arch 3423 Building Technology II: Building Structures

Term 1 2019-2020

Lecture 4 Structural materials and their properties

Major structural materials• Stone naturally occurring / quarried and formed into block or slab units by sawing,

splitting and chiseling / strong in compression - weak in tension / low strength to weight ratio / primary use as wall-pier or arch.

• Brick / Block formed structural units (generally smaller than stone) / standard unit sizes / strong in compression - weak in tension / primary use as wall-pier or arch.

• Concrete formed (cast) into many shapes / on-site (in situ) or off-site (pre-cast) / strong in compression - weak in tension (corrected by adding reinforcement) / excellent fire resistance / versatility: can be used for almost every type of structural member.

• Steel formed into a range of standard extruded shapes. Can also be cast / very strong in both compression and tension / can be assembled into many configurations by use of standard connecting techniques (bolting and welding) / versatility: can be used for almost every type of structural member because of its relative ease of fabrication, but due to its density and cost, steel usually is formed into thin linear shapes (beams, columns, rods/cables, plate structures) and therefore lends itself more to frames or tension structures (cable stayed/suspension) / must be protected from fire.

• Wood naturally occurring / renewable building material / formed into linear or planar elements by sawing (timber or lumber) or peeling (veneer) trees / lightweight and good strength to weight ratio / susceptible to fire and natural deterioration (rotting, insect infestation) / can be assembled into many configurations by use of standard connecting techniques (nail, screw, bolt, glue) / strong in tension and compression but dependent on orientation of grain (non-isotropic).

• Other Aluminium, Glass, PVC coated polyester (membranes), FRP (fiber reinforced plastic), compacted earth (adobe).

External Applied Force > Internal Force > Stress > Deformation

P, R, W, w, M, T FT, FC, V, M, T ft, fc, fb, fv, ft ∆ or deg

concentrated load P (kN) axial tension FT (kN) tensile stress (N/mm2) elongation (mm)

reaction force R (kN) axial compression FC (kN) compressive stress (N/mm2) shortening (mm)

resultant force W (kN) shear V (kN) shear stress (N/mm2) diagonal distortion

distributed load w (kN/m or kN/m2)

moment force M (kN-m) moment M (kN-m) bending stress (N/mm2) curvature/deflection

torsional force T (kN-m) torsion T (kN-m) torsional stress (N/mm2) twisting

Stress and Deformation

tensile stress (ft) causes stretching or elongation

compressive stress (fc) causes shortening

bending stress (fb) causes curvature

shear stress (fv) causes diagonal distortion (a rectangular area becomes a parallelogram

torsion stress (ft ) causes twisting

elongation and shortening

Stretching an elastic element will cause elongation but also necking, a secondary deformation. Compressing an elastic element will cause shortening but also bulging, a secondary deformation.

From Design-Tech: Building Science for Architects. Jason Alread and Thomas Leslie

Strength failure

Failure in tension (left). Failure in compression (right).

Stability failure

Failure due to buckling of the column.

ElasticityMost materials subjected to a stress such as tension, will stretch or elongate in a predictable manner and return to their original shape. This is referred to as elasticity. If a material is linearly elastic, doubling the amount of stress doubles the elongation.

StrainStrain is a term that is used to describe the amount of elongation or shortening in a material subject to stress. It is defined as the ratio of the change in length of an element (∆L) to its original length (L), or ∆L / L . This ratio known as strain is represented by the symbol epsilon or e. There are no dimensional units to strain as it is a ratio or percentage.

In the seventeenth century a scientist named Robert Hooke (1635-1703) discovered that for many materials there is a general relationship between stress and strain that is linear and constant. This relationship came to be known as Hooke’s Law and states that for an isotropic elastic material, the ratio of the amount of stress present in a body to the strain that is produced is a constant. This constant, which varies for different materials, is known as Young’s modulus after the 19th century British scientist Thomas Young, who investigated its application in mechanics. It is also known as the modulus of elasticity, E.

stress / strain = E

From Design-Tech: Building Science for Architects. Jason Alread and Thomas Leslie

stress / strain diagram for steel

Modulus of Elasticity (E)(mild steel)

E = 200,000 N/mm2

E = Stress / Strain

What is the strain (elongation of a specimen of steel)?

At a stress of 200 N/mm2

E = 200 N/mm2 / Strain or

Strain = 200 N/mm2 / 200,000 N/mm2

= 0.001 ( ∆L / L )

Stress-Strain Diagram for Mild SteelThe slope of the stress-strain diagram in the linear elastic region is E (Modulus of Elasticity).

