Steel Structures Made Simple: Properties and Connections for Your Loksewa Exam
Concrete hides its skeleton, but steel wears its skeleton on the outside - every beam, angle, and bolt is right there for you to see. That makes steel structures one of the most visual, most "get it once and never forget it" chapters in the whole syllabus.
This post walks through structural steel properties, rolled sections, and the three ways engineers join steel members together (bolted, riveted, welded) - with clear sketches, a few gaps filled in that the usual notes skip, and every leftover question from your notes answered in full.
1. What is structural steel, and why we use it
2. Properties of structural steel (with the stress-strain curve)
3. Grades of structural steel
4. Advantages and disadvantages
5. Types of rolled steel sections
6. Bolted connections and bolt spacing rules
7. Riveted connections
8. Welded connections and types of welds
9. Comparison of connection types
10. Failure modes
11. Design philosophy and safety factors
12. Corrosion and fire protection
13. A worked numerical example
14. Every previous-pattern question - answered
15. MCQs and interview questions - answered
16. The memory box
1. What Is Structural Steel?
Definition: Structural steel is a construction material manufactured to specified mechanical properties and used to carry structural loads in buildings, bridges, towers, industrial plants, and other engineering structures.
Think of the difference between concrete and steel like the difference between wet clay and a metal rod. Concrete is poured, cured, and gains strength slowly over weeks - it is strong in compression but needs steel bars just to survive tension. A steel member, on the other hand, arrives from the factory already at full strength, equally happy in tension or compression, and ready to be bolted or welded together the same day.
Why steel is used
- High strength for its weight
- High ductility - it bends and warns before it breaks
- Uniform, factory-controlled quality
- Fast construction
- Easy fabrication and site assembly
2. Properties of Structural Steel
(A) High Strength
Steel has much higher strength than concrete for the same cross-sectional area. This makes it the natural choice for high-rise buildings, long-span bridges, and industrial structures where a bulky concrete section would be impractical.
(B) Ductility
Steel undergoes large deformation before it finally fails. This is one of its most valuable properties structurally, because:
- A ductile member visibly sags or stretches before collapse, giving a warning
- Ductile behavior lets a structure absorb and dissipate earthquake energy instead of shattering suddenly
(C) Elasticity
Steel follows Hooke's Law within its elastic range - meaning stress is directly proportional to strain, right up until the yield point.
The stress-strain curve, explained simply added - fills a key gap
If your notes mention "elasticity" and "ductility" as separate bullet points, you are missing the one picture that ties both ideas together: the stress-strain curve. Every Loksewa answer about steel properties becomes stronger if you can sketch this.
Read the curve left to right: first steel stretches proportionally to the load (the straight elastic line, where E = 200 GPa applies) - remove the load here and it springs fully back. Push past the yield point and steel keeps stretching with very little extra load (the yield plateau) - this is the ductile warning phase. Push further and it actually gets stronger again (strain hardening) up to the ultimate stress, before finally necking down and fracturing. Concrete, by comparison, has almost no equivalent plateau - it simply cracks.
(D) Toughness
Toughness is the ability to absorb energy before fracture. This matters most under earthquakes, impact loads, and other dynamic loads, where a tough material survives sudden shocks that a brittle one would shatter under.
(E) Weldability
Steel can easily be welded, which makes fabrication - joining pieces into a finished structure - much easier and faster than with other materials.
(F) Recyclability
Steel is almost completely recyclable, which makes it one of the more environmentally friendly structural materials in use today.
3. Grades of Structural Steel added - commonly asked
Steel is manufactured and supplied in specific grades, each with a guaranteed minimum yield and ultimate strength. The grade you will see most often in Nepal and India for ordinary structural work is:
The "Fe410" name refers to the guaranteed minimum ultimate tensile strength (410 N/sq.mm), while "E250" refers to the guaranteed minimum yield strength (250 N/sq.mm). Higher grades with higher yield strength exist for specialized structures, but Fe410/E250 is the workhorse grade you should default to remembering.
4. Advantages of Steel Structures
- High strength-to-weight ratio
- Rapid construction
- Easy transportation
- Easy prefabrication
- Good seismic performance
- Reusable and recyclable
- Large clear spans possible
5. Disadvantages of Steel Structures
- Corrosion
- Fire vulnerability
- Buckling of slender members
- Regular maintenance required
- Higher initial material cost than reinforced concrete
6. Types of Rolled Steel Sections
(A) I-Section (Universal Beam)
The most commonly used beam section. Its shape puts most of the material as far from the center as possible (in the top and bottom flanges), which is exactly where bending needs it most. Used for beams and bridge girders.
