slinging training modules part 2 aug 2002

CRANEAGE: SLINGING - INTRODUCTION 2

TRAINING MODULE

SLINGING Modules 4 to 7

Sling LoadsReeving and Choking

Dunnage

Common Mistakes

NOTE: This is Part 2 of a 4-part module set covering Slinging.

Part 1 covers Sections 1 to 3. Part 3 covers Sections 8 and 9. Part 4 covers Sections 10 to 12.

This Training Module comes to you courtesy of:

Site Safe would like to acknowledge Fletcher Construction’s on-going support

SLINGING Modules 4 to 7

CRANEAGE: SLINGING - INTRODUCTION 2

CRANEAGE - SLINGING: BASICS PAGE 1

CRANEAGE - SLINGING BASICS

SECTION 4:

KNOW YOUR SLING LOADS

4.1 Know how to work out Sling Loads4.2 Sling Loads Formula4.3 Two-Legged Slings4.4 Three-Legged Slings4.5 Four-Legged Slings4.6 Four-Legged Slings: worked examples


GOOD SLINGING PRACTICE

BASIC RULES ... 4

4.1. KNOW HOW TO WORK OUT SLING LOADS.

You now know the importance of the included angle in slinging. It then becomes necessary to establish the loads actually being carried by slings. This can be done with nothing more complicated than a tape measure and a simple pocket calculator, and not having to become involved in degrees and angles.

W = weight

L = LengthV =

Height

Angle up to 90°Not more than 45° Slings

Load

S.W.L. x 1.4 = Safe Working Load.

Angle to the vertical

(Don't crowd the hook!)



BASIC RULES ... 4

4.2. KNOW HOW TO WORK OUT SLING LOADS.

This formula is very accurate, and allows you to use a tape measure for calculations, rather than having to use degrees and angles.

The formula is: T = W x LN V

T = Tension per leg in kg W = weight in kg N = number of legs L = sling length in metresV = vertical height in metres

Example:W = weight = 5,000 kg N = No. of legs = 2 V = vertical height = 2 metresL = sling length = 3.5 metres

T = 5000 kg x 3.52 2

= 17,5004

= 4,375 kg per leg.

W = weight

L = LengthV =

Height

You would need a two-legged sling rated at 5,000kg WLL capacity per leg.

This formula will work for any number of sling legs, but as we all know, a four leg sling system can often mean that only two legs are carrying all the weight, with the other two legs doing little more than acting as balancing legs.

Refer to Sheets 4.5 and 4.6 for details of four-leg slings.

Therefore, use the formula based on TWO legs only, to make sure that you are on the safe side.

It's too late to be checking your calculations when the load hasfallen to the ground.



BASIC RULES ... 4

4.3. SAFE WORKING LOADS - TWO-LEG SLINGS.

90° 10mm Chain Slings

Load

As an example:

1. Each leg of a two-leg sling has a WLL of 100 kg. The included angle between the legs of the sling is not more than 60°.

Safe working load = (100 / 0.58) = 172 kg.

2. Should the included angle between legs be increased to 120°; the S.W.L. = (100 / 1) = 100 kg.

3. The load is 100 kg, and the included angle is 30°. Sling load = 100 kg x 0.52 = 52 kg per leg.

The modern method of rating slings is by their working load limit (WLL), which rates the sling in pure tension (that is, vertical). The safe working load (SWL)includrefers ed angles on sling loads.to the rigging system being used - that is, sling angles, reeving, number of sling legs, and the like.

Using our Riggers' Guide, we find for this system: 1. the SWL = 4500 kg. 2. the WLL of each sling = 3200 kg. 3. the load in each sling leg will be

the actual load x 0.7. 4. the SWL = WLL of ONE sling leg

divided by 0.7.

Note: Older slings will be rated as SWL for straight lift or pull. In this case, use the SWL in your calculations.

1 tonne load

EFFECT OF INCLUDED ANGLEON SLING LOADS

30°

60°

0.5 t 0.5 t

0.52Loadintonnes

0.58

90°

120°

0.7

1.0

171° 6.02.0151°

This chart is in the Riggers Guide

Refer to the chart at right for the effect ofincluded angles on sling loads



BASIC RULES ... 4

4.4. SAFE WORKING LOADS - THREE-LEG SLINGS.

As an example:Sling legs are 3m long. Pitch diameter of the sling attachments is 3m. The effective included angle is 60° (30° from vertical). Using the approximation method, we assume the load is taken on two legs only. Therefore, from our Riggers Chart, we find the sling load at 60° included angle will be 100 kg x 0.58 (60° load factor) = 58 kg.

Assuming the load is equally distributed between the three legs, we find that the sling load =

100 kg x 0.58 (60° load factor) x

Assumed included angle = 90°. = 45° to vertical.

Load = 100 kg.

Three-leg slings need special consideration, because rarely do all three legs take an equal share of the load. It is best to assume that only TWO legs are sharing the load, with the third leg only acting as a "balancer".

