Snow load on structures: case studies on cableway masts and measures against snow gliding

Transcription

1 Hasliberg March 1999 Swiss Federal Institute for Snow and Avalanche Research Snow load on structures: case studies on cableway masts and measures against snow gliding Stefan Margreth/Mark Schaer Aosta, June 16 th 2011: SNOWPACK SLOW MOVEMENTS AND LOAD ON DEFENCE STRUCTURES

2 Contents: 1. Introduction 2. Snow pressure calculation according to the Swiss Guideline for Cableways 3. Examples of damage cases 4. Examples of design cases 5. Conclusions 2

3 1 Introduction Thousands cableway masts: no damage few cases: evidence of very strong snow force 3

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5 Snow pressure model according to the Swiss Guidelines (1990) 5

6 Snow pressure model according to the Swiss Guidelines (1990): S N : resultant snow pressure per unit length across the slope on a rigid wall!: average snow density (to m -3 ) g: acceleration due to gravity (m s -2 ) H: vertical snow depth (m) N: gliding factor, varies according to ground roughness and slope exposition between 1.2 and 3.2 K: creep-factor, depends on the snow density! (to m -3 ) and the slope angle " ( ). 6

7 Adapted snow pressure model for masts: efficiency factor: geometric term Bigger influence width for small obstacle: Consideration of end-effects Vertical snow depth (H) Angle of the slope (") c-factor: describes the snow glide intensity in relation of the ground roughness and the snow depth: - Small snow gliding: c=0.6 - Strong snow gliding: c=1.0 - Extreme snow gliding and H>2-3m: c=2.0 - Extreme snow gliding and H<2-3m: c=6.0 7

8 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) 4 December 1996 Slope inclination: 35-30, width of crack 80 m 13

9 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) 4 December 1996 Snow height < 2.0 m Snow density ~300 kg m -3 14

10 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Destruction of mast 10! Mast 10 Glide velocity: 12 cm per hour Glide velocity: 46 cm per hour 4 December

11 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) 6 December

12 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) 6 December

13 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Facts: location: Eastern Swiss pre-alps year of construction: 1984 height: 1460msl exposition SE slope height above pylon: 40m, open slope slope inclination: at the pylon 28, gradually increasing to 40 at the top of the slope ground cover: pasture with some cattle trails, generally dry snow and weather conditions: 18

14 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Strong snowfall First glide crack Failure of mast 19

15 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Structural analysis of mast: Failure of angle steel profile (80/80/8 mm) with uniformly distributed snow pressure per unit length of >85 97 kn m -1 Total failure load: > kn 20

16 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Snow pressure back-calculation with textbook parameters: H: vertical snow depth 1.8m!: average snow density 270kg/m -1 N: gliding factor 2.4 ": #slope angle 30 D $ F efficiency Factor " F =1+ c# 4.9 W! The pylon supports a 4m wide snow plate c=2, D=1.6m, W=0.8 S N,M = 16.9kN/m S tot = S N,M * D = 26.4kN 21

17 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Snow pressure back-calculation with recalibrated parameters: H: vertical snow depth 1.8m 2m!: average snow density 270kg/m kg/m -3 N: gliding factor ": #slope angle D $ F efficiency Factor " F =1+ c# W c=2, D=1.6m, W=0.8 c=5, D=2m! The pylon supports a 11m wide snow plate S N,M = 73kN/m S tot = S N,M * D = 123kN/m # 22

18 Case study 1: Skilift Bärenfall, ski resort Amden, northern pre-alps (Switzerland) Design now pressure calculation with textbook parameters: H: vertical snow depth 3.5m!: average snow density 270kg/m -1 N: gliding factor 2.4 ": #slope angle 30 D $ F efficiency Factor " F =1+ c# 8.6 W c=2, D=1.6m, W=0.8 S N,M = 58kN/m S tot = S N,M * D = 178kN 23

19 Case study 2: Chairlift Pleus, ski resort Elm, northern Alps 23 February 1999 Mast 7 25

20 Case study 2: Chairlift Pleus, ski resort Elm, northern Alps summer 1999 Mast 7 26

21 Case study 2: Chairlift Pleus, ski resort Elm, northern Alps summer 1999 Facts: location: Eastern Swiss pre-alps year of construction: 1982 height: 1900msl exposition SW slope height above pylon: 80m slope inclination: at the pylon 28, slowly increasing to 40 at the top of the slope ground cover: pasture, generally dry snow and weather conditions: avalanche winter 1999, snow cover was reaching a new record 27

