metals: steel framing

steel type:

cold formed steel

use case:

trusses/lighter weight framing & decking

steel type:

open web steel joist

use case:

floor support & trusses/lighter weight framing decking

steel type:

high strength, low alloy

use case:

beams and columns

steel type:

weathering steel

use case:

beams and columns

steel type:

wide flange

use case:

floor support & beams and columns

steel type:

stainless steel

use case:

corrosion resistant

steel type:

hss (hollow structural section)

use case:

beams and columns & trusses/lighter-weight framing

steel type:

cast iron

use case:

strongest in compression but most brittle

carbon content in iron alloy

• greater proportions of carbon generally increase the hardness and brittleness of the iron alloy

carbon content in iron alloy graph

a. strong in tension/malleable

b. most commonly used for “structural steel”

c. strong in tension/compression (not prone to failure)

d. most brittle, prone to sudden failure

wrought iron

• little or no carbon

• strong in tension, weaker in compression

• malleanle (easily shaped) and soft

mild steel (low carbon steel)

• most common alloy for modern structural steel

• not more than 0.3% carbon

• small amounts of other metals improve strength, toughness, and other qualities

medium/high carbon steel

• more than 0.3%, less than 2% carbon

• strong in both tension and compression

• ductile (not prone to sudden failure)

cast iron

• 2%-4% carbon

• strong in compression

• less strong in tension

• brittle (vulnerable to sudden failure)

steelmaking

• in a traditional steel mill, iron ore is the best raw ingredient

• first, it is processed into molten iron in a blast furnace

• then followed by conversion to steel in a second operation

steelmaking cont.

• molten steel being tapped from an electric arc furnace into a vessel called a “ladle”

casting (steelmaking)

• as the molten steel begins to solidify, it is cast into shapes ranging from plain rectangles or rounds to more complex cross sections, such as beam blanks, that approximate the shape of finished beam products

rolling mill

• structural shapes are produced in a rolling mill

• prior to rolling, the beam blanks are reheated to the necessary temp.

recycled content

• about 25-35% recycled content

• In North America, much of the structural steel is made from recycled scrap in mini mills using electric arc furnaces, converting scrap into new steel without needing to make iron from ore.

• primary products are flat-rolled stock, such as steel decking and other sheet products

structural shapes

• wide flange (w-shape): the most commonly used shape for beams and columns

• channels, angles, tees: ofr trusses, lighter weight framing, and other miscell. uses

shape designation

• W10×30

  • W: wide flange shape

  • 10: nominal/theoretical depth (inches)

  • 30: 30 lbs/lineal ft

wide flange shapes

• taller more narrow profiles are
  best for horizontally spanning
  elements such as beams and
  girders

• thicker, more balanced (same
  dimension both ways) used for
  vertical spanning elements such
  as columns.

aisc steel construction manual

• dimensions page: dimensional data

• properties page: shape properties related to structural stiffness and strength calculations

some exceptions in sizing

• size designations are
  nominal, not always an
  accurate indication of
  actual depth.

• a W14 x 873 is 23.6" deep!
  

other shape designations

• S: American Standard beam
  (“I-beam”)

• MC: Miscellaneous channel

• C: American Standard channel

• L: Angle

• WT: T-shape cut from W shape

• HSS: hollow structural section

hollow structural sections (hss)

• hollow square, rectangular,
  round, and elliptical shapes

• are used for trusses, structurally efficient
  column sections, and where
  the simple external profile is
  desirable

• Example designation:

  • HSS 8 x 8 x ½
    8"x8"x½" wall thickness

high strength, low alloy steels

• use of stronger steel alloys
  permits savings in weight and
  reductions in the size of
  structural elements, reducing
  costs

weathering steel

• surface oxidation
  adheres to base metal,
  limiting further rusting.

stainless steel

• added nickel and chromium

• forms self-protecting oxide
  layer that provides long
  lasting protection against
  corrosion

cold-formed steel

• deforming steel in its cold state
  causes realignment of the steel
  crystals and increases its strength.

  • corrugated steel decking (right)

  • steel studs and joists

  • high-strength wire for concrete
    prestressing strands

open-web steel joists (owsj)

• lightweight pre-manufactured
  trusses

• made from both hot- and cold-
  formed components

• depths range from 8 in. to 6 ft

• used for floor support

how do we join steel?

