1、土木工程概论土木工程概论Introduction to Civil Engineering第九章土木工程设计概述第九章土木工程设计概述 Chapter 9 Introduction to Design of Civil EngineeringIntroduction to Design of Civil Engineering What is Introduction to Design of Civil Engineering?It provides the students with the necessary background on terminology used in desig
2、n.With this chapter,entry-level students of civil engineering will better understand from the outset lectures on detailed subject areas.It will also prove beneficial for newly qualified professionals and others who want a concise guide to everyday design terminology.Introduction to Design of Civil E
3、ngineering Classification of loads:According to variation with time:Loads whose magnitude is constant,and not varying with time are called as static loads,and may vary with time are called as dynamic loads.According to occupation:The loads which remain constant in position are called dead loads.Acco
4、rding to distribution:The distribution where a heavy load distributed over a small area is called concentrated load.The load which are evenly distributed over a large area is called an uniformly distributed load.Introduction to Design of Civil Engineering Types of stresses and strainsIntroduction to
5、 Design of Civil Engineering TRUSS BRIDGE LABORATORY Bridges are essential to our nations infrastructure.A simple bridge can be made by spanning a gap with planks.As the gap becomes wider,however,the planks will begin to sag excessively even under the weight of a person.If the bridge is longer still
6、,the planks may break.When one of the planks,called a beam,is loaded,it bends as shown below.Lines are drawn on the beam for illustration.Introduction to Design of Civil EngineeringIntroduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY A close-up view of a short segment of the beam is s
7、hown below.The top part of the beam is being squeezed(in compression)and the bottom part of the beam is being stretched(in tension).The force in the beam actually changes continuously from the top of the beam to the bottom.That means that in the middle(top to bottom),it is neither in compression nor
8、 tension.These forces act in a bending manner on the beam.This bending force is referred to as moment,as shown in the diagram.Introduction to Design of Civil EngineeringIntroduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY If a plank bridge breaks,it is likely to splinter in the middle
9、 leaving the rest of the plank undamaged.This is because the center of the plank experiences much more moment than the ends,which experience none,because they are free to rotate without resistance.So the moment,or twisting force,varies continuously from zero at the left end to its highest value in t
10、he middle and back to zero again at the right end.The result is that although it is simple to build,a plank bridge does not make very efficient use of material.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY One way of making more efficient use of wooden beams is to stand them on
11、 edge.If you have ever been in an unfinished attic,you may have noticed that the floor beams(and the rafters)are in this configuration.The beams dont bend as much in the upright orientation.This is because of a property called moment of inertia.The basic principle of moment of inertia follows.Introd
12、uction to Design of Civil EngineeringIntroduction to Design of Civil EngineeringLow Moment of Inertia Use this for a diving board which you want to bend a lotHigh Moment of Inertia Use this for support beams which you want to be stiffIntroduction to Design of Civil Engineering TRUSS BRIDGE LABORATOR
13、Y The two beams above are called I-beams or wide flanges because of their shape(when looked at on end).The left beam would be made of steel and the right of concrete.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY These show how material is concentrated at the top and bottom of t
14、he beam.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY The more material and the farther away from the center it is,the higher the moment of inertia,and hence the stronger the beam.As nature would have it,achieving greater distance from the center is more beneficial than adding
15、more material,because the moment of inertia increases as the square of that distance.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY Obviously,we cannot remove all the material from the middle of the beam,because the top and bottom must be connected.The material in the middle als
16、o keeps the top and bottom from sliding with respect to each other in what is called shear.Yet there is a more efficient way to focus material at the top and bottom and provide resistance to shear.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY The middle part of the beam does no
17、t need to be solid and continuous,but can instead be made up of thin rods.This is shown in the figure below.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY This configuration establishes the basis for what is known as a truss.A truss is the oldest and most often used method of ma
18、king more efficient bridges,and you will be building one today.A truss is a structure made from straight links connected at joints.The joints are always at the ends of the links,never in the middle.Introduction to Design of Civil Engineering TRUSS BRIDGE LABORATORY The links are called members,and i
19、n your case,they are craft sticks with drilled holes.The joints are assembled with small bolts in your case.If the term members makes you think of a team,you are on the right track.When a load is applied to any joint,the members will share the load,although not equally.Introduction to Design of Civi
20、l EngineeringSome Concepts:COMPRESSION:This,as you would expect,describes a squeezing action or force on an object.TENSION:The opposite of compression,or a stretching action or force on an object.Introduction to Design of Civil Engineering STRESS:A measure of force per unit of area,i.e.lb./in2(or ps
21、i),kN/m2 STRAIN:A measure of deformation or elongation of a material,its units are inch per inch;it is the ratio of a change in length to the original length of a specimen.STRENGTH:The stress value at which a sample of material fails.Introduction to Design of Civil Engineering MODULUS OF ELASTICITY:
22、Relates stress to strain and visa versa.It is the ratio of the stress on a sample to the amount of stain that level of stress causes.It is also the slope of the straight line portion of the stress-strain curve for a specific material.ELASTIC RANGE:The portion of the stress-strain relationship for a