(B) Channel Section (C)
Used for purlins, secondary beams, and built-up members, often paired back-to-back for extra strength.
(C) Angle Section (L)
Used in roof trusses, bracing, and towers - anywhere a simple, light, easily-bolted member is needed.
(D) Tee Section (T)
Used for truss members and secondary framing; essentially half of an I-section split along its web.
(E) Hollow Sections
Available as circular, square, or rectangular hollow sections. Used for columns, towers, and space frames - their closed shape gives excellent resistance to twisting (torsion) and buckling in every direction equally.
I = I-section, C = Channel, A = Angle, T = Tee, H = Hollow
7. Steel Connections - The Big Picture
Structural steel members are connected using one of three methods:
- Bolts
- Welds
- Rivets (mainly seen in older, historic structures today)
8. Bolted Connections
Definition: A bolted connection joins steel members using steel bolts, nuts, and washers.
Types of bolts
Ordinary (bearing) bolts: the most common type, used in ordinary buildings and industrial structures. Under load they work by bearing against the sides of the bolt holes.
High Strength Friction Grip (HSFG) bolts: tightened to a very high tension so the connected plates are clamped together and the load transfers by friction between the plates rather than by the bolt bearing on the hole. Suitable for bridges, dynamic loading, and fatigue-sensitive structures.
Advantages of bolted connections
- Easy installation
- Easy inspection
- Easy replacement
- Suitable for site (field) work
Disadvantages of bolted connections
- Bolt holes reduce the net cross-sectional area of the member
- Ordinary bolts may loosen under vibration
Bolt spacing rules: pitch, gauge, and edge distance added - frequently tested numerically
This is the part of the bolted-connections topic most likely to show up as a numerical question, and it is usually the first thing left out of summary notes.
| Term | Meaning | Typical rule |
|---|---|---|
| Pitch (p) | Center-to-center spacing of bolts along the direction of load | Minimum 2.5 times the bolt diameter |
| Gauge (g) | Center-to-center spacing of bolts across the direction of load | Follows the same minimum spacing logic as pitch |
| Edge / end distance | Distance from the bolt center to the nearest edge of the plate | About 1.5 to 1.7 times the hole diameter, depending on whether the edge is rolled or sheared/cut |
| Maximum pitch | Upper limit to keep plates acting together | The smaller of 32 times the plate thickness, or 300 mm |
The logic behind all of these limits is simple: bolts placed too close together risk tearing the plate between the holes, and bolts placed too far apart let thin plates buckle or separate between fasteners instead of acting as one solid piece.
9. Riveted Connections
A historical method that uses hot-driven rivets, formed by heating a metal pin red hot, inserting it through aligned holes, and hammering the protruding end into a second head while it cools and shrinks - which pulls the plates tightly together.
Applications
- Old bridges
- Railway bridges
- Heritage steel structures
Advantages
- Reliable when properly installed
- Good fatigue performance
Disadvantages
- Labour-intensive
- Slow construction
- Rarely used in modern steel construction
10. Welded Connections
Definition: joining steel members by fusion using heat, so the two pieces of metal actually melt and merge together rather than being clamped by a separate fastener.
Fillet Weld
The most commonly used weld type, deposited into the corner formed by two members set at an angle (typically 90 degrees). Used for angle connections and plate connections.
Butt Weld
Two members are joined edge-to-edge, in the same plane, providing a smooth, direct transfer of force straight through the joint.
Plug Weld
Used to join overlapping plates by filling a circular hole punched through the top plate with weld metal, fusing it to the plate beneath.
Slot Weld
Similar to a plug weld, but the hole is an elongated slot rather than a circle, giving a longer line of fusion for the same connection.
Advantages of welded connections
- No bolt holes, so no loss of net section
- Better appearance
- Efficient force transfer
- Rigid connections
Disadvantages of welded connections
- Requires skilled labour
- Quality control is essential
- Residual stresses may develop from the heating and cooling cycle
11. Comparison of Connections
| Bolted | Welded | Riveted |
|---|---|---|
| Easy installation | Permanent connection | Mostly obsolete |
| Easy inspection | Better appearance | Labour-intensive |
| Easy replacement | Skilled labour required | Rare in modern work |
| Common for field work | Common in fabrication shops | Historic structures |
12. Failure of Bolted Connections
Common modes:
- Bolt shear failure - the bolt itself shears through
- Bearing failure of the plate - the hole elongates as the bolt crushes into the plate material
- Net section tension failure - the member tears across the row of bolt holes, where the cross-section is weakest
- Block shear failure - a block of material around the bolt group tears out along a combined shear-and-tension path
13. Failure of Welded Connections
Common modes:
- Weld throat failure - the weld metal itself shears through its thinnest section
- Base metal failure - the parent plate fails rather than the weld
- Heat-affected zone failure - the zone next to the weld, weakened by heating during welding
- Fatigue cracking - cracks that grow slowly under repeated loading, often starting at a weld defect
14. Design Philosophy
Modern steel structures are designed using the Limit State Method. The checks fall into two broad categories:
Strength (Ultimate Limit State): tension, compression, shear, bending.