• riveting: not widely used today

• bolts: faster assembly on site

• welding: more cost effective

riveting

• white-hot fastener is inserted
  through holes in members to be
  fastened

• fastener is hammered to
  produce a head on the plain end

• as the metal cools, it contracts,
  and tightly clamps the steel
  members

• mostly found in historic
  structures. Rarely used in
  modern construction

bolted connections: carbon steel bolts

• relatively low strength

• limited uses, such as fastening
  light framing elements or
  holding temporary connections

bolted connections: high strength bolts

• carbon steel bolts not used
  often.

• instead, we tend to use high-
  strength bolts:

  • stronger than common bolts

  • used for fastening primary
    structural members

bearing-type connection

• body of bolt resists movement between connected members by bearing directly against sides of bolt holes (left).

• bolt is stressed in shear.

  • connection will slip before it reaches full strength.

slip-critical connection

• bolt is tensioned to such an extent that movement in the joint is resisted
  by friction between the adjoining "faying" surfaces of the members
  themselves (right).
  • bolt is very highly stressed, but in tension.
  • no slippage

bolt tensioning

• with slip-critical connections, proper bolt
  tension must be assured.

• turn-of-nut method: Nut is tightened
  some additional fraction of a turn
  after achieving a snug condition.

load indicator washers

• when bolt is adequately tensioned,
  protrusions on the washer are flattened.

• bolt tension is verified by inserting a gauge between bolt head and washer.

squirt dye for bolt tensioning

• some load indicator washers squirt dye
  when adequate tension is achieved, making inspection easier

tension control bolt

• when bolt is adequately tightened, the splined end snaps off

• bolt tension is verified by visually inspecting for splines.

• t.c.b. also allow tightening with access to only one side of the bolt.

welding

• steel surfaces to be joined are heated to a molten state

• additional molten metal is added from the electrode

• in finished joint, members are fully fused

welding terms

• fillet welds: easy to make, little
  joint preparation required

• groove welds: require properly
  shaped and spaced joints

• puddle welds: fasten metal
  decking to structural members

structural stability of welding

• welds critical to structural stability may be inspected/tested (ultrasonic, magnetic, etc.) to ensure their soundness and freedom from hidden flaws.

details of steel framing

• connection types

  • shear (simple connection)

  • moment (fully restrained)

• steel frames

  • braced frame (simple connection)

  • moment connection (fully restrained)

  • shear walls

shear connection

• angles, plates, or tees connect web of beam to side of column.

• angles are usually joined to beam in the fabricator's shop.

• beam/angle assembly is bolted to the column in the field

shear connection (simple connection)

• transfers gravity loads from beam to columns.

• not sufficiently rigid to transfer bending forces.

• modeled structurally as if it is free to rotate, or hinged.

• AISC Simple Connection (SC)

seated connection (shear conn.)

• another version of shear connection is a
  ‘seated connection’ which relies on an angle below the beam and above.

• this is used when connecting a beam to a column web

moment connections

• transfers gravity loads and bending forces.

• beam flanges are joined to column.

• column is reinforced to carry bending forces.

• beam is restrained from even small rotations.

• AISC Fully Restrained (FR)

beams/columns in mom. conn.

• a pair of beam-column moment
  connections

• full penetration welds join top and bottom beam flanges to column and column reinforcing plates

dog bone cut (mom. conn.)

• "dog bone" cut creates a zone of weakness.

• in extreme seismic event, this area will deform plastically, protecting the more brittle welds from failure.

• single common bolt holds beam temporarily prior to welding.

stabilizing building frame

• rectangular geometry of the building frame must be made stable against lateral forces by:

  • diagonal bracing

  • moment-resisting frame

  • shear walls

braced frame

• diagonal bracing creates stable, triangular geometry

• can be constructed with shear connections between beams and columns (free to rotate).

moment resisting frames

• some or all of the beam-column connections are moment connections
  capable of resisting rotations between the members, making the frame stable

shear walls

• rigid vertical walls or core structures resist lateral forces.

• remainder of frame relies on shear connections.

• usually concrete (occasionally heavy steel plate)

bolted web and welded flanges (connection)

• the shear tab is deep enough to position
  the bolts clear of the column flanges for easy access.

