Structural Engineering Rules of Thumb Design

Structural Engineering

Design Notes

&

Rules of Thumb

Author

- 1 -

Caroline MacVey

- 2 -

1 GENERAL 3

1.1 CONVERSIONS 31.2 3

2 LOADING 4

2.1 RULES OF THUMB - SIMPLE MATHS 42.2 GENERAL NOTES 52.3 FLOOR LOADING 52.3.1 DEAD LOADS 52.4 ROOF LOADING 72.5 OTHER 8

3 STEEL 9

3.1 RULES OF THUMB 93.2 FORMULA 103.3 DEFLECTIONS 113.4 COMPOSITE DESIGN 113.5 GENERAL NOTES 12

4 CONCRETE 14

4.1 RULES OF THUMB 144.2 FORMULA 154.3 GENERAL NOTES 17

5 MASONRY 18

5.1 GENERAL NOTES 18

- 3 -

6 TIMBER 19

6.1 RULES OF THUMB 19

7 GEOTECHNICAL & FOUNDATIONS 20

7.1 RULES OF THUMB 207.2 GENERAL NOTES 21

- 4 -

1 General o Tension (-ve) --->-----<---o Compression (+ve) ---<----->---o Always check wind load paths reach the

foundations

1.1 Conversions o 1 Pascal = 0.001 kN/m2

o 1 kPa = 1 kN/m2

o 1 Tonne = 1000kg = 10kNo 1m2 = 10,000cm2 = 1,000,000mm2

1.2

- 5 -

2 Loading

2.1 Rules of Thumb - Simple Maths

1. 10 Gravity; Institutional Office DL + LL2. 8 General Office Floor DL + LL3. 3 Deafened Timber Floor DL + LL4. 2 Timber Roof DL + LL5. 1.5 Average Load Factor6. 2.5 Reasonable Zone A Wind Load7. 2.0 Reasonable Zone B Wind Load8. 1.0 Reasonable Zone C Wind Load9. 4 4 x Mx ≈ Zxx (S275)10. 3 3 x Mx ≈ Zxx (S375)

- 6 -

2.2 General Notes o 3 Main combos

a. 1.4DL + 1.6LLb. 1.2DL + 1.2LL + 1.2WLc. 1.0DL + 1.4WL

2.3 Wind Loading

o Don’t combine NHF with wind loadso Wind Loading- Suction

o Wind Loading- Pressure

- 7 -

-1.3

-0.3+0.6 -0.5

-1.3

+0.2+0.6 -0.5

2.4 Floor Loading

2.4.1 Dead Loadso Raised Floor = 0.5 kN/m2

o Ceilings = 0.15 kN/m2

o Services (Typical) = 0.25 kN/m2

o Services (Plant) = 0.5 to 0.75 kN/m2

o Finishes (Typical) = 0.15 kN/m2

o Finishes (60mm Screed) = 1.5 kN/m2

o SW Steel = 0.3 kN/m2

o Blockwork = 3.02 / 2.5 kN/m2

o Brickwork = 2.16 kN/m2

2.4.2 Imposed Loadso Construction = 1.5 kN/m2o Stud Partitions = 1.0 kN/m2

o

- 8 -

2.5 Roof Loading

2.5.1 Dead Loadso Trocal & Outer Sheet = 0.05 kN/m2

o Insulation = 0.02 kN/m2

o Inner sheet = 0.03 kN/m2

o Ceilings = 0.15 kN/m2

o Services (Typical) = 0.25 kN/m2

o Services (Plant) = 0.5 kN/m2

o Timber = 1.0 kN/m2

o Slate = 1.0 kN/m2

2.5.2 Imposed Loadso Snow (min) = 0.6 kN/m2

o Snow Fresh = 0.94 kN/m3

o Snow Compacted = 3.14 kN/m3

- 9 -

2.6 Other o 38mm Asphalt = 0.85 kN/m2

o Cladding = 0.15 / 0.2 kN/m2

o Curtain Walling = 1.0 / 0.5 kN/m2

o Windows = 0.8 kN/m2

o Tenement Rehabilitation Contracts - Floor Loading Joists (265x63 @ 450 c/c) = 0.25 kN/m2

