On The Use Of Pickets And Flukes As Snow Anchors

70 Pages·2009·2 MB·English

OOnn tthhee UUssee ooff PPiicckkeettss aanndd FFlluukkeess
aass SSnnooww AAnncchhoorrss
ART FORTINI
Sierra Madre Search & Rescue
International Technical Rescue Symposium
Denver, CO
November 2002
PPiicckkeettss
Placements
(cid:194)
Traditional vertical
Horizontal
Sierra picket
Behavior
(cid:194)
Strong snow
Powdery Snow
FFlluukkeess
Rake angle
(cid:194)
Behavior
(cid:194)
Strong snow
Powdery Snow
MMuullttii--TTooooll AAnncchhoorrss
Parallel anchors
(cid:194)
Series anchors
(cid:194)
Crossed tools
(cid:194)
Equalization
(cid:194)
methods
EEffffeeccttss ooff ssnnoowwppaacckk
Mechanical properties
(cid:194)
Compressive strength
Tensile strength
Shear strength
Layering
(cid:194)
Weak layers
Icy layers
Disturbance and settling time
(cid:194)
AAnncchhoorr FFaaiilluurree
System breaks at the weakest link
(cid:194)
Software failure Snow failure Hardware failure
rope webbing σ A σ A biner hardware
c c s s
compression shear
SShheeaarr vvss CCoommpprreessssiivvee FFaaiilluurree
σ > σ
(cid:194)
c s
Determined by
shear
snowpack
A
s
A << A
(cid:194)
c s
A = LW
c
A = f(d,W)
W
s
Failure
(cid:194) compression
A
c
shear if σ A > σ A
c c s s
comp if σ A < σ A
c c s s
CCAAUUTTIIOONN !!
Building a man-rated system out of snow
(cid:194)
Material is extraordinarily weak
– σ < 1 psi (snow)
s,t
– σ < 10 psi (snow)
c
– σ > 100,000 psi (steel)
t
Material properties vary tremendously
Material properties typically unknown when building
the anchor
Fortunately, the applied loads are typically small
CCoommpprreessssiivvee FFaaiilluurree
Seen only when
(cid:194)
compressive stress is higher than compressive strength
and
shear stress is less than shear strength
Dominant effect in
(cid:194)
traditional picket placements in any snow
all anchors in weak, powdery snow
SShheeaarr FFaaiilluurree
Surface area of stress cone
(cid:194)
Much larger than area of tool
Stress cone extends
top view
– horizontally at ~45o
– upward at ~30o
Dominant effect in strong snow
(cid:194)
– flukes, mid-clipped pickets
side view