*CIV 6215

Structural Design

Autumn 2012University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering6 week course Assessment:Design project: 30%Exam (Week 6): 70% Lecturers: Dr. Jurgen Becque j.becque@sheffield.ac.uk Dr. Colin Smith

*University of SheffieldDepartment of Civil & Structural Engineering1. Limit State Design Principles

*University of SheffieldDepartment of Civil & Structural EngineeringPhilosophy of design What is Structural Engineering ?Structural engineering is . the art of moulding materials we do not really understand .. into shapes we cannot really analyse ..so as to withstand forces we cannot really assessin such a way that the public does not really suspect.

*University of SheffieldDepartment of Civil & Structural EngineeringAims of structural designSafe structure able to fulfil intended purpose:Able to resist predicted loading with adequate safety margin,In- service deflections acceptable,Other forms of behaviour acceptable.

Other considerations include: Economy, safety, erection, transportation & sustainability.

*University of SheffieldDepartment of Civil & Structural EngineeringDesign processStructural design consists of three interactingaspects:Decision making or conceptual design,Structural analysis,Proportioning members and subsequent detailed design.

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural EngineeringRelationship between stress and strength at all critical points in a structure

*University of SheffieldDepartment of Civil & Structural EngineeringThree basic design methods: Permissible stress design Load factor design Limit state design

*University of SheffieldDepartment of Civil & Structural EngineeringLimit state designLimit state design applies partial safety factors, both to the loads and to the material strengths.This design approach overcomes many of the disadvantages of previous two methods. Limit state philosophy forms the basis of the design methods in most modern codes of practice for structural design.

*University of SheffieldDepartment of Civil & Structural EngineeringTwo principal types of limit state:Ultimate limit statesServiceability limit states

*University of SheffieldDepartment of Civil & Structural EngineeringUltimate limit statesStrength yielding, rupture, crushing of concreteStability bucklingFracture due to fatigue

*University of SheffieldDepartment of Civil & Structural EngineeringServiceability limit statesDeflection the appearance or efficiency of any part of the structure must not be adversely affected by deflections. Cracking local damage due to cracking and spalling must not affect the appearance, efficiency or durability of the structure. Durability this must be considered in terms of the proposed life of the structure and its conditions of exposure. Vibrations/Sway.

*University of SheffieldDepartment of Civil & Structural EngineeringCharacteristic material strengths

*University of SheffieldDepartment of Civil & Structural EngineeringCharacteristic loadsIdeally it should be possible to assess loads statistically as: In most cases it is the maximum loading on a structural member that is critical and the upper, positive value given by this expression is used

*University of SheffieldDepartment of Civil & Structural EngineeringPartial safety factors for materials,The strength of the material in an actual member The severity of the limit state being considered

*University of SheffieldDepartment of Civil & Structural EngineeringPartial safety factors for loads,The importance of the limit state under considerationDifferent type of loadingThe probability of particular load combinations

*University of SheffieldDepartment of Civil & Structural EngineeringBasic Structural Concepts

*University of SheffieldDepartment of Civil & Structural EngineeringSequence of load transfer between elements of a structure Sequence of load Transfer: Ceiling/floor loads

*University of SheffieldDepartment of Civil & Structural EngineeringDetermination of the design loads acting on the structureStructural design process

*University of SheffieldDepartment of Civil & Structural EngineeringDesign loads acting on structuresDead (permanent) loadsThe loads acting on a structure are divided into three basic types:Imposed (live) loadsWind loads

*University of SheffieldDepartment of Civil & Structural EngineeringDead loads include:The weight of the structure itself, and all architectural components such as exterior cladding, partitions and ceilingsThe weight of equipment and static machineryThe weight of permanent fixtures

*University of SheffieldDepartment of Civil & Structural EngineeringSchedule of unit mass of building materials (based on EN 1991-1-1)

*University of SheffieldDepartment of Civil & Structural EngineeringImposed loads include:The weight of the occupants The weight of furnitureRoof loads including snow

*University of SheffieldDepartment of Civil & Structural EngineeringImposed loads (based on NA EN 1991-1-1) officesclassroomsstairs

*University of SheffieldDepartment of Civil & Structural EngineeringLoad combinations for the ultimate limit state In most of the simple structures the worst possible combination will arise due to the maximum dead and maximum imposed loads acting on the structure together

*University of SheffieldDepartment of Civil & Structural EngineeringLoad combinations for the serviceability limit state

*University of SheffieldDepartment of Civil & Structural EngineeringThe load combination should be arranged to produce the worst possible effect on the structure in terms of bending moments, shear forces and deflections Load combination

*University of SheffieldDepartment of Civil & Structural EngineeringDesign loads acting on elementsHow the loads transmit down to the ground To calculate the design loads acting on individual elements the designer needs to make assumptions regarding:The support conditions

*University of SheffieldDepartment of Civil & Structural EngineeringTypical beams and columns support conditions

*University of SheffieldDepartment of Civil & Structural EngineeringThese assumptions will considerable simplify calculationsAll the joints in the structure are pinnedThe sequence of load transfer occurs in the order: ceiling/floor loads to beams to columns to foundations to groundTwo (usually) conservative assumptions:

*University of SheffieldDepartment of Civil & Structural EngineeringStructural analysisThree common methods for structural analysis:Computer methodsEquilibrium equationsFormulae

*University of SheffieldDepartment of Civil & Structural EngineeringFormulaeBending moments, shear forces and deflections for various standard load cases

*University of SheffieldDepartment of Civil & Structural EngineeringWork example WE 1-1: Fig. WE 1-1 shows a generic 37.5m x 37.5m normal-weight reinforced concrete structure comprising ten floors with a 4.5m storey height and five 7.5m x 7.5m bays in each direction. The building is designed to Eurocode 2, and represents a commercial office building. The characteristic loads are assumed to be: Self-weight (including slabs, beams and columns): 7.5kN/m2 Raised floor: 0.5kN/m2 Permanent ceiling and services: 0.5kN/m2 Temporary partitions: 1.0kN/m2 Imposed load: 2.5kN/m2

Calculate the maximum design bending moment and shearing force for the beam at position B3-C3 and design axial loads of the ground floor columns at position A1 and C3 for both ultimate and serviceability limit states.

Fig. WE 1-1 Concrete structure layout.

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural EngineeringWork example WE 1-2: Fig. WE 1-2 shows a generic 27m x 18m composite steel-framed structure comprising three floors with a 4m storey height and three bays in each direction. The composite steel decking floors are one way spanning over the secondary beams. The building is designed to Eurocode 3, and represents a commercial office building. The characteristic loads for each floor are assumed to be: Permanent: 4.5 kN/m2 Variable: 4.0 kN/m2

Calculate the maximum design bending moment on secondary Beam A-B, primary Beam C-D and axial force of column C-E for both ultimate and serviceability limit states.

Fig. WE 1-2 Composite steel-framed building.

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering

*University of SheffieldDepartment of Civil & Structural Engineering