Structural Engineering Art and Approximation. A Talk By Hugh Morrison

Transcription

1 Structural Engineering Art and Approximation A Talk By Hugh Morrison

2 Any Questions? Please Speak Up! And turn off mobile phones.

3 Summary of Talk Why and How the book came to be written Illustration: Historic and Contemporary structure. Samples from the book Conclusion: the benefits of sketching and approximation.

4 WHY and HOW the book came to be written Teds Ram STAAD GSA NO COMPUTER Desert Island Engineering? I m an engineer get me out of here!

5 ENGINEERING IS THE ART OF APPROXIMATION ART APPROXIMATION CREATIVE THOUGHT and self- expression PROPOSE and TEST OPTIONS: sketches and rough calculations Culminating in a wholesome, pleasing solution DESIGN is finding a solution within acceptable limits

6 Ove Arup Quote Engineering problems are under-defined, there are many solutions, good, bad and indifferent. The art is to arrive at a good solution. This is a creative activity, involving imagination, intuition and deliberate choice.

7 SUMMARY With simplified methods one can be assured that: THE PROBLEM IS DEFINED THE ASSUMPTIONS ARE CLEAR THERE IS A ROUGH SOLUTION ONE KNOWS APPROXIMATELY HOW THE STRUCTURE WILL BEHAVE THE COMPUTER MODEL IS VERIFIABLE

8 One thing to be sure, I will not be presenting anything like this

9 A REVIEW OF AN HISTORIC AND A CONTEMPORARY STRUCTURE GOOD EXERCISE: Look at Built Structures UNFAMILIAR STRUCTURES: To avoid preconceptions SKETCHES ASSUMPTIONS LISTED APPROXIMATE CALCULATIONS

10 R100 AIRSHIP Chief designer: Barnes Wallis Lead Stress Engineer: Neville Norway Shute

11 DESCRIPTION OF STRUCTURE Hydrogen filled buoyancy bags top centre. Powered by 3 diesel gondolas 16 side polygon with steel radial ties circa mm dia. Duralumin transverse and longitudinal frames Trussed approx. 680mm deep Circa 15 transverse frames at 13.5m (45ft)

12 Transverse Frames Maximum net pressure Assumed 1kPa Transverse Frame Max 40m diameter

13 ASSUMPTIONS UNIFORM SUCTION: RING INEFFECTIVE Radial Cables Resist Load UNIFORM INWARDS PRESSURE: RADIAL CABLES INEFFECTIVE Ring Resists Load ASYMMETRICAL LOADING: (SUCTION AREA) CABLES IN TENSION RING IS INEFFECTIVE IN BENDING

14 HAND CALCULATIONS IN EASY STEPS CONSTANT SUCTION 1kPa CONSTANT PRESSURE 1kPa UNEQUAL PRESSURE 1kPa

15 R100 CONCLUSION STAGE 2: MORE DETAILED DESIGN CHECKS LEVEL OF REFINEMENT: DECIDE WHAT IS APPROPRIATE? FOR EXAMPLE: Cable stress close to yield examine further 35m diameter? Check Ring with combined bending/cable tension. Computer analysis of a 2D half frame linear/non-linear Buckling checks? Longitudonal Frame checks?

16 NEVILLE SHUTE: A Delighted Engineer... After literally months of labour, having filled perhaps fifty foolscap sheets with closely pencilled figures, after many disappointments and heartaches, the truth stood revealed, real, and perfect, and unquestionable; the very truth.

17 ISTRUCTE STAIR HQ BASTWICK STREET Expedition Engineers Hugh Broughton Architects June/July 2015: Design Feature IstructE magazine REVIEW OF DESIGN: alternative load path CONSIDERED

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19 ISTRUCTE STAIR BASTWICK STREET Torsion Increases down the stair Torsion and Shear resisted By wall No cantilever bending Tread loads accumulate Traditional Cantilever Stair Load path

20 ASSUMPTIONS 1. GLASS balustrade ineffective 2. BARS transfer shears/torsion into wall 3. 50% STAIR loaded onto wall 4. 50% STAIR and balustrade load cumulatively onto outer stringer 5. LOCAL TORSION resisted at each tread by STRINGER DOES NOT ACCUMULATE 6. LANDING supports upper half of FLIGHT

21 FREE BODY DIAGRAM Alternative Load Path Balustrade/stringer/half tread each 1kN Cumulating loads onto stringer/landing x10 treads max = 10kN Local applied Torsion (over tread width) = (10-9)x0.25 = 0.25kNm 1OkN Resolving Shears at support R1= T/0.08m+0.8/2=3.6kN (down) R2=-T/0.08m+0.8/2=-2.7kN(up)

22 INITIAL MODEL - VERIFICATION Concrete treads 40mm thick (plates) 75x30mm stepped stringer (steel) LIKE WOBBLY TEETH WALL SUPPORTS with VERY LOW PULL OUT STIFFNESS REACTIONS +3.7/2.8 OK STRINGER AXIAL FORCE 10.6kN OK