Elasticity versus Plasticity

Past a certain point in the stress/strain test a test specimen will continue to to elongate or stretch under no appreciable additional load. This is referred to as the plastic range. Once in the plastic range the material undergoes a permanent deformation. For ex., if the test specimen is unloaded and its length measured there will be a change: the length will have increased.

Yield pointThis is the point on the stress / strain curve at which the material yields and begins to stretch with no extra load applied. It marks the beginning of the plastic range. Not all materials have a yield point. Steel has a very distinct yielding point and this limit is used to determine safety margins in design under the working stress method of steel design.

Strain hardening

Beyond the yield point the material stretches for a certain amount and then becomes stiff again. Further load may be applied and the material will offer resistance. This range of capacity beyond the yield point is known as the region of strain hardening.

Ultimate StrengthA test specimen subjected to increasing load in the strain hardening range will offer resistance and continue to gradually elongate until the limit of the material tensile strength is reached. At this point the specimen will abruptly begin to stretch. Some necking or reduced cross sectional area occurs in the specimen before the material actually breaks.

Ductility versus Brittleness

The property that allows a material to undergo large amounts of deformation before breaking is known as ductility. Rubber, for ex. is extremely ductile. And so is steel although it is a much harder material. A material that can not stretch easily and breaks with very little elongation is called brittle. Glass is an example of a brittle material. Also concrete and cast iron.

GPa = kN/mm2

MPa = N/mm2

Stress-strain diagrams for various materials.

19

Appendix A Density of Materials

Materials Density (kN/m³)

Plain 23.6 Reinforced 24.5

Concrete (normal weight aggregate, with or without PFA)

Prestressed 24.5 Brick work 21.7 Brick and block work

Concrete blocks 20.6 Aluminium 27.2

Brass 83.3 Bronze 87.7 Copper 87.7

Iron (cast) 70.7 Iron (wrought) 75.4

Lead 111.0 Steel 77.0

Metals

Zinc 70.0 Cement mortar 23 Gypsum mortar 18

Lime-cement mortar 20

Mortar

Lime mortar 18 Granite 29 Marble 27 Basalt 30

Sandstone 25

Natural stone

Slate 28 Timber Refer to suppliers specifications

Hardboard 11 Chipboard 8 Plywood 6

Blockboard 5

Wood

Wood-wool 6 Other materials Glass 26

Soil 20 Acrylic sheet 12 Asphaltic concrete 25 Mastic asphalt 18 Hot rolled asphalt 23

http://www.bd.gov.hk

From Introduction to Architectural Technology. Chart developed by Mike Ashby, Cambridge University, Department of Engineering.

steel

Steel production

When iron is smelted from its ore, it contains more carbon than is desirable. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. In the past, steel facilities would cast the raw cast iron product into ingots which would be stored until use in further refinement processes that resulted in the finished product. In modern facilities, the initial product is close to the final composition and is continuously cast into long slabs, cut and shaped into bars and extrusions and heat treated to produce a final product. Today only a small fraction is cast into ingots. Approximately 96% of steel is continuously cast while only 4% is produced as ingots.

The ingots are then heated in a soaking pit and hot rolled into slabs, billets or blooms. Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods and wire. Blooms are hot or cold rolled into structuralsteel sections such as universal beams (I-beams), channels, angles, etc. In a modern steel mill these processes occur in one assembly line, with ore coming in at the start and finished steel products coming out the end.

Steel bloomsSquare or rectangular about 500mm in length

Steel billetsSquare (approx. 180mm x 180mm). Length varies

Steel slabsFlat and up to 3000mm wide and up to 320mm thick

Use of steel in building structure

Cast iron 1780 – 1850Wrought iron 1850 – 1900Steel 1880 - present

Sainte Genevieve Library (1850) Eiffel Tower (1889) Shukhov Radio Tower (1922)Henri Labrouste (Paris) Gustav Eiffel (Paris) Vladimir Shukhov (Moscow)

Design strength: variation in the yield strength of various steel grades

Basic Design Strengths for Steel*

BS 4360 Grade Thickness: less than Design Strength (Yield Strength)

or equal to (mm) (N/mm2)

____________________________________________________________

43 16 275

A, B, & C 40 265

100 245

50 16 355

B & C 63 340

100 325

55 16 450

C 25 430

40 415

*reproduced from BS 5950 The Structural Use of Steelwork in Building

Basic permissible (allowable) stresses for steel grade 43 (BS 4360)*

Stress type Basic Permissible Stress (Grade 43 steel)

____________________________________________________

Tension 155 N/mm2

Compression 155 N/mm2

Bending 165 N/mm2

Shear (average) 100 N/mm2

Bearing 190 N/mm2

Factor of Safety:

If the yield stress of Grade 43 steel (thickness 41 – 100 mm) is 245 N/mm2 and the permissible stress in tension is 155 N/mm2, then what is the factor of safety using this value?

