LOADING LOADING
A. LOADING GENERALITIES
6.3.7. Ultimate load for a standard lashing (4 straps)
Let‘s now consider a standard lashing composed of four straps (1 Forward, 1 Aft, 2 Up) fastened
to 4 single stud fittings (see next figure).
As done previously, it is possible to establish the ultimate load in each direction (Forward, Aft,
Up, Left, Right).
With: STF = Strap Forward TDLF = Tie-down point Left Forward STA = Strap Aft TDLA = Tie-down point Left Aft STU = Strap Up TDRF = Tie-down point Right Forward
TDRA = Tie-down point Right Aft
a) Angles definition
The force breakdown needs to be calculated for each tie-down point. So for each point the angles α and β need to be defined.
α : angle between the force direction and the floor.
β : angle between the force direction and the forward direction.
Note : for the following example we assume that each strap shall make a maximum angle of 30 degrees with the direction of restraint.
A. LOADING GENERALITIES
. Left Tie Down Points
Restraint UP
TDLF STU : α= 60°, β=90°
TDLA STU : α= 60°, β= 90°
Restraint FORWARD
TDLF STF : α= 30°, β= 30°
Restraint AFT
TDLA STA : α = 30°, β = 150°
. Right Tie Down Points
Restraint UP
TDRF STU : α = 60°, β = 270°
TDRA STU : α = 60°, β = 270°
Restraint FORWARD
TDRF STF : α = 30°, β = 330°
Restraint AFT
TDRA STA : α = 30°, β = 210°
LOADING LOADING
A. LOADING GENERALITIES
b) Force breakdown
Then for each tie-down point the force breakdown is : FX = F.cos(α).cos(β) FY = F.cos(α).sin(β) FZ = F.sin(α)
. Left Tie Down Points
TDLF : 2 straps : 1 UP and 1 FORWARD Fx UP = 0 Fx FORWARD = 0.75 F FY UP = 0.5 F FY FORWARD = 0.43 F FZ UP = 0 86 F FZ FORWARD = 0 50 F
TDLA: 2 straps : 1 UP and 1 AFT Fx UP = 0 Fx AFT = - 0.75 F FY UP = 0.5 F FY AFT = 0.43 F FZ UP = 0 86 F FZ AFT = 0 50 F
. Right Tie Down Points
TDRF : 2 straps : 1 UP and 1 FORWARD Fx UP = 0 Fx FORWARD = 0.75 F FY UP = - 0.5 F FY FORWARD = - 0.43 F FZ UP = 0 86 F FZ FORWARD = 0 50 F
TDRA : 2 straps: 1 UP and 1 AFT Fx UP = 0 Fx AFT = - 0.75 F FY UP = - 0.5 F FY AFT = -0.43 F FZ UP = 0 86 F FZ AFT = 0 50 F
Note : F being the maximum breaking strength allowed at the tie-down point.
c) Maximum load
So the maximum loads in each direction are: FX FORWARD = 0.75 F + 0.75 F = 1.5 F FX AFT = -0.75 F - 0.75 F= - 1.5 F FY RIGHT = - 0.5 F - 0.43 F - 0.5 F - 0.43 F = - 1.86F FY LEFT = 0.5 F + 0.43 F + 0.5 F + 0.43 F = 1.86 F Fz = 0.86 F + 0.5 F + 0.86 F + 0.5 F + 0.86 F + 0.5 F +0.86 F + 0.5 F = 5.44 F
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