23 INITIAL MODEL DEFLECTIONS DEFLECTIONS HIGH NO GLASS 10mm ADDING GLASS DEFLECTION 0.50mm

24 REVISED MODEL FIXED SUPPORTS By fixing supports shears at support model corresponded to Expedition design EXPEDITION MODEL HORIZONTAL SHEARS EXPEDITION MODEL NO GLASS DEFLECTION 1mm

25 REVISED MODEL NATURAL FREQUENCY Natural Frequency at 14.9Hz similar to Expedition site observations EXPEDITION MODEL 14.9Hz

26 DOES THE ALTERNATIVE LOAD PATH MODEL HELP? POST DRILLED ANCHORS POSSIBLE (pre-forming holes complicated construction) SINCE GLASS BALUSTRADE INCLUDED natural frequency > 14.9Hz

27 CONCLUSION Sketches and hand calculations used to understand problem Support loads may be estimated Elements may be Initially Sized and Checked Initial COMPUTER MODEL built from FIRST APPROXIMATIONS

28 EXAMPLES FROM THE BOOK Structural Engineering: Efficacy, Balance and Grace Simply Supported Beams Learning from Failure Cantilever Beams Continuous Beams Framed Structures Trusses and Vierendeel Structures Tension and Suspension Structures Arches, Vaults and Domes Torsion Structures and Ring Beams Plate Structures Deep Beams, Load Carrying Walls and Diaphragms Dynamics

29 TRIANGULAR LOADING ASSUMING UDL 2/3 PEAK Classic Bending Moments/Shears UDL in place of Triangular Loading Equivalent UDL 2/3 peak +33% Bending +0% max shear +33% deflection

30 ASSUMING CENTRAL POINT LOAD WHEN ACTUAL LOAD OFF CENTRE Classic Bending Moments/Shears Use When Point Load off-centre (up to Middle Third Zone) Equivalent Assume Central Point Load +11% Bending -140% max shear calculate separately! +15% deflection

31 CANTILEVERS Comparison of Cantilever with Simply Supported Cantilever with Backspan Point Load Cantilever with Backspan UDL Increasing backspan To 4L 4xencastre Increasing backspan UDL backspan increased influence

32 FRAME STRUCTURES Wind Frame Traditional Design Wind Loading Virtual Pins Columns Design Beams Assuming Simply Supported Quick estimation of Wind Bending

33 TRUSSES AND VIERENDEELS KING POST TRUSS Kingpost Truss Statically Indeterminate Combination of Beam and Truss Action

34 TENSION STRUCTURES FABRIC TENSION ROOF STRUCTURE NON LINEAR ANALYSIS Form Finding (dynamic relaxation) LINEAR ESTIMATION POSSIBLE Assume line load in Fabric (say 3kN/m) Goodwood Racecourse Cable loads from T=PR (utilise railway curves) External reactions from collected cable loads

35 TENSION STRUCTURES FABRIC TENSION ROOF STRUCTURE Internal Force Diagram External Forces Net Effect on Supporting Structure

36 COMPRESSION - CATHEDRAL BUTTRESSING THE MIDDLE THIRD RULE! Cathedral Section Estimate Buttress (and Pinnacle) weight Check Foundation thrust -line in middle third Simple thrust from vaulted roof Geometric check resolved buttress counterweight

37 DEEP BEAMS Quick Check Table Reinforcement Estimate Lever Arm 0.62d Check Shear overall rectangular section Or resolve by STRUT-TIE model

38 DYNAMICS DISCOMFORT/PERCEPTION ASSESSMENT Fundamental Natural Frequencies tendency for slender structures to resonance Summary of terms Fundamental Frequencies Damping Ratios Simple Excitation Calculations Discomfort Criteria Lateral Frequencies Tall Buildings

39 FOOTBRIDGE/RAMP 10m SPAN Actual 10m span structure with screed floor fn 2.6Hz NATURAL FREQUENCY 2.6Hz. Resonance LIVELY! GLASS BALUSTRADE DAMPING

40 STADIUM CANTILEVER TIER RESPONSE CALCULATION based DYNAMIC MAGNIFICATION Response Chart Simplified STADIUM tier 3.5Hz NATURAL FREQUENCY Check further < 6 Hz

41 EXERCISE BOOK A POCKET COMPANION 15 EXERCISES RELATED TO REALISTIC PROBLEMS FOLLOWS DESIGN PROCESS CROSS REFERS TO MAIN BOOK

42 AND FINALLY STRUCTURAL CURIOSITIES. WHAT NO SPREAD?!

43 STABLE?

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45 THE TALK.. Why and How the book came to be written Illustration: Historic and Contemporary structure. Samples from the book Conclusion: the benefits of sketching and approximation.

46 THANK YOU ANY QUESTIONS?