Using our Riggers' Guide, we find for this system: 1. the SWL = 4500 kg. 2. the WLL of each sling = 3200 kg. 3. the load in each sling leg will be

the actual load x 0.7. 4. the SWL = WLL of ONE sling leg

divided by 0.7.

10mm chain slings

If loads are critical and equally shared we can find accurate sling loads by calculation - e.g; using the same scenario, 10mm chain slings with a WLL of 3200 kg and effective 90° included angle has a SWL of (3200 / 0.7) = 2286 kg per leg. = 6857 kg for the three legs.

2 legs 3 legs = 39 kg.



BASIC RULES ... 4

4.5. SAFE WORKING LOADS - FOUR-LEG SLINGS.You will have noticed that the Riggers Chart treats 2, 3 and 4 leg slings as having the same SWL for various configurations.

With a 4-leg sling, unless a load-sharing device exists in the system, the load will ALWAYS be taken by TWO slings only. The other two will only serve to balance the load.

Included angle is between two opposite legs.Angle to

vertical

The reason for this situation is that it is virtually impossible for the length of the slings to be exactly equal, and the placement of the slings to load attachment points to be such that the distance from their underside to the crane hook is exactly equal. If the load is flexible, some sharing of the load will take place as the load twists. The degree of sharing is dependent on the stiffness of the load. If the effect of this stiffness is unknown, we MUST assume it to be perfectly stiff, and NO load sharing takes place.

Sling calculations are therefore the same as for two-leg slings (see Sheet 4.3).

Of special concern with precast slung from four points, is the capacity of the pin anchors.

Let's see, now. Ten tons to pick up -4 x 2.5 tonne pins = 10 tonnes. 10mm chains ought to do. Sweet, eh!

OH, REALLY?Remember that one leg at least will be slack, and the load will then be carried by only two slings. Therefore, the included angle must be 90°.

Sling angle = 90°. Sling load must be 7.0 tonnes. Therefore, 4 x 10-tonne pins are required, and 16mm chains. Some difference!

8.5m across diagonals

10 TONNES

6m

10mm chain slings

2.5 pin anchors

CRANEAGE - SLINGING BASICS PAGE 7


Alternative -use a snatch block.

One side slung with fixed slings, the other side using a delta plate to ensure all slings are equally loaded.

Use short two-leg slings and a further set of two-legs, to ensure equal load sharing.

Use a lifting beam with two sets of two leg slings, to ensure all legs are equally loaded.

BASIC RULES ... 4

4.6 LOAD SHARING DEVICESThese are examples of load sharing devices which distribute a load equally among sling legs.

SWL 6.0 T

BEAUFORT WIND SCALE PAGE 8

BeaufortNumber Description Observed Features Velocity

in MPHVelocityin KPH

0

1

Calm Smoke rises vertically Lessthan 1

Lessthan 1.6

LightAir

Smoke drifts downwind. Winddoes not move wind vane. 1 - 3 2 - 5

2 LightBreeze

Wind felt on face. Leaves rustle.Wind vane moved by wind. 4 - 7 6 - 11

3 GentleBreeze

Leaves and twigs in constantmotion. Wind extends light flag. 8 - 12 13 - 19

4 ModerateBreeze

Raises dust and loose paper.Small branches are moved. 13 - 18 21 - 29

5 19 - 24 31 - 39FreshBreeze

Small trees in leaf begin tomsway.Crested wavelets form on inland waters.

6 25 - 31 40 - 50StrongBreeze

Large branches in motion;whistling heard in power wires.

7 32 - 38 52 - 61ModerateGale

Whole trees in motion.Inconvenience felt in walkingagainst wind.

8 39 - 46 63 - 74FreshGale

Twigs break off trees, progressgenerally impeded.

9 47 - 54 76 - 87StrongGale

Slight structural damage occurs(roof tiles and chimney potsremoved).

10 55 - 63 88 - 101WholeGale

Seldom experienced inland;trees uprooted, considerablestructural damage.

11 64 - 75 103 - 121Storm Very rarely experienced; causeswidespread destruction.

12 Over75

Over121Hurricane

BEAUFORT WIND SCALE

Note: Velocity is measured approx. 6 metres (20ft) above ground level.

GEN

ERAL

LY L

IMIT

FO

R S

AFE

CR

ANE

OPE

RAT

ION

. R

EFER

TO

SPE

CIF

IC C

RAN

E M

ANU

AL.



SECTION 5:

REEVING AND CHOKING

5.1 Know the Effect of Reeving on Slings5.2 The Nip Angle



BASIC RULES ... 5

5.1. KNOW THE EFFECT OF REEVING ON SLINGS.

"Reeving" is the practice of wrapping a sling around an object, and either doubling it back on the hook, or passing one eye of the sling through the other and then to the hook. This practice is also known as "choking", or "nipping".