Boarding (25mm T&G) = 0.15 kN/m2

Plaster Ceiling = 0.35 kN/m2

Deadening (100mm Ash Fill) = 0.75 kN/m2

Total Dead = 1.5 kN/m2

Imposed = 1.5 kN/m2

o Tenement Rehabilitation Contracts - Roof Loading Slates = 0.5 kN/m2

Felt & Sarking = 0.10 kN/m2

Trusses = 0.15 kN/m2

Plaster Ceiling = 0.35 kN/m2

Total Dead = 1.1 kN/m2

Imposed = 0.6 kN/m2

Storage = 0.25 kN/m2

- 10 -

3 Steel

3.1 Rules of Thumb

1. Grade S355 for major structures2. Beams Light Loading – Span/25; Medium –

Span/20; Heavy – Span/153. Frame Form Economics and ease of calculation4. Stanchions Portal Leg D ≈ H / 105. Columns D ≈ No Storeys * 100 / 4 (rnd 50) NOT

< 2036. Economics Deeper the Beam the more

economical?7. Eurocodes As above

- 11 -

3.2 Formula o Tension Capacity, Pt = t Ae

o Steel Required =

o For quick element sizing based on criteria

o Torsion Moment,

o Natural Frequency, where = Self weight

+ dead + 10% imposed loado Stress, = M*y / I

- 12 -

3.3 Deflections o Portal Eaves – Metal cladding ≤ h/150o Portal Eaves – Masonry ≤ h/300o Portal Apex – ≤ S/200o Dead Floor ≤ 15mmo Imposed ≤ S/360o Total ≤ S/250 (S/300 Mangan Factor)

3.4 Composite Design o In composite design for 19mm studs then

minimum size of reinf is 10mm barso Composite beams

Slab Perpendicular Studs per trough Slab Parallel Studs @ 200mm c/c

o Effective Width Secondary = Spacing or Span/4 Primary = 0.8*Spacing or Span/4 Edge Beams – Half of above values plus

projection of slab beyond the C/L of beam

- 13 -

3.5 General Notes o Buckling – Intermediate restraint be careful of

effective lengtho For cellbeams 8 out of 10 engineers use 1.0L for

effective lengtho For a cantilever beam connected to the flange of

a column use the distance from the point load to flange face as lever arm and not the middle of column. Use the middle when finding the moment on the column itself.

o Hit & Miss Welds – generally 150mm weld then 400mm gap. Generally used for angles on box sections as full weld would damage the box.

o When there are moments about the minor axis of a UB section. Better to adopt a box section instead.

o For long span rafters the top flange is restrained sufficiently by the purlins. Be more careful of bottom flange for uplift loads i.e. add rafter stays

o Check angle for load P2. Check RHS for load P1 & P2. P2 will cause a moment about axis y-y therefore check RHS for torsion.

- 14 -

d

P2

RHS

P1 Y

X

o If a channel has multiple point loads it would be advisable to switch to a UB section.

o For beam end shears ≤ 50kN state on dwgs as 50kN.

o Capacity Columns = 80% (MF) Beams = 75% (MF) Beams = 90% Non composite Beams = 85% Composite

o Fabsec Beams Flange Max 60mm wide, 100mm thick Web max 1500mm Deep, 75mm thick

o Small cr i.e. < 4 carry out a 2nd order analysiso Box section cost twice as much to manufacture

than UB/UC’so For holding down bolts the tolerance is 25mm in

all directionso Metsec says spans of side rails & purlins are not

efficient until 3 – 3.5m lengthso Purlins – Small projects generally single or

double span. For Large buildings use HEB system

- 15 -

4 Concrete

4.1 Rules of Thumb

1. Beams Simple span – Span/20; Continuous – Span/26

2. Slabs Span / 26 – generally continuous – span / 20 if not

3. Columns D ≈ No Storeys * 100 / 3 (rnd +50) NOT < 200

4. Walls Keep D > 200mm5. Economics Deeper the Beam more

economical?6. Eurocodes As above

- 16 -

4.2 Formula

o

o ≤ 0.95d

o

o K’ = 0.156 when redistribution <10%o

o

o Mu = 0.156fcubd2

o ≤ 0.95d

o &

o Shear, ≤ or 5N/mm2

o

o Modification Factors for deflection Tension Reinforcement:

Mod F =

Where

- 17 -

Compression Reinforcement:

Mod F =

- 18 -

4.3 General Notes o Stair checks done as simple beam for simplicityo Factor of Safety

Uplift = 1.4 Sliding = 1.5 Overturning = 2.0 (1.5 GS)

o Foundations Sizing, Bearing capacity & Uplift calcs use

unfactored loads Area of steel use factored loads

o Steel reinf strength, fy = 500N/mm2

o Min area of steel = 0.13(bh/100) (b=1000)o Concrete Grade

Foundations – C35 Internal Slab – C30

o Design ground beams as continuous Span min. = clear span + effective depth

o Columns Biaxial bending min. moment =

0.05xdimension but ≤ 20mm Min. steel = 0.4% Longitudinal bars 12mm and max

spacing = 250mmo If Mu < M then compression reinf requiredo Also if K > K’ then compression reinf requiredo Reinforcement Spacing

Min Spacing = hagg +5mm Max Spacing ≤ 47000/fs ≤ 300

o Pre-cast stairs ideally need 180-200mm bearing. Minimum of 165mm. Creagh say 100mm min.

- 19 -

5 Masonry

5.1 General Notes

o Freestanding walls and wall panels are 2 different things

o Typical Masonry Valueso Brick

102.5mm thk Density = 20kN/m3

Mortar Type 3 Water Absorption = 7%

o Block 140mm thk Density (H) = 18kN/m3

Density (M) = 15kN/m3

Mortar Type 3 Compressive Strength = 7N/mm2

o Partial Safety Factor = 2.5 (Special)o Blockwork does not have a good lateral strength

in comparison to brickworko Wall Panels with H/L < 0.3 then wall will tend to

span verticallyo Wall Panels with H/L > 1.75 then wall will tend to

span horizontallyo Stress, = Load / Wall Area

- 20 -

6 Timber

6.1 Rules of Thumb

1. Roof Joists Span (mm) / 20 – RND + 25mm2. Roof Joists Span (mm) / 20 +25 mm – RND +

25mm3. Racking Model to suit published tables4. Hybrids – Steel & Timber go well together5. Section Stability Engineered products vs.

natural products6. Creep / Settlement Engineered products vs.

natural products

- 21 -

7 Geotechnical & Foundations

7.1 Rules of Thumb

1. Cu Cohesive approx SBC = 2*Cu

2. N Granular approx SBC = 10*N

3. Concentric Find ways to remove eccentricity

4. Getting Close Foundations & Adjacent

Buildings – Cantilever so you don’t undermine

5. Piles Size & capacities approx SWL = 2*D (mm)

6. Tension Piles Friction only – surface area,

surface texture, method – big & fat

7. Vibro Know its limitations – not for tall

buildings

8. Pile Settlement Take care – Clean holes?

Friction Failure? FOS – Make sure your spec is

clear

9. Mass Footing Min Depth = width = breadth –

45° spread

- 22 -

7.2 General Notes

o Overburden pressure is the existing load on the

soil

o Gross Pressure is the new load on the soil

o Net pressure is the change in load

o Secant piled walls are 3x more expensive than

contiguous

o Large diameter bored pile are required when

retaining 6m + to get enough rock penetration

o Typical density, γ = 18 kN/m2

o Ø’ = 30°

o Ka = 1-sin Ø’ / 1+sin Ø’

o Kp = 1 / Ka

- 23 -