245 N/mm2 / 155 N/mm2 = 1.58

or the amount of the capacity for stress below the yield point used is 63%

*In Hong Kong, Grade 250 steel refers to a grade of steel with a nominal yield strength of 250 Mpa and having a similar chemical composition and mechanical properties to Grade 43.

Other issues

• Processing: Finished steel products are formed through different processes which include: casting (cast steel has <2% carbon versus cast iron alloys), rolling (hot rolled sections, wire rod, re-bar,rails, sheet piling and finished flat products) and forging (compressive force applied to hot steel by forging tools that improves the strength).

• Fabrication | Forming: Forming involves the bending or shear deformation of a piece of solid steel, generally a flat sheet. Various cutting processes include shearing, sawing, drilling, milling and grinding. New cutting methods include CNC (computer numeric control) cutting using laser, mill bits, torch or water jet.

• Connections: Steel is most commonly connected by bolting or welding. Bolts replaced an earlier technology of riveting. In some rare cases, steel members may connected by interlocking shapes that remove the need for a third component.

• Grades of steel: steel varies in grade according to chemical content of the alloy (the presence and amount of carbon, e.g.). The chemistry will alter the properties of steel such as strength, plasticity, toughness, weld-ability, and many other characteristics.

• Fire protection: various fireproof materials and assemblies are used to protect steel from exposure to high temperatures.

• Corrosive protection: galvanizing (zinc coating).

• Recycling: steel is an easily recycled building material. Steel structural elements can be dis-assembled and re-used or they can be reformed into new steel products.

concrete

Concrete consists of cement, aggregate and water.

The proportions of these components determine the strength (maximum compressive stress) and the workability (flow or viscosity).

Aggregate consists of both fine aggregate (sand) and coarse aggregate (stone).

Various mix proportions (by volume) and the associated compressive strengths.

stress / strain diagram for concrete concrete cylinder test

stress / strain diagram for concrete

From Structures Design Manual for Highways and Railways. Third Edition, 2006. Published by the Highways Department of the Government of the Hong Kong Special Administrative Region.

Slump test for consistency of concrete. Amount of water by volume required in concrete.

A slump of between ½ to ¾ the height of the cone is acceptable. Below ½ there is probably too much water.

Properties of Concrete. Curing time.

Left: effect on ultimate compressive strength

Right: time needed before removing formwork and props

PFA refers to “Pulverized Fuel Ash”. PFA is a by-product of coal-powered power stations. It is a pozzolanic material. In the presence of lime (contained in Portland cement) it has hydraulic (cementing ) properties.

wood

Wood Use

3.4 billion m3 of wood (logs) cut down in 2000. 12-15 million hectares of forest are lost every year (existing forest area is 3.9 billion ha).

55% used for energy, 45% for other uses (products such as lumber, paper, etc.).

Of the 45% (1.5 billion m3) of wood used for other products: 28% converted to lumber, 12% converted to products such as strand-board, particle board, etc., and 21% to paper and 39% to other uses. (therefore, approximately 18% of all trees cut down is used in the production of building products)

Wood use

Lumber

Engineered Wood

Paper

Other uses

Energy

Ecological and Constructional Advantages of Wood Construction

Sustainability: wood occurs naturally, the forest is society’s friendliest factory. Use of wood instead of other less environmentally friendly materials (e.g. concrete and steel) protects and preserves the environment. Wood for use in the building industry can be provided by tree farms, thus saving ‘old growth’ forests. Adherence to the Forest Principles of the “Earth Summit” in Rio de Janeiro (1992).

Climate change: use materials that help reduce greenhouse gases. Wood as a naturally occurring resource requires less energy to produce. Wood (trees) converts CO2 to carbon (stored in the material) and oxygen (released in the air) thereby reducing greenhouse gas. This process occurs through forest growth, therefore trees need to be harvested and replaced to allow for continued extraction.

