2.1 Rules of Thumb - Simple Maths 1. 10 Gravity; Institutional Office DL + LL 2. 8 General Office Floor DL + LL 3. 3 Deafened Timber Floor DL + LL 4. 2 Timber Roof DL + LL 5. 1.5 Average Load Factor 6. 2.5 Reasonable Zone A Wind Load 7. 2.0 Reasonable Zone B Wind Load 8. 1.0 Reasonable Zone C Wind Load 9. 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 + 1.6LL b. 1.2DL + 1.2LL + 1.2WL c. 1.0DL + 1.4WL 2.3 Wind Loading o Don’t combine NHF with wind loads o Wind Loading- Suction 1.3 +0.6 0.3
0.5
o Wind Loading- Pressure 1.3 +0.6 +0. 2
-7-
0.5
2.4 Floor Loading 2.4.1 Dead Loads o 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 Loads o Construction = 1.5 kN/m2 o Stud Partitions = 1.0 kN/m2 o
-8-
2.5 Roof Loading 2.5.1 Dead Loads o 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 Loads o Snow (min) = 0.6 kN/m2 o Snow Fresh = 0.94 kN/m3 o Snow Compacted = 3.14 kN/m3
3.1 Rules of Thumb 1. Grade S355 for major structures 2. Beams Light Loading – Span/25; Medium – Span/20; Heavy – Span/15 3. Frame Form Economics and ease of calculation 4. Stanchions Portal Leg D ≈ H / 10 5. Columns D ≈ No Storeys * 100 / 4 (rnd 50) NOT < 203 6. Economics Deeper the Beam the more economical? 7. Eurocodes As above
- 11 -
3.2 Formula o Tension Capacity, Pt = ρt Ae σAc o Steel Required = 0.95 fy
o For quick element sizing based on δ criteria I req =
5wL4 384 Eδ L
FeL 2 18 o Natural Frequency, f = where δ = δ
o Torsion Moment, M T =
Self weight + dead + 10% imposed load o Stress, σ = M*y / I
3.4 Composite Design o In composite design for 19mm ∅ studs then minimum size of reinf is 10mm bars o 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 length o For cellbeams 8 out of 10 engineers use 1.0L for effective length o 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 -
P2 P1
Y RHS X d
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 λcr i.e. < 4 carry out a 2nd order analysis o Box section cost twice as much to manufacture than UB/UC’s o For holding down bolts the tolerance is 25mm in all directions o Metsec says spans of side rails & purlins are not efficient until 3 – 3.5m lengths - 15 -
o Purlins – Small projects generally single or double span. For Large buildings use HEB system
- 16 -
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 > 200mm 5. Economics Deeper the Beam more economical? 6. Eurocodes As above
- 17 -
4.2 Formula M × 10 6 f cu × bd 2
o
K=
o
Z = d 0.5 + 0.25 − K ≤ 0.95d 0.9
o
As =
M ×10 6 0.95 f y Z
o K’ = 0.156 when redistribution <10% o K ' = 0.402( βb − 0.4 ) − 0.18( βb − 0.4 ) 2 o
M − M u ×10 6 M ×10 6 + As ' & As = u 0.95 f y Z 0.95 f y ( d − d ')
o Shear, ν = o
V ×10 3 ≤ 0.8 bd
1
1
f cu
100 As 3 400 4 νc = 0.79 γm
bd
d
or 5N/mm2 1
f 3 × cu 25
o Modification Factors for deflection Tension Reinforcement: ( 477 − fs ) ≤ 2 0.55 + M Mod F = 120 0.9 + 2 bd ( 2 fyAs req ) 1 × Where fs = 3 × As prov β b
Compression Reinforcement: - 18 -
100 A' s prov bd Mod F = 1 + 100 A' s prov S + bd
- 19 -
≤ 1.5
4.3 General Notes o Stair checks done as simple beam for simplicity o 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 = 250mm o If Mu < M then compression reinf required o Also if K > K’ then compression reinf required o Reinforcement Spacing Min Spacing = hagg +5mm - 20 -
Max Spacing ≤ 47000/fs ≤ 300 o Pre-cast stairs ideally need 180-200mm bearing. Minimum of 165mm. Creagh say 100mm min.
- 21 -
5
Masonry
5.1 General Notes o Freestanding walls and wall panels are 2 different things o Typical Masonry Values o 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 brickwork o Wall Panels with H/L < 0.3 then wall will tend to span vertically o Wall Panels with H/L > 1.75 then wall will tend to span horizontally o Stress, σ = Load / Wall Area
- 22 -
6
Timber
6.1 Rules of Thumb 1. Roof Joists Span (mm) / 20 – RND + 25mm 2. Roof Joists Span (mm) / 20 +25 mm – RND + 25mm 3. Racking Model to suit published tables 4. Hybrids – Steel & Timber go well together 5. Section Stability Engineered products vs. natural products 6. Creep / Settlement Engineered products vs. natural products
- 23 -
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 - 24 -
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
