CIVL1810 Chap.8 Steel Sections and Bolted Connections
Steel Sections and Bolted Connections
Steel is chosen because it is cheap for what it does, lasts, leaves the designer free, is simple, goes up in any weather, is easy to mend and can be melted down again, and because a lighter floor lets a building go taller for less money in less time.
The design content is short and precise: a ductile stress strain curve reduced to a yield stress and a tensile strength, AS 4100 for design, fabrication, erection and modification by the limit states method, cold formed sections under AS NZS 4600, and a compact, non compact or slender classification that is really a statement about whether the plate elements can reach yield before they buckle locally.
The rest of the week is connections, because that is where steel structures are designed and where they fail.
What this chapter covers
- 01
Seven reasons for building in steel, grouped as material, design and site operation properties
- 02
Steel framing for mid rise and tall buildings and for bridges of long span, with the columns inside the plan taking most of the load
- 03
The stress strain curve: elastic, plastic, strain hardening and necking, and the elastic plastic approximation
- 04
Yield stress used in design and tensile strength used in design, and the two grades tabulated
- 05
AS 4100 as the scope standard for design, fabrication, erection and modification by limit states
- 06
AS NZS 4600 for cold formed sections, and the cold formed residual stress classification
- 07
Reading a universal section designation as nominal depth and mass per metre
- 08
Compact, non compact and slender classification as defined in AS 4100
- 09
Section buckling as local instability of the plate components, such as the flanges of an I section
- 10
Composite beams and composite columns, and the role of the shear connection
- 11
Bolts in tension and bolts in shear, and the threads and shank distinction
- 12
Single shear across one plane and double shear across two, with each plane carrying half the force
- 13
Connection elements and configurations: end plates, splices, cleats, continuity, beam to beam and column bases
- 14
Four detailed connections and the bolt loading each one produces
- 15
Coping the flange and web, with a minimum radius at the cope corner
Why a splice uses cover plates on both faces
- +1Count the planes. A lap puts the two plates in contact along one interface, so the bolt is cut across one plane and the whole force P crosses it. A symmetric splice sandwiches the member between two cover plates, so the bolt is cut across two planes.
- +1Divide the force. In the symmetric arrangement the force divides, so each plane carries P over two. For the same bolt and the same total force the shear stress therefore halves, and the connection carries roughly twice the load before the bolt shears.
- +1Name the geometric benefit. A lap loads the joint eccentrically, because the two plate forces are offset, so the joint tends to rotate and pull the plates out of line. The symmetric arrangement removes that eccentricity, which is a second and independent reason to prefer it in a splice.
Key terms
- Yield stress and tensile strength
- The two values that characterise a steel grade. AS 4100 defines both for design purposes, and between them they bracket the simplified elastic plastic curve that replaces the real one.
- Section buckling
- Failure in one of the plates that make up a profile, the flange of an I section being the standard case. It is a local event in a single plate, not the whole member bowing over its length.
- Compact, non compact and slender
- The classification of a section as defined in AS 4100, which states in effect whether the plate elements can reach yield before they buckle locally.
- Single shear
- A bolted arrangement in which the whole applied force is transferred across one plane, typically a lapped joint. It also loads the joint eccentrically.
- Double shear
- A bolted arrangement in which the member is sandwiched between two plates, so the force divides and each of two planes carries half.
- Web side plate
- A plate welded to a column and bolted to a beam web. Although often thought of as a simple shear connection, the bolt loading is combined shear and tension, and torsion in the beam puts the bolts into tension.
- Coping
- Cutting away part of the flange and web of an incoming beam so it can pass a column flange or web. A minimum radius is required at the cope corner, because a sharp re entrant corner concentrates stress and starts a crack.
Steel Sections and Bolted Connections FAQ
Is a web side plate a shear connection?
Not purely. The tutorial solution classifies both the web side plate into a column flange and the web side plate into a column web as being subjected to combined shear and tension, and notes that if the beam is subjected to torsion the bolts are subjected to tension loading.
The eccentricity between the bolt group and the supported load is what generates the tension component, so treat nominally simple connections as combined unless the geometry genuinely removes that eccentricity.
What loading does a column base plate produce in the holding down bolts?
Tension. Vertical load passes from the column into the footing largely by bearing, with the plate spreading the concentrated load over an area of concrete the footing can carry, so the bolts are not the primary path for compression. What they resist is uplift and the tension side of any base moment, which for an exposed frame is generated by wind.
What does a designation such as 610 UB 125 tell you?
That it is a universal beam of nominal depth 610 millimetres weighing 125 kilograms per metre. The word nominal matters: the tabulated depth of that section is 612 millimetres, so the designation is a label and the table is where the real dimensions live.
Comparing a universal beam with a universal column of the same nominal depth also shows the column is much heavier, because it is a stockier profile intended to carry axial load.
What makes a beam and slab composite rather than merely adjacent?
A connection that transfers longitudinal shear across the interface. Without it the steel beam and the concrete slab deflect together while sliding at their contact face, and no composite action exists. With it, the slab takes compression at the top of the section and the steel takes tension at the bottom, and the slab also restrains the compression flange so local buckling is suppressed.
Exam move
Learn the connections as a small table with three columns: where the detail sits, how the load transfers, and whether the bolts are in shear, in tension or in both. That is exactly the tutorial task, and it is how the paper asks about it. For the shear question, practise the sketch until it takes under a minute: two plates and one plane, then three plates and two planes with the force labelled P and P over two.
Separate local from global instability in your mind, since calling a bowed member section buckling loses the mark. Finally, be able to state the material argument for steel in three groups rather than as a list of seven adjectives, because grouped reasons are easier to recall under pressure and read as understanding.
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