Chapter One General Concepts of Building Structures What do Architect/Artist do? What is an Architect Engineer? • Engineer – Mathematics of design • Architect/Artist – Vision – Aesthetics of design • Mediator – Liaison between parties on a project • Salesman – Must sell your idea, yourself • What is a structure? – A system designed to resist or support loading and dissipate energy • Building Structures – Houses – Skyscrapers – Anything designed for continuous human occupation • Non-building Structures – Bridges – Tunnels – Dams Forces • Influence on an object that causes a change in a physical quantity • Considered “vectors” – magnitude and direction • Static Force – Unchanging with time • Walls • Floors • Dynamic Force – Changing with time • People • Furniture Forces • Axial Forces – Acting along one axis, directly on a point or surface • Momential (Bending) Force – Acting along an axis, at a certain distance from a point, causes a folding motion – M = F*d F Forces • Tensile Force – Pulling on an object – stretching it – Steel shows “necking” when too much tensile force is applied • Compressive Force – Pushing on an object – collapsing it – Concrete crushes when too much compressive force is applied Forces • Strain – Tensile-related property – Deformation / Length • Stress – Compression-related property – Force / Area • Compare using stressstrain graph What constitutes loading? • Loading is a force being enacted on the structure – Many sources of load • • • • • Gravity/Weight Wind Snow Earthquake Man-made – Two Types of Structural Loading • Dead Loads – static, ever-present (i.e. Walls, Floors, etc) • Live Loads – dynamic, changing (i.e. People, Desk, etc) What should we build our structures out of?? • Common Structural Materials – Timber – Masonry – Concrete – Steel – Composites How do we judge the materials? • Common Material Properties – Strength – Tensile/Compressive – Density – Hardness – Ductility / Brittleness – Elasticity – Toughness Strength • Ability of a material to withstand loading – Tensile strength – ability of a material to withstand a pulling force • Steel is good at this, but concrete performs very poorly. – Compressive strength – ability of a material to withstand a pushing force • Wood, concrete, steel, and masonry perform well Density • Mass per unit volume of a material – Units – mass/vol - kg/m3 or lb-m/ft3 – Typically, materials with a high density are very strong and offer great protection. – However, a high density means that they are heavy and difficult to work with. Hardness • Ability of a material to resist permanent deformation under a sharp load – Relates to the elasticity of a material – Diamond is a very hard substance. If we built a wall out of diamond, we could be sure that very few things would scratch it. – However, Diamond is incredibly expensive and not as tough as other engineering metals. It wouldn’t stand up as well in impact loading versus other materials. Ductility / Brittleness • Ability of a material to deform without fracture – We want materials with high ductility, because they will indicate structural failure without a sudden collapse. Elasticity • Ability of a material to deform and return to it’s original shape. – Important quantity • Young’s Modulus • Ratio of stress to strain – Stress = Force / Area (lbs./in2 or N/m2) – Strain = Deformation / Length (unitless) • Generates a stress-strain graph • Related to the ductility of a material Toughness • Ability of a material to resist fracture when stressed (amount of energy absorbed per unit volume) – Area under the stress-strain curve, evaluated from 0 to the desired strain. So, we know what properties are important in structural materials. How do the common materials stack up against each other? Timber • Advantages – Cheap, renewable resource – Good in Tension – ~40 MPa • Disadvantages – Susceptible to fire, nature – Not very hard – Not very strong – Limits on shape, size Masonry • Concrete blocks, clay bricks – Advantages • Large compressive strength • Cheap • Good thermal properties – holds heat well – Disadvantages • Not a cohesive material. The strength could depend on the mortar, other factors • Poor tensile strength, unless reinforced • Heavy material, requires skilled laborers to use • Height restriction • Susceptible to the weather Concrete • Combination of water, cement, small aggregate, and large aggregate. • Advantages – Very versatile – can be modified with admixtures for different effects – High compressive strength – Fire resistant – Many diverse sizes and shapes - formwork Concrete • Disadvantages – Long curing time – Low tension strength – Fails in shear, unless reinforced – Fairly heavy material to work with Steel • Advantages – High tensile and compressive strength (A36 Steel) – Many varieties, depending on your need • Carbon steel • Stainless steel • Galvanized steel – Elastic material – Ductile material – Many shapes, sizes Steel • Disadvantages – Expensive – limited quantities / competition – Susceptible to fire, rust, impurities Put them together and… • Reinforced Concrete – Concrete with steel reinforcement • Concrete handles compression • Steel takes the tension – Can handle nearly 4 times the loading that concrete alone can handle – More expensive material Composites • Engineered compounds that have different physical or chemical properties – FRP – Fiber reinforced polymers – CFRP – Carbon-fiber reinforced polymers – Plastics – Categories of Glass – Categories of Wood So, now we know what material will best suit our needs.. What should we build with it? Structural Shapes • Rectangle / Square • Triangle – Interested in stability • Truss • Geodesic Dome Shape Stability Exercise • Split into teams of 5 • Build a triangle and square • See which shape is the most stable – Can the unstable shapes be made stable? – How? Rectangle • Advantages – Proficient in resisting vertical load. • Disadvantages Need another bar for lateral support! --BRACING-- – No lateral (horizontal) load support Triangle • Advantages – Able to withstand lateral & vertical loading – Many triangular shapes available • Disadvantage – Wide base Truss • Combination of square and triangle Truss • Combination of square and triangle Squares Truss • Combination of square and triangle Triangles Truss • Combination of square and triangle – Both vertical and lateral support Geodesic Dome Domes Domes • Advantages – Very strong shape, gets strong as the dome size increases – Perfect load distribution – No need for structural supports – Great aerodynamic performance Structural Components • • • • • Beams Girders Columns Floors Foundations Column Girder Beam Load Path • Floor • Beams • Girders • Columns • Foundation • Soil/Bedrock Foundations • Support the building – Typically attached to columns • Types – Shallow • Spread footing – concrete strip/pad below the frost line • Slab-on-grade – concrete pad on the surface – Deep • Drilled Shafts • Piles Columns • Carry the load from floors to the foundation – Never want the columns to fail COLLAPSE – Typically reinforced concrete or steel – Many sizes and shapes Girders • Attached columnto-column – Take the load from the beams – Transfer it to the columns – Generally shaped as an I-Beam Beams • Attached between the girders – Take load from the flooring system – Transfer it to the girders – Generally solid squares, I-beams Quiz-I (5%) Split into teams of 5 Q1.Build a triangle and square, see which shape is the most stable? And Can the unstable shapes be made stable? How? Q2. Let say you are the designer; and one organization wants to order you to design a large diameter of circular roof. Which type of Structural Shapes you recommend for this? Why? Q3. Explain in detail, with neat sketch, about the load path of a building starting from roof to Soil/bed-rock.