• AISC Fully-Restrained (FR)

construction process timeline

1. Preparation of Structural Drawings (Structural Engineer)

2. Preparation of Shop Drawings (Fabricator)

a) Detailed fabrication & erection drawings
b) Detail each piece and connections

3. Submission & Approval

4. Order “stock” lengths

5. Fabricate each piece (after Shop Drawing Approval)

6. Ship to Jobsite

7. Erection (Erector)

steel framing plans

• shows sizes and locations of
  steel members

• dimensions have been omitted
  from the plan in pic

steel framing plans examples

• W30 girder-column connection: girder to column flange

• W27 beam-column connection: beam to column web

• W18 to W30 connection: coped beam-girder

steel detailing and farbication

• fabricator makes shop drawings based on contract documents

  • each component is dimensioned

  • each connection fully detailed with dimensions of all connection angles, holes, bolt diameters and types of welds

  • shop or field connections identified

  • type of bolted connection

• after modifications by GC, A & E, materials ordered from rolling mill

• components fabricated in shop, shipped to site for erection

fabricator’s role

• the fabricator prepares shop drawings showing the configuration of each member.

• drawings are reviewed by
  architect and structural engineer

• also shares responsibility for design of the steel connections, based on more general requirements provided by the structural engineer.

building information modeling

• more recently, using bim systems, steel
  fabrication information and details
  made by the structural engineer in the building model can replace fabricator shop drawings.

steel fabricator

• fabricator prepares steel members and delivers them to the construction site

• workers cut to length, cope, drill, punch, weld, and add tabs, angles, plates and other accessories to members from the approved shop drawings.

• band saw cuts heavy members to size

• fabricated members are stacked using overhead crane, awaiting transportation to the construction site

• members are individually labeled to correspond to information on the erection drawings.

steel erector

• assembles steel members delivered to the construction site.

• may or may not be same entity
  as fabricator.

• workers are called ironworkers

steel frame erection sequence

• erect columns

• install beams and girders

• plumb structure

• complete (weld or tighten) all connections - including diagonal bracing

• install edge angles & decking

• start next tier

cranes used for steel frame erection

mobile, tower, and climbing cranes

structural steel erection (1st tier columns)

• column base plate

• distribute loads

• attachment to fdn.

• often shop welded

• holes must match

• anchor bolts in ftg

coulmn set to proper elevation

Options:

• Leveling Plate

  • set in grout prior to column erection

• Leveling Nuts

  • nuts set to elevation prior to steel erection

• Shims

  • metal shims set to proper elevation

plumbing structures

• as the frame is erected, temporary cables with turnbuckles are used to plumb up (make vertical) the frame.

steel decking

• corrugated steel decking laid over
  the framing is the most common
  floor and roof decking material.

• the decking is puddle welded to
  framing members

• steel decking comes in a variety of
  profiles and depths to suit different
  load and span conditions.

metal decking attachments

• mechanical fastener (self-tapping screws)

• welding (common for floor deck)

shear studs

• welded to tops of beams, through metal decking

• once concrete is poured, the beams, decking, and concrete act together structurally as another form of composite construction

composite metal decking

• functions as formwork for concrete and steel reinforcement

• often in combination w/ shear studs

• creates a shear connection between
  deck & frame

• increases carrying capacity

• produces lighter, stiffer, & less costly
  frame

fireproofing

• above roughly 500-600 F,
  steel rapidly looses strength

• fireproofing acts as insulation, protecting steel from the heat of fire

fireproofing materials

• concrete

• plaster

• gypsum board

• gypsum wallboard

• spray-applied insulation

• loose insulation within
  column cover

spray-applied fire resistive material (sfrm)

• most common approach

• to achieve an equal level of protection, lighter members need more insulation than heavier ones, since lighter members heat up more quickly

• spray-applied fireproofing, in progress, applied to framing in a steel high rise structure (in pic) (thickness is greater on columns)

• metal decking does not require additional fire protection

steel & building codes

• without fire protection:

  • building heights & areas severely limited

• with proper fire protection:

  • unlimited building heights & areas permitted for most occupancy groups

corrosion: 2 basic causes

• ferrous metals oxidize rapidly when exposed to moisture, salt, chemicals

• galvanic action between different metals

corrosion protection approaches

• stainless steel (Type 316) should be used in critical exposures/applications

• isolate/insulate incompatible metals

  • e.g., plastic washers

• rust-inhibitive protective coatings

• reduce ambient moisture & humidity; increase local air circulation

• sacrificial galvanic anodes (temporary/time based)

corrosion protection approaches (temp. & perm.)

• temporary protection

  • prime coat (red-brown color)

• permanent protection (method varies with environment, and type of steel)

  • large exposed structural steel

    • acrylic, epoxy

    • polyurethane coating most permanent for exterior application

  • cold formed or light structural steel

    • galvanizing (zinc coating metallurgically bonded to steel)

early detection of corrosion: how?

• currently, when accessible, via E-M
  current & ultrasound technologies

• being studied:

  • fiber optics

  • “smart paint” which changes color in the presence of corrosion

  • sensors