Recycling: As a biodegradable substance, wood is easier to dispose of or to recycle, requiring less energy. The three main ways that wood is disposed of are: biological decomposition, material recycling, and as a source of energy.

Advantages as a construction material: high strength to weight ratio, good insulator, easy to form and use. A natural building material with aesthetic and visually pleasing qualities.

Embodied energy is the total energy required for the extraction, processing, manufacture and delivery of building materials to the building site. It does not include the energy consumed for the placement or disposal of materials.

Energy consumption produces CO2 which contributes to greenhouse gas emissions.

Selection of materials based on energy consumption should also consider:

• Durability of materials (long life)• Use of locally sourced materials (minimize energy cost of transportation)• Use of recycled materials• Specifying standard sizes of materials (reduces waste)• Use of materials manufactured using renewable energy sources (human labor, wind/solar power,

etc.)• Consideration of ease of disassembly for purpose of recycling and disposal

Embodied energy of common building materials in MJ/kg

Air-dried sawn hardwood 0.5Kiln-dried sawn hardwood 2Plywood 10-15Glue-laminated timber 11.0Laminated veneer lumber (LVL) 11.0Imported dimensioned granite 13.9Clay bricks 3Concrete blocks 1.5In situ concrete 1.9Glass 12.7Steel (common with recycled content) 20Galvanized Steel 38Vinyl flooring 65Aluminium 170

Strength to Weight ratio of WoodWood is an anisotropic material with a relatively high strength to weight ratio.

Strength to weight ratio of various materials

tensile density specific strengthstrengthN/mm2 g/cm3 kN-m/kg

Concrete 2-5 2.30 5.22Steel (mild) 365 7.87 46.4Nylon 78 1.13 69.0Steel (HY) 600 7.85 75Aluminium 310 2.70 115Oak wood 90 0.74 120Titanium 1250 8.28 151Balsa wood 73 0.14 521Spider silk 1400 1.31 1069Glass fiber 3400 2.60 1307Carbon fiber 4300 1.75 2457Kevlar 3620 1.44 2514

Potential of forming and variety of shape

Type Product Forming methods

Sawn dimensional lumber chainsaw, band saw, circular sawlog, squared timber, plank, board batten

Laminated plywood rotary cutting of log + adhesive

laminated veneer lumber (LVL) rotary cutting of log + adhesive

laminated strand lumber (LSL & PSL) sliced veneer strands + adhesive

glue laminated lumber (Glu-lam) squared lumber + adhesive

cross laminated lumber (CLL) squared lumber + adhesive

Pressed oriented strand board (OSB) compressed strands/flakes + adhesive

particle board compressed waste wood chips + adhesive

medium-density fiberboard pressure formed wood fibers

masonite pressure formed wood fibers

Glu laminated timber or glulam

Probably the oldest engineered wood product. Earliest patented version around 1906. Invention of resin glues after 1928 that are waterproof and have high adhesive strengthled to the widespread development of glulam products. Also the invention of finger joints enabled larger contact area for gluing further contributing to strength.

Glulam elements are made from gluing relatively small pieces of timber together to makelarger sections. Hence, the product can be created from fast growth sustainable lumber.The number of laminations depends on the size. In most structural elements, such as

beams, the bending stresses are highest on the outermost portions of the element. Therefore, in thisregion the glulam uses higher strength (and more expensive) timber with lower strength andless expensive wood placed in the center.

Different cross sectional shapes are possible with glulam. Elements can be designed to act as beams, columns or slabs. Frames and arches of varying cross sections and diverse profiles are obtainable.

Cross laminated timber (CLT) was developed in 1990’s and has become a major wood structural product. CLT panels can be used for floor systems as well as walls. A CLT panel has a minimum of three layers, normally of softwood. Layers can be replaced with other products such as OSB (Oriented Stranboard) or PLV (Parallel Veneer lumber) for increased strength.

PlywoodAlso an early engineered wood product. Modern development in the 1930’s.

Odd number of layers of veneer with grain of wood alternating in direction. Grain in outer layers parallel to long dimension (strongest direction).Size: some variation but standard is 4’ x 8’ (1200mm x 2400mm) and 1/4” (6mm) to 1 ¼”

(32mm)Roofing (5/8” or 15mm) Flooring (>3/4” or 18mm)

Composition: different types of wood used such as spruce, redwood or Douglas fir (soft/average quality), birch (hard/high quality),

Laminated veneer lumber (LVL or Microstrand) Parallel strand lumber (PSL) Laminated strand lumber (LSL or Timberstrand)

LVL uses 30% more of log vs. lumber (veneers from logs approx. 400-800mm ø)LSL uses >70% of the log (220-300mm ø)Similar uses: posts, beams and headers.