22 Case study 2: Chairlift Pleus, ski resort Elm, northern Alps Structural analysis of mast: 5.2 m 1.2 m Passive earth pressure P P Total weight of mast W Friction resistance R 2.3 m Foundation slides if the resisting forces (friction between base and soil, passive earth pressure) are smaller than the driving force (snow pressure). With a total weight of 674 kn the total snow pressure on the mast was at least 472 kn. With a snow thickness of 3.5 m the resulting snow pressure per unit length was 135 kn m

23 Case study 3: Chairlift Käserstatt-Hochsträss, ski resort Hasliberg, Bernese Oberland 26 February

24 Case study 3: Chairlift Käserstatt-Hochsträss, ski resort Hasliberg, Bernese Oberland Facts: location: Central Swiss pre-alps year of construction: 1985 height: 1860msl exposition S slope height above pylon: 60m slope inclination: at the pylon 25, slowly increasing to 40 at the top of the slope ground cover: pasture snow and weather conditions: avalanche winter 1999, snow cover was reaching a new record 30

25 16 March

26 Case study 3: Chairlift Käserstatt-Hochsträss, ski resort Hasliberg, Bernese Oberland 17 March

27 Case study 3: Chairlift Käserstatt-Hochsträss, ski resort Hasliberg, Bernese Oberland 33

28 Case study 3: Chairlift Käserstatt-Hochsträss, ski resort Hasliberg, Bernese Oberland Mean snow density: 390 kg m -3 Ground temperature: +0.2 C Inclination: 25 Snow thickness: 2.7 m 34

29 Case study 4: Cableway Monte Baldo, Italy

30 Case study 4: Cableway Monte Baldo, Italy Facts: location: Italian Alps year of construction: 2002 height: 1600msl exposition WSW slope height above pylon: 100m slope inclination: at the pylon 30, increasing to steep rocks at the top of the slope ground cover: pasture/gravel snow and weather conditions: - early, important snow cover in fall - winter snow cover was reaching a new record (up to 3.5m) - very dense snow in spring (500kg/m 3 ) anti avalanche wedge

31 Case study 4: Cableway Monte Baldo, Italy

32 Case study 4: Cableway Monte Baldo, Italy

33 Case study 4: Cableway Monte Baldo, Italy

34 Case study 4: Cableway Monte Baldo, Italy

35 Case study 4: Cableway Monte Baldo, Italy

36 Case study 4: Cableway Monte Baldo, Italy

37 Case study 4: Cableway Monte Baldo, Italy

38 Case study 4: Cableway Monte Baldo, Italy rocky outcrop A B anti avalanche wedge Stopping area: the snow plate is locked between the wedge and the rocks. Below this zone the gliding accelerates again there are tension cracks.

39 Case study 4: Cableway Monte Baldo, Italy Stopping area: The snow plate is supported by a trail crossing the slope. Snow bulges on this flatter area. anti avalanche wedge

40 Case study 4: Cableway Monte Baldo, Italy a =4.0 m S A b =15.6 m 50 S An S Bn S B S B S A S Ap S Bp 60 S A =245kN 10 wedge side A=23m R=1197kN S B =955kN

41 Application of the snow pressure model: 47

42 Case study 5: Chairlift Jochstock, Engelberg : Andi Jacomet 48

43 Case study 5: Chairlift Jochstock, Engelberg Facts: location: Swiss central Alps year of construction: 2001 height: 2450 msl exposition NW slope height above pylon: 50m slope inclination: at the pylon 32, ground cover: gravel

44 Case study 5: Chairlift Jochstock, Engelberg Design now pressure calculation with textbook parameters: H: vertical snow depth 6.3m!: average snow density 320kg/m -1 N: gliding factor 1.6 ": #slope angle 33 D $ F efficiency Factor " F =1+ c# 5 W c=0.6, D=1.6m, W=0.8 S N,M = 53kN/m S tot = S N,M * D = 280kN # 50

45 Case study 5: Chairlift Jochstock, Engelberg Design now pressure calculation with textbook parameters: H: vertical snow depth 6.3m!: average snow density 320kg/m -1 N: gliding factor 1.6 ": #slope angle 33 D $ F efficiency Factor " F =1+ c# 5 W c=0.6, D=1.6m, W=0.8 S N,M = 53kN/m S tot = S N,M * D = 280kN! very large loads: do we need to be even more conservative? # 51

46 Measures against snow gliding 52

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48 Measures against snow gliding 54

49 Measures against snow gliding 55

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51 Conclusions I: A simple calculation model for snow pressure forces on narrow obstacles as masts was developed on the base of the Swiss Guidelines. In comparison to an infinitely long obstacle, end-effect forces also have to be considered. The corresponding influence zone was in the 3 case studies wider than the diameter of the mast. The largest uncertainties consist in the assessment of the snow gliding and efficiency factor. Not all damage cases will be avoided by the Swiss approach, but a more conservative approach is very expensive 58

52 Thank you for your attention! Questions?! 60