Module 4: Structural Steel Construction Systems

Steel

Iron-based alloys having a carbon content less than cast iron and more than wrought iron

Steel qualities

Strength, hardness and elasticity

Steel measured in

Weight to volume

Bessemer process

1855, removes impurities from iron through oxidation

Cheap steels with high tensile and compressive strength available

About 1870

Wrought and cast iron

Met most of the demand for iron-based building products until 1880

Reliable mild steel era

1880

First steel-framed building in the US

Rand McNally Building, Chicago, 1890, Burnham and Root

Cold-formed steel

Sheet steel formed into shapes and sizes similar to dimensional lumber

Roll forming

Sheet steel passed through a series of rollers to form the bends that make the shape

Cold-formed steel strength

Studs and joists are made stronger than the original sheet steel

Hot-rolled steel

Raw steel rolled through compressive force at a temperature above 926°C

Hot-rolled steel tolerances

Not the same as cold-rolled due to thermal expansion

Hot-rolled steel shapes

Round bars, sheets, plates, I-beams, channels

Alloy steel

Carbon steel with chromium, cobalt, copper, manganese, molybdenum, nickel, tungsten or vanadium added

Stainless steel

Minimum of 12% chromium

HSLA steel

Low-carbon steel with less than 2% alloys, for increased strength, ductility and corrosion resistance

Weathering steel

High-strength, low-alloy steel that forms an oxide coating when exposed to rain or moisture

Tungsten steel

10% to 20% tungsten, for increased hardness and heat retention

Structural steel

Low- to medium-carbon steels used in elements 1/4 in. thick or more

Structural steel uses

Flexural members, columns, trusses, bearings and bearing plates, bracing, hangers, bolts and nuts, shear connectors, wedges, shims

Carbon steel alloy maximums

Manganese 1.65%, silicon 0.60%, copper 0.60%

HSLA yield strength

Larger than 40 ksi, without heat treatment

Heat-treated high-strength carbon steels

Heat-treated to achieve specified high strength and toughness

Heat-treated constructional alloy steels

Alloying elements in excess of carbon steel limits, heat-treated for high strength and toughness

Ductility

Percent elongation, permitting redistribution of stresses at points of high local stresses

Toughness

Capacity of steel to absorb energy

Cold working

Forming plates or structural shapes into other shapes at room temperature

Creep

Gradual change in strain under constant stress, usually not significant for structural steel framing in buildings

Hardness

Used to estimate tensile strength and check uniformity of tensile strength

Fatigue resistance

Material's ability to resist initiation and propagation of cracks from repeated cyclic loading

W shape

Wide flange (American standard)

S shape

American standard

C shape

American standard channel

L shape

Equal and unequal leg angles

WT shape

Structural tee cut from W shape

Structural tubing

Square or rectangular

Structural tubing (circular pipe)

Round

Bars

Square, round and flat

W shape designation

Example W8 x 31: nominal depth in inches, nominal weight in pounds per foot

Common yield strength of W shape

50 ksi

C shape designation

Example C7 x 14.7: nominal depth in inches, nominal weight in pounds per foot

Common yield strength of C shape

36 ksi

Rectangular HSS designation

Example HSS8 x 6 x 1/2: side 1, side 2 in inches, thickness in inches

Common yield strength of rectangular HSS

46 ksi

Round HSS designation

Example HSS7.00 x 0.50: diameter in inches, thickness in inches

Common yield strength of round HSS

42 ksi

Structural steel connections

Workshop or site connection, according to where fabrication takes place

Site connections

Mostly bolted

Workshop connections

Often welded

Connection type and number

Selected by the structural engineer according to the connection strength to be achieved

Column-to-foundation connection

Holding down bolts, base plate, levelling wedges or shims, grouted after final levelling

Welding

Fabrication process that joins materials, usually by using high heat to melt the parts together

Brazing and soldering

Lower-temperature metal-joining techniques that do not melt the base metal

Oxy-acetylene welding

May be used to fuse together light steel sections

Electric arc process

Preferred, more effective and efficient, applies an expendable steel electrode to fuse parts by high amperage current

Weld strength depends on

Base metal, filler metal, type of weld, throat and weld size

Shielded-metal-arc welding

Fuses parts by the heat of an electric arc between a coated metal electrode and the base metal

Submerged-arc welding

Fuses parts by the heat of an electric arc between a bare metal electrode and base metal

Gas-metal-arc welding

Fuses by the heat of an arc between a filler-metal electrode and base metal, shielded by gas or gas and flux

Electroslag welding

Uses a molten slag to melt filler metal and surfaces of the base metal

Electrogas welding

Maintains an electric arc continuously, uses an inert gas for shielding, electrode provides flux

Stud welding

Fuses metal studs or similar parts to other steel parts by the heat of an electric arc

Tack weld

Temporary weld used to hold parts in place while more extensive, final welds are made

Continuous weld

Extends continuously from one end of a joint to the other

Stitch weld

Series of welds of a specified length spaced a specified distance from each other

Fillet weld throat thickness

0.7 x weld size

Full penetration groove weld

Groove weld with full penetration through the joint

Structural bolt

Specialized, heavy type of hex bolt for large structures such as buildings and bridges

Hardened steel washers

Spread the pressure from the bolt-tightening process over a larger area, and may cover an oversized or slotted hole

Bolted joint failure modes

Bearing, fracture, yield, shank

Metal decking

Metal sheet as slab support and integral formwork substitute