Serviceability: deflection, vibration, durability.
Partial safety factors added
The Limit State Method does not use one single blanket "factor of safety" the way older working-stress methods did. Instead it applies separate partial safety factors to the material strength depending on which failure mode is being checked - typically a smaller factor for yielding-governed checks and a somewhat larger factor for checks governed by ultimate (rupture) strength. This is why two different capacity values are often calculated for the same bolted or welded joint, and the lower, more conservative one governs the design.
15. Corrosion and Fire Protection added - supports the interview question below
Since corrosion and fire vulnerability are listed as disadvantages of steel, it is worth also knowing how engineers actually address them:
- Corrosion protection: painting, galvanizing (coating with a protective zinc layer), or use of weathering steel that forms a stable, self-protecting oxide layer
- Fire protection: fireproof cladding or spray-applied fire protection coatings, intumescent paints that expand into an insulating char when heated, and keeping steel members enclosed within fire-rated construction where required
16. Engineering Applications
Steel structures are widely used in industrial buildings, bridges, transmission towers, stadium roofs, aircraft hangars, metro stations, and high-rise buildings.
17. Worked Numerical Example added
Problem: A steel plate is connected using bolts of 20 mm diameter (giving a hole diameter of about 21.5 mm after allowing standard clearance), arranged at a pitch of 60 mm. Find the efficiency of the joint.
Solution:
Efficiency = (60 - 21.5) / 60
Efficiency = 38.5 / 60
Efficiency = 0.6417, or about 64.2%
Answer: the joint is about 64.2% efficient - meaning the connected section can only carry about 64% of the strength of the original unpunched plate, because roughly a third of the width has been removed by the bolt hole. This is exactly why bolt holes are listed as a disadvantage of bolted connections, and why designers always check the net section, not just the gross section.
18. Previous Loksewa Subjective Questions - Answered
Answer: Steel members are joined using three main methods. Bolted connections use bolts, nuts, and washers - either ordinary bearing bolts for everyday buildings or High Strength Friction Grip (HSFG) bolts for bridges and fatigue-sensitive work - and are valued for being easy to install, inspect, and replace on site. Riveted connections use hot-driven rivets hammered into place while red hot, and are now mostly seen only in older or heritage structures because the method is slow and labour-intensive. Welded connections fuse members together using heat, most commonly as a fillet weld at an angled joint or a butt weld between edge-to-edge members, with plug and slot welds used for overlapping plates; welding gives a rigid, permanent connection without any loss of section to bolt holes, but demands skilled labour and careful quality control. In an exam, sketch one small joint for each type exactly as shown in the diagrams above.
Answer: Use the comparison table in Section 11. In short: bolted connections are easy to install, inspect, and replace, and are the standard choice for field (site) connections. Welded connections give a permanent, rigid, better-appearing joint with no bolt holes, but need skilled labour and quality control, and are mainly done in fabrication shops rather than on site. Riveted connections are now mostly obsolete, being slow and labour-intensive, and are chiefly encountered when maintaining historic bridges and structures rather than in new construction.
Answer: Advantages include a high strength-to-weight ratio, rapid construction, easy transportation and prefabrication, good seismic performance thanks to steel's ductility, full reusability and recyclability, and the ability to achieve large clear spans that would be impractical in concrete. Disadvantages include vulnerability to corrosion (addressed through painting, galvanizing, or weathering steel), vulnerability to fire (addressed through fireproof cladding or intumescent coatings), a tendency for slender members to buckle under compression, the need for regular maintenance, and a higher initial material cost compared to reinforced concrete.
Answer: The five common rolled sections, best remembered with the trick "I C A T H", are: the I-section, the most common beam and bridge girder section, which puts material in flanges far from the center where bending needs it most; the Channel (C) section, used for purlins and secondary beams; the Angle (L) section, used in trusses, bracing, and towers; the Tee (T) section, used for truss members and secondary framing; and Hollow sections (circular, square, or rectangular), used for columns, towers, and space frames because their closed shape resists torsion and buckling well in every direction.
