs?

A linear plate with results of bending moments.

To be accurate what is needed for finite element analysis for slab designs?