Grain is oriented in same direction.

Advantage: uses fast growth young trees of smaller diameter and age but produces an equivalent size (or larger) to lumber with much greater strength, dimensional stability andabsence of defects. PSL uses lower grade trees that would otherwise not produce usable

lumber.

PSL and LSL resist moisture-induced warping better than LVL.

Front to back: LVL, LSL, PSL Sizes LVL: up to 90mm thick and up to 25m in lengthPSL: up to 280mm thick x 510mm wide and up to 20m in lengthLSL: up to 140mm thick x 2500mm wide and up to 15m in length

Oriented strand board (OSB)Particles of wood (strands) are glued together in layers. Orientation may be parallel (on exterior face) or transverse (on interior layers).

Size: thickness 6-40mm, l x w 1220-2620mm x 2440/5000mm.

Use: sheathing and some dry load bearing applications.

Particle boardSmall timber particles pressed together with adhesives or mineral binders. Strength of the board is controlled by the orientation and physical characteristics of the particles and the binding agents (adhesive, cement, gypsum).

Use: non-load bearing applications. Sometimes use as webs for structural members.

Size: thickness 2-38mm l x w 1250mm x 2500mm (common).

Medium density fiberboard (MDF)Wood fibers pressed together in either a dry or wet process (high temperature and

pressure). Bond created by interlocking of fibers and the self-adhesiveness of the fibers. Generally isotropic (strength equal in all directions).

Uses: non-load bearing applications, substrates for veneers (cabinetry), models and mock-ups.

Size: thickness 3-25mm, l x w 1250mm x 2500mm.

Hardboard (Masonite)Method of production is steam-cooked and pressure molding of wood fibres. Process

patented by William H. Mason in 1924.

Denser than MDF (950 kg/m3 compared to 650 kg/m3)

Size: thickness 5-16mm, l x w max. 2100mm x 2500mm.

Uses: non-loading bearing protective sheathing

Fiberglass reinforced plywoodSheets of plywood are bonded between tough glass fiber reinforced resin surfaces.

Advantages: Strong but lightweight. The thin and non-porous FRP surface provides color, weatherability and additional durability.

Uses: not generally used in the building construction field but could have applicability for siding.

Polystyrene woodExtruded product from virgin or recycled plastic. Can have “wood-like” appearance. May

contain some sawdust or other non-plastic elements. Can be colored and has long lasting durability.

Size: varies

Uses: Non-load bearing such as decking, siding, etc. Can be used for planks if span is short.

Wood polystyrene compositeSandwich panel formed from sheets of wood with a core of polystyrene.

Advantages: Sandwich panel has depth which improves bending strength but is relatively lightweight. Polystyrene provides additional insulation.

Uses: roof decking and non-load bearing walls.

Calculating stress in a linear element subject to either tension or compression.

Internal Force: tension force in a hanging rod Internal Stress: tensile stress in a hanging rod

Cross section of rod

Area (A) = π r2 = (3.14)(10mm)2 = 314mm2

Tensile Stress (ft) = Ft / A

If W = 10 kN (approx. 1000kg) then:

ft = 10 kN / 314mm2

= 31.8 N / mm2 (or 31.8 Mpa)

ft << 400 N / mm2

(400 N/mm2 = ultimate tensile strength of steel*)

Internal Force: compression force in a column Internal Stress: compression stress in a column

Cross section of column

Area (A) = 300mm x 200mm

= 60,000mm2

Compression Stress (fc) = F / AIf P = 1000 kN (approx. 100,000kg)

then 3P = 3000 kN (at bottom of column)

fc = 3000 kN / 60,000mm2 = 50 N/mm2

since ultimate stress of concrete = 20.7 N/mm2

and 50 N/mm2 > 20.7 N/mm2

then column cross section is too small.

Reference

Design and Control of Concrete Mixtures 13th ed.. Steven H Kosmatka and William C. Panarese. Portland Cement Association, Stokie, IL, 1992.

http://www.bd.gov.hk/english/documents/code/steel/e_steel.htm

http://www.bd.gov.hk/english/documents/code/concrete/e_concrete.htm

Materials in Construction 2nd ed. G. D. Taylor. Longman Press, UK, 1994.

Introduction to Architectural Technology. Pete Silver and Will McLean. Laurence King Pub, London, 2008.

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