👷 Engineering Rules of Thumb

Essential guidelines for structural and civil engineering design
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📋 General Guidelines

Essential Engineering Principles

Cost Reality Check

Reinforced concrete cost: $150-$800/m³

Key Point: Rules of thumb are only as good as their background context. Use carefully and verify with detailed design.

Concrete Placement

Rule: Place concrete vertically into the face of concrete already in place. Never allow concrete to fall more than 1.0-1.5m

Drawing Standards
  • Prepare drawings properly & accurately
  • Label each bar and show its shape for clarity
  • Indicate proper cover (clear, nominal, or effective)
  • Show enlarged details at corners and intersections
  • Avoid bar congestion at member intersections
Memory Aid: "CAD-BARS"
Clear cover • Avoid congestion • Detailed openings • Bar sizes (minimize variety) • Augmented details • Required joints • Splicing rules
🧱 Concrete Design

Reinforced Concrete Guidelines

Span-to-Depth Ratios (Quick Reference)
L/26 Beam (non-cantilever)
L/7 Beam (cantilever)
L/24 One-way slab
L/30 Two-way slab

DO's - Construction Best Practices

Reinforcement Placement
  • Use commonly available bar sizes
  • Minimize different sizes in a single member
  • For bundled bars: splice one bar at a time, stagger splices
  • Ensure hooked bars have adequate concrete protection
  • Keep higher diameter bars away from neutral axis
Joint Details
  • Mark expansion, construction, and contraction joints on plans
  • Construction joints: at mid-point, vertical formation
  • Bonded reinforcement: never across expansion joints

DON'Ts - Critical Restrictions

Prohibited Practices
  • Don't terminate flexural reinforcement in tension zones
  • Don't bundle bars larger than 36mm diameter
  • Don't use lap splices for bars >36mm (except welded)
  • Dowel diameter: max 3mm larger than column bars
Cover Requirements

Minimum cover:

  • General concrete: 25mm
  • Prestressed concrete: 25mm
  • Slab fire resistance: 120-150mm

Expansion Joints

Movement Joint Spacing

Recommended spacing:

  • Concrete structures: 100-200ft (30-60m)
  • Steel structures: 200ft (60m)
  • Movement allowance: 15-25mm (200mm if seismic)
Joint Location Factors
  • New building adjoining existing
  • Long low building abutting higher building
  • Wings adjoining main structure
  • Intersections at 'L', 'T', or 'U' shaped buildings
  • Different foundation types in structure
⚖️ Beams & Slabs

Beam and Slab Design

Beam Sizing

Non-cantilever: d (mm) = span (mm)/26 + 300 (round to 25mm)
Cantilever: d (mm) = span (mm)/7 + 300 (round to 25mm)
Beam width (b):
span < 6000mm: 300mm
6000-9000mm: 350mm
9000-12000mm: 400mm

Span-to-Depth Ratios

Member Type Reinforced Concrete Max Span Prestressed
Rectangular Beam (b>250mm) L/10 to L/14 8m L/13 to L/20 (12m)
Flanged Beams L/12 to L/18 - -
Cantilever L/2 to L/6 5m -
Band Beams L/18 to L/20 8-12m L/25 to L/30 (14m)
Important Notes
  • Higher ratios = light loading (~1.5 kPa)
  • Lower ratios = heavy loading (~10 kPa)
  • Beams need depth for sufficient reinforcing
  • Check detailing early

Slab Design

Span-to-Depth Ratios:
One-way: Single L/24, Continuous L/30, Cantilever L/7
Two-way: Single L/28, Continuous L/36
Slab Reinforcement
  • Max spacing: 250mm for main reinforcement
  • Minimum roof reinforcement: 0.24% (temperature variations)
  • Fire resistance: minimum 125mm thickness
  • Punching shear: thin slabs require reinforcement at columns
Memory Aid: "One-Two-Cantilever"
One-way slab: L/24 (single), L/30 (continuous)
Two-way slab: L/28 (single), L/36 (continuous)
Cantilever: L/7 (both slab types)

Shear Design

Shear stress: v = V/(b×d)
Ideal: < 2 N/mm² (avoid congestion)
Shear Reinforcement
  • Only closed stirrups for torsion or seismic loading
  • Face reinforcement: depth >750mm (no torsion), >450mm (with torsion)
  • Compression reinforcement reduces deflection
🏗️ Columns & Walls

Column and Wall Design

Column Sizing

Height-Based Sizing
  • Interior columns: H/10-20
  • Edge columns: H/7-9
  • Corner columns: H/6-8
  • Minimum section: 200-250mm
Load-Based Sizing (Stocky Columns):
Ac = N/15 (1% reinforcement)
Ac = N/18 (2% reinforcement)
Ac = N/20 (3% reinforcement)
(N in Newtons, N32 concrete)
Moment Multipliers

Multiply axial load from floor above by:

  • Interior columns: 1.25
  • Edge columns: 1.50
  • Corner columns: 2.00
Reinforcement Limits
  • Maximum: 3% (6% absolute max, 10% at laps)
  • Minimum: 6 bars for circular sections
  • Rectangular: 4 bars (one at each corner)
  • Preliminary design: aim for 1-2%

Eccentricity

Minimum eccentricity = [(unsupported length/500) + (lateral dimension/30)]
Minimum: 20mm