Answer: A bolted connection can fail in four main ways: bolt shear failure, where the bolt itself shears through; bearing failure of the plate, where the bolt hole elongates as the bolt crushes into the surrounding plate material; net section tension failure, where the member tears across the reduced cross-section at the line of bolt holes; and block shear failure, where a block of material around the bolt group tears out along a combined path of shear and tension. A complete design check considers all four modes and is governed by whichever gives the lowest capacity.
19. MCQ Practice (with answers)
A. 20 GPa B. 50 GPa C. 200 GPa D. 500 GPa
Answer: C
A. Plug weld B. Slot weld C. Fillet weld D. Spot weld
Answer: C
A. Angle section B. Tee section C. I-section D. Channel section
Answer: C
A. Only light loads exist B. Fatigue resistance and slip resistance are important C. Temporary structures D. Timber structures
Answer: B
A. Bolted B. Welded C. Riveted D. HSFG bolted
Answer: C
A. 100 N/sq.mm B. 250 N/sq.mm C. 410 N/sq.mm D. 500 N/sq.mm
Answer: B
A. 1.5 times the bolt diameter B. 2.5 times the bolt diameter C. 5 times the bolt diameter D. 10 times the bolt diameter
Answer: B
20. Interview Questions - Answered
Answer: Welding is faster, needs no pre-drilled holes (so the full cross-section stays intact), produces a cleaner appearance, and creates a rigid, continuous connection. Riveting, by comparison, is slow, labour-intensive, requires a large site crew to heat and hammer each rivet, and has largely been made unnecessary by the arrival of reliable, quicker welding and bolting methods. Riveting survives mainly in the maintenance of existing historic structures, not in new design.
Answer: Bolted connections are preferred for site (field) connections, where skilled welders and controlled conditions are hard to guarantee; where the connection may need to be inspected, tightened, or replaced later; where fast erection is a priority; and where avoiding the residual stresses and heat distortion that come with welding matters, such as in connections made in cold weather or on structures sensitive to distortion.
Answer: HSFG bolts are tightened to a very high tension, clamping the connected plates tightly together so load transfers through friction rather than through the bolt bearing on the hole. This gives excellent resistance to slip and very good performance under the repeated, fluctuating loads (fatigue) and dynamic loading that bridges experience from traffic, making HSFG bolts far better suited to bridges than ordinary bearing bolts, which can loosen under vibration over time.
Answer: Steel's high strength-to-weight ratio means a steel member can span a much greater distance while carrying its own self-weight than an equivalent concrete member, which becomes impractically heavy and bulky at long spans. Steel is also equally strong in tension and compression, so it does not need the same reinforcement strategy concrete does, and steel sections like the I-section put material efficiently where bending needs it most, further reducing weight for a given span.
Answer: Corrosion gradually eats away the steel section, reducing its effective strength over time if left unprotected; it is controlled through painting, galvanizing (a protective zinc coating), or using weathering steel that forms its own stable protective oxide layer. Fire is a serious concern because steel loses strength rapidly and can soften significantly at high temperatures, potentially leading to sudden loss of load capacity; it is controlled using fireproof cladding, spray-applied fire protection coatings, or intumescent paints that expand into an insulating char when heated, along with proper detailing to keep members enclosed within fire-rated construction where required.
Memory Box - Quick Revision Before the Exam
- Steel: high strength, ductility, toughness, weldability, and recyclability
- E is approximately 200 GPa
- Common structural grade: Fe410 / E250, fy = 250 N/sq.mm, fu = 410 N/sq.mm
- Rolled sections - "I C A T H": I-section, Channel, Angle, Tee, Hollow
- Most common beam section: I-section. Most common weld: fillet weld
- Bolted: easy install/inspect/replace, but holes reduce net section
- HSFG bolts: used where slip resistance and fatigue performance matter (bridges)
- Riveted joints: mostly found in older/heritage structures today
- Welded: rigid, no holes, but needs skilled labour and quality control
- Minimum pitch: 2.5 x bolt diameter. Maximum pitch: smaller of 32t or 300 mm
- Joint efficiency = (pitch - hole diameter) / pitch
- Modern design method: Limit State Method, checking both strength and serviceability
- Corrosion protection: painting, galvanizing, weathering steel
- Fire protection: fireproof cladding, intumescent coatings
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