Download presentation
Presentation is loading. Please wait.
1
CHAPTER 1: INTRODUCTION part B
BFC Structural Steel and Timber Design Assoc. Prof. Dr DYeoh 2014
2
Course content Assoc. Prof. Dr DYeoh 2014
3
Outline of lecture Design concept and design process
Stability of structure Actions on structure Quantifying the “effects” on structure How to determine the worst load combination for design? Assoc. Prof. Dr DYeoh 2014
4
Design Philosophy LIMIT STATE DESIGN (LSD) Ultimate Limit State (ULS)
Serviceability Limit State (SLS) Assoc. Prof. Dr DYeoh 2014
5
ULS Conditions considered: Strength Stability Collapse
Assoc. Prof. Dr DYeoh 2014
6
SLS Conditions considered: Deflections Vibration Assoc. Prof. Dr DYeoh
2014
7
Limiting deflection Assoc. Prof. Dr DYeoh 2014
8
Design process Flow diagram in structural design: Define problem
Site survey and investigation Assess loads Analyse Investigate alternatives Element design and detail design Detailed development Assoc. Prof. Dr DYeoh 2014
9
Design concept A structure (e.g. a building/ bridge) is a 3D solid made of: 1D members (beams/columns, cables) 2D members (floors/walls, shells, membranes) Those elements are assembled in order to make the 3D structure. Assoc. Prof. Dr DYeoh 2014
10
Design concept The connections between members and with the foundation can be pinned or rigid Assoc. Prof. Dr DYeoh 2014
11
Stability The structure must be stable (i.e. at least statically determinate). Any type of instability, in-plane and out-of-plane, must be avoided! IN-PLANE STABILITY NO! YES YES Assoc. Prof. Dr DYeoh 2014
12
Stability IN-PLANE STABLE OUT-OF-PLANE UNSTABLE! OUT-OF-PLANE STABLE z
x IN-PLANE STABLE OUT-OF-PLANE UNSTABLE! OUT-OF-PLANE STABLE Assoc. Prof. Dr DYeoh 2014
13
Important features of Stability in a Structure
Static equilibrium Resistance to horizontal forces Sway stiffness – second order or P-delta effect Assoc. Prof. Dr DYeoh 2014
14
Lateral resistance for stability Assoc. Prof. Dr DYeoh
2014
15
Actions (Loads) on structure
Actions – things that produce stresses Loads Settlements Accelerations (earthquake) Fire Loads – forces Self-weight You and me Wind Snow etc etc All loads are actions Not all actions are loads! Assoc. Prof. Dr DYeoh 2014
16
Will My Structure Work? Design for limit states
Ultimate limit states (life safety) Strength – must not collapse under applied actions* Stability – must not buckle, overturn or move* Cyclic plasticity And others Serviceability limit states (use-ability) Deflections* Comfort Appearance Also others – accidental, fatigue, vibration etc. etc Assoc. Prof. Dr DYeoh 2014
17
Will My Structure Work? E<R The BIG PICTURE – design inequalities
R=Resistance of structure Need to know size of actions to calculate this side E<R Need theory of structures to calculate this side E= Effect of actions E determined from size of pt load and self-weight R given by bending resistance of beam Assoc. Prof. Dr DYeoh 2014
18
Design Values of Actions
Design value, Fd Typical distribution of permanent loads Characteristic Value, Fk Design value, Fd Typical distribution of imposed loads Probability density Narrow distribution design load, Fd=γFk γ =partial safety factor γ, large for variable loads γ, small for permanent loads Characteristic Value, Fk Wide distribution Value of Load Assoc. Prof. Dr DYeoh 2014
19
Classification of Actions
Permanent Self-weight* Settlements Variable Imposed loads* Wind loads* Snow loads* Accidental Fire Blast Impact Assoc. Prof. Dr DYeoh 2014
20
LOAD PATH – THE SHORTEST ROUTE DOWN
Assoc. Prof. Dr DYeoh 2014
21
ACTIONS - Variable Assoc. Prof. Dr DYeoh 2014
22
ACTIONS - Variable Assoc. Prof. Dr DYeoh 2014
23
ACTIONS - Variable 1.5 kPa = 1.5 kN/m2 = 150 kgf/m2 floor live load 1
Assoc. Prof. Dr DYeoh 2014
24
Wind actions This actions is variable and beyond human control. Wind loads are the positive or negative pressures exerted on a building when it obstructs the flow of moving air. Assoc. Prof. Dr DYeoh 2014
25
Will all Loads Occur Together?
Unlikely to have huge crowd, hurricane, 3m snow and blast altogether therefore consider realistic combinations of loads apply reduced values of some loads Full permanent load and imposed load with 70% wind load Or permanent and 70% imposed plus fire Also unlikely to have full imposed load over a large area – reductions sometimes used. Assoc. Prof. Dr DYeoh 2014
26
Recap what have been said...
