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FUNDAMENTALS OF STRUCTURES 2013 3 STRUCTURAL MATERIALS page 1

Budapest University of Technology and Economics

Department of Mechanics and Materials of Structures English courses General course /2013 Fundamentals of Structures BMEEPSTG201 Lecture no. 3:

Structural materials Characteristics, testing, strength evaluation


Introduction Structural materials are used to construct the loadbearing structural part of building constructions. Loadbearing structures of buildings should safely support loads acting on the structure (without demage, rupture, loss of stability, falling over or down), and transmit loads to the subsoil under the building. Their most important characteristic is having adequate strength. Strength is the greatest force per unit area that the material cab bear without demage, rupture. Tensile strength, compression strength and shear strength can be distinguished, according to the way of actuation the force is acting on to the cross-section of the investigated member. Considering a cross-section of a short linear member, the above strengths can be determined approximately as indicated on the figure:


1. General requirements of structural materials structural requirements non-structural requirements strength space limitation tensile strength (ft) thermal insulation compression strength (fc) sound insulation shear strength (fτ) economicity, prize deformability elastic, elastic-plastic, plastic, brittle behavior ultimate deformation density, specific weight (γ) fire resistance durability, resistance to corrosion remodelation possibilities


2. Mechanical characteristics of some structural materials Materials Strength Deformability fd /γ ratio compression tension at σ=fd at rupture (kN/m2)/(kN/m3)= N/mm2 mm/m=%o =m Sun-dried brick 0,2 0 Cer.ic brick+lime m. 0,2-1 0 0,8 4 120 Cer. brick+cement m. 0,8-3,5 0 0,8 4 Natural stone 1-8 0 0,2 2 350 Concrete light normal 10-50 0,7-4 1 2,5-3,5 320 high strength Timber (parallel to fibre direction)

15-30 15-30 2 4 1900 Iron 50 25 Steel mild 180-280 180-280 1-2 25 2500 high strength steel 1400-1800 1400 1800 7-9 15 17200 Reinforced concrete see concrete see concrete 1200


(Continuation) Materials Strength Deformability fd /γ ratio compression tension at σ=fd at rupture (kN/m2)/(kN/m3)= N/mm2 mm/m=%o =m Glas 120 75 very small Aluminium 150 200 4 20 7900 Fibre-reinforced plastics 3 1500 Numerical example for better understanding Gman≈0,8 kN= 800 N

Apalm ≈100x100 =10 000 mm2

σsoil≈ 1,010000

800AG

soil

man ≈= N/mm2

Compare this stress You can feel with material strengths indicated above!


1. Uniaxial stress-strain relationships of structural materials Definitions:

Stress = σ = force per unit area = AF (N/mm2)

Stress at rupture= strength (compression or tensile strength according to the direction of F)

Strain = ε = specific deformation =length

ncontractioor alongation =llΔ

Plastic behaviour: the material deforms under compression or tension without stress increment

Characteristic mechanical behaviours in uniaxial stress state


elastic rigid-plastic elastic-plastic nonlinear Hook's law describes the linear elastic mechanical behaviour: ε=σ E where E: modulus of elasticity (N/mm2) Definition of E: the value of the stress to be applied to double (or – in case of compression – to halve) the length of the member (ε=1)


Characteristic σ-ε relationships of some important structural materials:

timber steel concrete Idealized stress-strain relationships:

timber steel concrete


Test specimens used for uniaxial tests

timber steel concrete


Static bending test of a timber specimen Tension test of steel specimen


Mechanical behavior of the tested materials Timber Strength perpendicular to grains is different (very low). Linear elastic behavior. Strength is similar to compression strength of concrete. Steel After linear elastic behavior yield produces great plastic deformation. The same strength in compression or tension. The strength is approximately 10 times greater than that of timber. Concrete Compression and tensile strength is very different. After short linear behavior non-linear σ-ε curve can be observed. Ultimate deformation is much smaller than that of steel. Brickwork Negligible tensile strength. The strength of brickwork is much smaller, than that of ceramic bricks. Compression strength is at about by one order of magnitude smaller than that of concrete.


2. Strength as probability variable 2.1 Statistical strength data no. of occurance relative occurence A=1 An strength f (N/mm2) strength Strength distribution diagram Density function of strength (histogram) distribution

The probability of not exceeding fnom= nn A

AA

=


2.2 Statistical evaluation of test results (Statistical strength evaluation9 Characteristics of the strength distribution

Mean value: fm=nfiΣ n: number of test data (min 10, if

qualification is based on unknown scatter)

Scatter: s=1n

)ff( 2mi

−−Σ

Threshold value (characteristic or nominal value): fk= fm – ts where t is the so called Student factor, which, for n=10 and 5% risk is t=1,79 (5% threshold value: the probability of the occurence of a strength value smaller than fk is 5%)


Relation of characteristic and design value relative occurance An fd fk f =fm strength (N/mm2)

Design value: fd= m

kfγ

The probability of fd not being exceeded is 0,1% γm for some important structural materials 1,15 for reinforcement (steel in reinforced concrete) 1,3 for timber 1,5 for concrete


Example: test data evaluation of a timber test specimen subjected to axial compression in direction of the grains. Specimen data: 2x2x3 cm timber No. Cross-section Rupture load Rupture strength A Nu fu fui- fm (fui- fm)2 mmxmm kN N/mm2 N/mm2 N2/mm4

1 20 x 19,8 26,7 67,42 1,62 2,62 2 20 x 20 27 67,50 1,70 2,89 3 19,7 x 19,2 24,5 64,77 -1,03 1,06 4 20 x 19,6 25,4 64,79 -1,01 1,02 5 20 x 20 28,2 70,50 4,70 22,09 6 20 x 20,6 24,8 60,19 -5,61 31,47 7 20 x 20 27,0 67,50 1,70 2,89 8 20,7 x 20,5 25,3 59,62 -6,18 38,19 9 19,6 x 19,3 26,0 68,73 2,93 8,58 10 20 x 20 26,7 66,75 0,95 0,90 =−Σ 2

mui )ff( 111,71 Mean value of the rupture strength:


8,6510

77,657nf

f um ==

Σ= N/mm2

The scatter:

s= 1n

)ff( 2mi

−−Σ = 52,3

971,111

= N/mm2

The characteristic compression strength: fk= fm- ts= 65,8 – 1,79· 3,52 = 59,5 N/mm2 for n= 10 and 5% risc the Student factor: t=1,79 Design compression strength:

fd= m

kfγ

= 76,453,15,59= N/mm2

the safety factor for timber: γm= 1,3


TESTING OF PREFABRICATED REINFORCED CONCRETE BEAMS IN.2005 IN THE LABORATORY

OF THE DEPARTMENT OF STRENGTH OF MATERIALS AND STRUCTURES

structural materials characteristics, testing, strength ... courses/0_fundamentals of... · lecture...

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