Concrete Grades

Material Selection
  • Axial load predominant: Higher grade concrete
  • Moment predominant: Higher section properties
  • Service load stress: ~0.3f'c
  • Gravity load stress: ~0.15f'c

Walls

Wall Thickness
  • General walls: H/30-45
  • Tilt panels: H/50
  • Shear walls: span-to-depth ratio 7
  • Minimum thickness: 200mm
Column vs Wall

Column changes to wall at about 4× thickness

Fire consideration: If fire can reach all four sides, treat as column

🏛️ Foundations

Foundation Design

Retaining Walls

Thickness Guidelines

Cantilever retaining walls:

  • Stem thickness: H/10-14
  • Sheet pile penetration: ~equal to unsupported height
Footing Dimensions
T-shaped (horizontal fill): Footing length = 0.46×height
(1/3 in front of exposed face)
L-shaped (horizontal fill):
All footing at toe: 0.65×height
All footing at heel: 0.55×height
Stability Check

Use "traditional" dimensions first. Balance heel and toe properly. Oversized elements may satisfy stability but create strength problems.

Slab on Grade

Application Subgrade Minimum Thickness Reinforcement
Domestic Medium good 100mm SL72 WWF
Domestic Poor 125mm SL82 WWF
Commercial Medium good 130mm SL82 WWF
Industrial Medium good 180mm SL92 WWF
Industrial Poor 200mm Two layers SL72
Joint Spacing
  • Rule of thumb: 24-36× thickness (in mm)
  • Dowel sawn joints: Max 21m apart
  • General joints: Max 8m apart
  • Aspect ratio: Max 2:1 length to width
Thickness Variation

Keep variation to -5mm to +10mm

Subgrade: no abrupt changes >15mm in 1.2m

Memory Aid: "Joint Spacing Rule"
Joint spacing = thickness × 30 (use 30 as middle value)
Example: 150mm slab → 150×30 = 4500mm (4.5m spacing)
🔩 Steel Structures

Steel Design Guidelines

Beam Systems

Element Typical Span/Depth Typical Span
Plate girder 10-12 25m
Floor joist 17 6-9m
Castellated UB 14-17 12-20m
Primary beam 10-15 12m
Secondary beam 15-25 10m
Portal frame leg 35-40 60m
Castellated Beams

Avoid for high point loads. Increase required I by 1.3×

Deflection: ~25% greater than equivalent solid beam

Expansion Joints (Steel)

Joint Spacing
  • Steel industrial buildings: 100m typical, 150m max
  • Steel commercial buildings: 50m typical, 100m max
  • Steel/tilt-up building: 50m
  • Steel roof sheeting: 20m down slope

Design Principles

Key Rules
  • Deeper is cheaper: Deeper wide-flange beams more economical
  • Cantilever rule: Don't exceed 1/3 of back span
  • Truss depth: Optimal ratio 1/10 of span
  • Transportation: Keep member lengths <15m

Slenderness Limits

Maximum Slenderness Ratios
  • Load bearing members: λ ≤ 180
  • Self-weight + wind only: λ ≤ 250
  • Tie members (reversal): λ ≤ 350

Connections

Welding
  • Weld strength should not exceed connected part
  • Fillet weld size: ≤6mm (single pass)
  • Relative costs: Downhand 1.0, Vertical 2.0, Overhead 3.0
  • Select fillet over full-penetration when possible
Bolting
  • Simple connections: Grade 8.8, 20mm bolts
  • Moment connections: 20mm or 24mm bolts
  • Holding down bolts: Grade 4.6
  • Standard sizes: M16×300, M20×450/600, M24×450/600, M30×450/600, M36×450/600/750
Economy Rules
  • More pieces = more expensive
  • Don't over-economize single connections (>25-30%)
  • Use popular flat bar sizes
  • Standardize member sizes for bulk purchasing
  • Supporting beam should equal/deeper than supported beam
Memory Aid: "The 1/3 Rule"
Cantilever length should not exceed 1/3 of back span
Example: 9m back span → max 3m cantilever

Earthquake Loading

Base Shear Estimate:
V = 0.1 × W
(W = total building weight)

Lateral Load Systems

Stories Lateral System
<30 Rigid frame
30-40 Frame - shear truss
41-60 Belt truss
61-80 Framed tube
81-100 Truss-tube w/ interior columns
101-110 Bundled tube
111-140 Truss-tube, no interior columns

Quick Reference Cards

Beam Depth
Non-cantilever: L/26 + 300mm
Cantilever: L/7 + 300mm
Slab Depth
One-way: L/24-30
Two-way: L/28-36
Column Height
Interior: H/10-20
Edge: H/7-9
Corner: H/6-8
Expansion Joints
Concrete: 100-200ft
Steel: 200ft
Movement: 15-25mm
Don't Bundle
Bars >36mm diameter
Minimum Cover
General: 25mm
Prestressed: 25mm
Fire: 120-150mm
Reinforcement Limits
Max: 3% (6% absolute)
Min: 0.24% (roof slabs)
Column: 6 bars circular

Safety Factors

Critical Warnings
• These are guidelines only
• Verify with detailed design
• Consult qualified engineer
• Consider local codes/standards

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Note: These rules of thumb are for preliminary design and estimation only. Always verify with detailed analysis, local codes, and qualified structural engineering judgment for actual construction.