Obtain “characteristic” values for actions Apply appropriate partial safety factors Possibly apply reductions as worst case for each action will not occur together in time in space Use actions to estimate stresses, forces, moments etc. that must be resisted Check “resistance” great than “load effect” Repeat for each limit state Assoc. Prof. Dr DYeoh 2014
27
Eurocode 0 and 1 Eurocode 0 (EN 1990) – Basis for Design
Design philosophy Safety factors Eurocode 1 (EN 1991)– Actions on Structures Part 1: General actions – self-weight, imposed loads etc Part 3: Snow loads Part 4: Wind loads (later) Other parts deal with fire, bridges, accidental loads etc. etc. Assoc. Prof. Dr DYeoh 2014
28
Assoc. Prof. Dr DYeoh 2014
29
Characteristic Loads Also known as Working Loads
It is not a factored load Statistically, working loads have a 95% chance of not being exceeded Assoc. Prof. Dr DYeoh 2014
30
Characteristic Permanent Loads
Self-weight – look it up in Section 4 of Eurocode 1 Annex A Eurocode 1 Assoc. Prof. Dr DYeoh 2014
31
Example 1 – Permanent Loads
Structure supporting a hotel balcony Frames at 6m centres Require vertical reaction at supports Cross-section=75cm2 Self-weight (EN Table A4) Beam: 75/1002*10*78.5= kN Columns: 100/1002*(5*2)*78.5= 7.8kN Load per support kN Note: Section masses normally obtained directly from section tables. Cross- Section =100cm2 5m 10m Assoc. Prof. Dr DYeoh 2014
32
Characteristic Variable Loads
Imposed loads Given in Section 6 of Eurocode 1 May differ for each country – take care Need to determine category and then look up load Reduction allowed to account for large areas and multiple storeys ( 6.2.1, NA.2.5 and NA.2.6) Some complications for roofs and car-parks Assoc. Prof. Dr DYeoh 2014
33
Example 2 – Variable Loads
Structure supporting a hotel balcony Frames at 6m centres Require vertical reaction at supports Imposed loads from occupants, furniture etc. Imposed Load (EN ) Category A7 Table NA.2 Load intensity 4kN/m2 Table NA.3 Total load: 4*10*6=240kN Load per support =120kN Assume no reduction factor 6.2.1(4) 5m 10m Assoc. Prof. Dr DYeoh 2014
34
Partial Safety Factors
Section 6 and Annex A1 of Eurocode 0 Apply appropriate partial safety factors and combination factors to account for possible variability in loads Example for the ultimate limit state (Eqn 6.10): Sum of permanent actions Sum of accompanying variable actions Leading variable action Design effect Prestressing action (not relevant for this course) Assoc. Prof. Dr DYeoh 2014
35
Unfavourable means action that results in greater effect / load resultant
Favourable means action that results in smaller effect / load resultant Assoc. Prof. Dr DYeoh 2014
36
Example 3 – Counting design effect
For the example there is only one Permanent action (self-weight) and only one variable action (imposed Load) so this simplifies to Partial factor on self-weight Partial factor on imposed load Characteristic Dead load = Permanent action = Gk =6.9kN from before Characteristic Imposed load= Variable action=Qk =120kN from before Partial factor on permanent action=γG =1.35 A1.3.1(5) and Table NA.A1.2(B) Partial factor on variable action= γQ = A1.3.1(5) and Total load per support for this condition= 1.35* *120 =189kN Assoc. Prof. Dr DYeoh 2014
37
Wind Loads Wind loading complex – many details
See separate part of EN1991 For now we assume characteristic values Assoc. Prof. Dr DYeoh 2014
38
Example 4 – Wind Wind loads mainly horizontal Some vertical component
Assume acting on cladding and transferred to “nodes” Assume characteristic value of 1kPa Resultant force 5m Wind load of 1kPa Acts over surface Characteristic Wind Load Resultant force = 1*6*5=30kN 10m Assoc. Prof. Dr DYeoh 2014
39
Other Actions or Loads Snow Earthquake Assoc. Prof. Dr DYeoh
2014
40
Example 5 – Loads Cases or Combinations
A roof has the following loads applied: Permanent load, Gk = 1.0 kN/m2 Variable actions, Qk = 0.5 kN/m2 Wind load: Wuplift = 1.25 kN/m2 Wdown = 0.4 kN/m2 Snow, S = 0.6 kN/m2 Determine the most critical load combination. Assoc. Prof. Dr DYeoh 2014
41
Assoc. Prof. Dr DYeoh 2014
42
Assoc. Prof. Dr DYeoh 2014
43
Assoc. Prof. Dr DYeoh 2014
44
Assoc. Prof. Dr DYeoh 2014
45
Assoc. Prof. Dr DYeoh 2014
46
End of Introduction part B
Assoc. Prof. Dr. DYeoh 2014
47
Recap what have we learned?
Assoc. Prof. Dr DYeoh 2014
48
Recap what have we learned?
Design concept and design process Stability of structure Actions on structure Quantifying the “effects” on structure How to find the worst load combinations? Assoc. Prof. Dr DYeoh 2014
Similar presentations
