University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Modes of Failure solids held together by bonds between their atoms these bonds can be compressed or extended 1/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 l tension l compression l shear l bending l stress pattens may be complex but consist of only 3 basic states of stress tension - compression - shear Modes of Failure l buckling 2/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Tension l state of stress where material l tends to be pulled apart l cable with weight becomes longer under pull l lengthening depends on X-section, length & load l larger the diameter - smaller the elongation 3/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Compression l state of stress in which particles l pushed against the others l column under load shortens l squashes l shortening of material a steel column under compression shortens as much as a rod of same steel lengthens in tension under same stress 4/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Compression (cont.) l can have no tension elements but must have compression elements l materials weak in tension often strong in compression l with modern materials of high compressive strength, e.g. steel can build columns much slimmer l but slenderness introduces new problem l compression elements very common must channel loads to ground 5/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Buckling l buckling is a basic design factor for slender elements in compression l buckling occurs even if load perfectly central l as compressive load increases, reach value where slender elements instead of shortening buckle & usually break l buckling load depends on: material, length, shape of X-section, restraints at ends 6/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Shear l state of stress in which particles of material slide relative to each other rivets tend to shear a hole puncher uses shear to punch out holes in paper load on short cantilever tends to shear beam from wall 7/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Bending l consider plank loaded as shown l upper fibres lengthen l plank ends move down l section between stones deflects up l curve is arc of circle l lower fibres shorten l middle fibres remain original length - Neutral Axis 8/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Bending (cont.) l concrete beam fails in tension due to bending l may fail in diagonal tension due to shear due to bending 9/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Behaviour of Materials l stress l strain l elasticity - plasticity - brittleness l safety factors l selecting appropriate materials 10/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Modes Of Failure - Under Stress l tension l compression l buckling l shear l bending stress patterns complex but consist only of three basic states of stress tension - compression - shear 11/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 General Load-Deformation Properties Of Materials application of load produces dimensional changes in a member l member undergoes change in size or shape or both l deformation may be reversible or irreversible elastic or plastic 12/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Stress l internal forces developed within a structure due to action of external forces l stress is force intensity -force per unit area 13/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Fi = Fe Fe Stress (cont1.) consider member in tension l stress is force intensity - force per unit area Fe XX Stress = Force / Area f = F / A Fe 14/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Stress (cont2.) stress is force per unit area 1 pascal = 1 newton per square metre 15/22 A load of 1 N on each square metre represents an average stress of 1 N/m 2, or 1 Pa 1m 1N

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Stress (cont3.) we use stress in megapascals (MPa) for most materials 1 MPa = 10 6 N/m 2 = 1 N/mm 2 we use stress in kilopascals (kPa) for floor loads and foundation pressures (loads distributed over an area) ( remember 1 m 2 = 10 6 mm 2 ) 16/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Stress (cont4.) internal force not concentrated at single spot stress developed DOES NOT DEPEND ON MATERIAL OF MEMBER distributed over entire cross-section stress in a member depends only on force applied and cross-section f = F / A 17/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 Strength of Members strength depends on many factors in tension, failure will occur by pulling apart at weakest location weak spot (point of reduced X-section) determines capacity of whole member f = F/A higher because of smaller A if material can sustain stress member will carry load as load increases stress increases eventually material fails (pulls apart) 18/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 ratio of change in size or shape of element to original size or shape Strain response to stress have stress --> get strain strain to do with change in size or shape 19/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 STRAIN (cont.1) for member subject to simple tensile force dimensionless - millimetre / millimetre strain = increase in length original length e = LL L 20/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 STRAIN (cont2.) determined by: except for rubber bands, strains very small usually not visible more a material strains under load - more the structure deflects taking member of known length subjecting it to a known load measuring elongation 21/22

University of Sydney – BDes Design Studies 1A - Structures Modes of Failure Mike Rosenman 2000 STRESS & STRAIN SUMMARY force stress causes strain puts material under deformation results in 22/22