23、material where if the specimen loaded and then unloaded,it will return to its original undeformed shape.The straight line portion of the stress strain curve.Introduction to Design of Civil Engineering NEUTRAL AXIS:A line which runs along the length of a beam where stress and strain are equal to zero
24、.MOMENT OF INERTIA:This is one measure of the stiffness of a beam.It relates cross sectional area and the distance from the neutral axis at which the majority of the area is located to the ease in which the beam is bent.Introduction to Design of Civil Engineering Example:An I beam has a greater mome
25、nt of inertia than a flat plate of the exact same cross sectional area.CANTILEVER BEAMA beam(a member whose main action is bending)supported,or fixed,at only one end.i.e.an overhang or a diving board configuration.Introduction to Design of Civil Engineering STABILITY AND SIMPLE TRUSSES There is an i
26、mportant characteristic of a useful truss:it must be stable,which is to say that it should not move freely in any direction.Below are some configurations of members joined at the ends.The first shown is the most basic triangular truss.The left support only allows connected members to rotate.The righ
27、t support additionally allows horizontal movement.This configuration is stable,because there is no motion which can freely occur.Introduction to Design of Civil Engineering STABILITY AND SIMPLE TRUSSES Two members connected at a joint form a hinged arch,as shown below.A hinged arch may be added to a
28、ny stable truss to form another stable truss,as long as the angle of the arch is other than 180.A truss which can be assembled in this manner is called a simple truss.Introduction to Design of Civil Engineering STABILITY AND SIMPLE TRUSSES Lastly,we see that a pentagonal configuration is also unstab
29、le,because as points A and B move apart,point C is free to move down.What is the smallest number of members required to make this stable?In a similar fashion,all but the triangle will be unstable,so the triangle is basic unit of any truss structure.Introduction to Design of Civil Engineering THE LON
30、G AND SHORT OF IT Another special feature of trusses is that the members dont bend.They get pulled apart(in tension)and pushed together(in compression),but they dont bend like the plank does when you stand on it.The members stay straight from end to end until they break.This does not mean the bridge
31、 will stay straight,though.As heavier loads are put on the bridge,it will still sag in the middle.This is because the individual members of the truss are getting longer(if they are in tension)and shorter(if they are in compression).Introduction to Design of Civil Engineering A BELT ISNT THE ONLY THI
32、NG THAT BUCKLES Many materials,in theory,have the same strength when being squeezed together(in compression)as they do when pulled apart(in tension).The problem is that if you press the two ends of a thin member(like a ruler)together,it doesnt simply stay straight and get shorter,but instead it bend
33、s out to the side.This is called buckling,which is the way that most tall,skinny things break when compressed end-to-end.Introduction to Design of Civil Engineering HOW CAN MY TRUSS FAIL?There are three ways your truss can fail.If a member buckles enough,it will bend and break in the direction in wh
34、ich the craft sticks have a low moment of inertia.This may be prevented if the loading frame supports partially buckled members.Another type of failure is that a craft stick pulls apart in the middle in tension.The third type of failure possible is joint break-out.This is when the craft stick breaks
35、 right where the bolt is connected.Introduction to Design of Civil Engineering Students in the Structures lab Building EnvelopePerformanceLaboratoryFull scale wall thermal resistanceexperimentIntroduction to Design of Civil EngineeringStructures LaboratoryIntroduction to Design of Civil EngineeringE
36、nvironmental LaboratoryIntroduction to Design of Civil EngineeringWaterResourcesLaboratoryIntroduction to Design of Civil EngineeringBuildingAerodynamicLaboratoryIntroduction to Design of Civil EngineeringAcoustics LaboratoryIntroduction to Design of Civil EngineeringIndoor AirQuality andVentilation
37、LaboratoryIntroduction to Design of Civil EngineeringThermalEnvironmentand ControlsLaboratoryIntroduction to Design of Civil EngineeringComputerLaboratoriesIntroduction to Design of Civil EngineeringIntroduction to Design of Civil Engineering ENGINEERING RESEARCH The role of the research engineer is
38、 achieved after many years of study.Students wishing to pursue this field should take courses that emphasize advanced mathematics and statistics.Use of the computer is essential;computer languages such as FORTRAN and C+aid the research engineer in completing projects.Students can start early as a st
39、udent assistant for a professor in the undergraduate major.Introduction to Design of Civil Engineering ENGINEERING RESEARCH Most of the skills necessary to work in the field,however,will come with research completed in graduate studies.Most colleges and universities do not offer a research sub-disci
40、pline in the Civil Engineering major.Students should choose courses which they wish to specialize in.Introduction to Design of Civil Engineering STRUCTURAL ENGINEERING All structures,regardless of their function,are subjected to forces caused by the natural environment(such as wind and earthquakes)a
41、nd by man(such as cargo and automobile traffic),and they must be designed to withstand these forces.These structures can be as varied as buildings,bridges,pipelines,machinery,automobiles and spacecraft.Introduction to Design of Civil Engineering STRUCTURAL ENGINEERING The job of the structural engin
42、eer is to create these new designs or to evaluate and improve the load resistance capabilities of existing structures which may have been damaged during an earthquake.In order to accomplish this the structural engineer must be knowledgeable about the behavior of deformable bodies,about the sources,m
43、agnitudes and probability of occurrence of applied loads,about material properties,design philosophies and governmental design codes,and about computer programming and usage.Explain the terms stress and strain.State Hooks Law.Why is a factor of safety provided for a design?What is meant by working stress?Define(i)Poissons ratio(ii)hydrostatic pressure and(iii)volumetric strain.Introduction to Design of Civil EngineeringReview Questions