This practice is perfectly safe when done properly, BUT you MUST realise that the act of reeving creates an "included angle" within the sling which reduces the permissible load on that sling.

And NEVER bang the hook down on to the load. You are NOT making the sling more secure - in fact, you are creating larger loads in the sling by increasing the included angle. It also increases the crushing force on the load.

T = Tension in each slinging leg. L = Length of each leg. R = Rise. W = Load to be lifted.

T = 1/2 W x LR

R

L

Load = W.

Note protection on sharp corners.

To work out the sling loads for a reeved sling, use the following formula. This will work for any reeved sling with an included angle LESS that 120°, which must be the absolute maximum angle. An example has been shown.

Any sling reeved around a load needs to have a SWL which is TWICE that of a vertical sling.



BASIC RULES ... 5

5.2. KNOW THE EFFECT OF REEVING ON SLINGS.When a load is lifted by a pair of equal-length slings, the load in each leg of a sling increases as the angle between the legs increases. We then have the seemingly ridiculous situation where the load on a sling can be many times the actual weight of the load - a frequent cause of sling failure.

2,000 kg

1,000 kg

120°90°

60°

1,155 kg 1,414 kg 2,000 kg 11,470 kg

170°

LOAD ON ANGLED SLINGS

1 tonne

Bridled sling

Single leg sling

Angle A

0.5 tonne

0.5 tonne

1 tonne

1 tonne

Basket or cradle sling

Reeved, nip, or choker sling

Reeving a sling around a load with one eye (or bight) through the other, actually halves the safe working load of that sling.Keep in mind also, that the angle of the sling has already reduced the capacity of the sling. If it were possible to reeve the sling and keep the nip well above the load, this load would not be so great, but safety requires the nip to be well down on the load.

A sling which is to be reeved around a load needs to have a safe working load of AT LEAST TWICE that of a sling which would otherwise be used.

1,00

0 kg

1,00

0 kg

2,000 kg 2,000 kg 2,000 kg 2,000 kg

2,00

0 kg

2,00

0 kg

2,00

0 kg

2,00

0 kg

1 tonne 1 tonne



SECTION 6:

KNOW YOUR DUNNAGE

6.1 Know how to use Dunnage properly



BASIC RULES ... 6

6.1. KNOW HOW TO USE DUNNAGE.

1/5 length1/5 length 3/5 length

Note dunnage is kept aligned.

NEVER use dunnage on its edge. This is unstable, and the load could easily shift and drop.

NO!!Example of simple but effective dunnage for storage of pipes or reinforcing bars. The inclined sides prevent the material from rolling about.

ALWAYS use dunnage to allow the slings to be removed after lowering the load into place. Use the "fifth-point" rule as shown. The common materials for dunnage are timber offcuts, which are generally fine. NEVER use hollow light-wall tubes or pipes which could collapse under the load weight, and NEVER use material which is to be used on the job - the load could damage them beyond use.



SECTION 7:

SOME COMMON MISTAKES

7.1 3- and 4-leg Slings7.2 Load sharing traps



BASIC RULES ... 7

7.1. KNOW HOW TO SLING SAFELY.

We repeat - whenever four (or often, three) legged slings are used for a load such as a precastslab, it will often be the case that only TWO of them are taking the load, and the other sling or legs only serve to balance the load. Always assume this will be the case.

! !

A. DO THREE- OR FOUR-LEGGED SLINGS SHARE THE LOAD EQUALLY?

In this typical lift of a site fuel tank, the front rope will often only be acting as a balance. Do NOT assume that the load is being divided three ways - often, only TWO legs are actually taking the load. This has a massive significance on sling selection.

B. SURELY, THREE-LEGGED SLINGS WILL TAKE EQUAL LOADS?

Slinging, as you are well aware now, is more than just attaching slings to a load and waving it away. There are however some wrong assumptions which can be made in common slinging situations, and some of them are put right:

(What hidden hazard exists in this lift?)



BASIC RULES ... 7

7.2. KNOW HOW TO SLING SAFELY.Another wrong assumption is that a four-leg sling set will be twice as strong as a two-leg set. Here's why it is not.

Let's see, now. Ten tonnes to pick up - 4 x 2.5 tonne pins = 10 tonnes. 10mm chains ought to do. Sweet, eh!

OH, REALLY?Remember that one leg at least will be slack, and the load will then be carried by only two slings. Therefore, the included angle must be 90°.

Sling angle = 90°. Sling load must be 7.0 tonnes. Therefore, 4 x 10-tonne pins are required, and 16mm chains. Some difference!

8.5m across diagonals

10 TONNES

6m 90°

Measured between diagonals!

At least one leg will be slack!

Alternative -use a snatch block.One side slung with fixed

slings, the other side using a delta plate to ensure all slings are equally loaded.

Use short two-leg slings and a further set of two-legs, to ensure equal load sharing.

Use a lifting beam with two sets of two leg slings, to ensure all legs are equally loaded.

slinging training modules part 2 aug 2002

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