This article was first published in 2010.
Very few people interested in working with their hands don’t at some stage or another need to carry home a load. The load might be on a trailer, it might be on the back of a ute or it might even involve a truck. But how you tie down the load so it doesn’t fall off? That’s a deceptively difficult question – and correctly answering it is a vital safety issue. Loads that fall off moving vehicles can kill or badly injure people.
The following article is based on material drawn from the current edition of the Load Restraint Guide, published by the National Transport Commission and the Roads and Traffic Authority of NSW. It’s an excellent book – and a recommended purchase if you frequently carry large loads – that can also be freely downloaded chapter by chapter at http://www.ntc.gov.au/viewpage.aspx?documentid=00862
The text, diagrams and photographs drawn from the book are used here for training and educative purposes.
A load that is restrained so it doesn’t shift is required to withstand forces of at least:
When moving, a vehicle and its load are subjected to forces caused by changes of speed, direction or slope. These forces result from braking, accelerating, cornering or travelling over cambered, undulating or uneven road surfaces and air flow.
The load can shift forward when driving forward and braking, or accelerating in reverse.
The load can shift rearwards when braking in reverse, or accelerating forward.
The load can shift sideways when cornering. The amount of force needed to prevent the load shifting will increase as the speed increases and as the corner gets tighter.
The force on the load when travelling over undulating or hilly roads will increase as the slope of the road increases.
The force on the load when travelling over cambered roads increases as the camber of the road increases.
When a vehicle is travelling at high speed or in windy conditions, the force caused by air flow can shift a load, especially lightweight objects with large surface areas.
When a vehicle is travelling over rough surfaces, an unsecured load can shift or fall off the vehicle.
The weight of the load alone cannot provide enough friction to restrain it during normal driving. Additional restraint must be used.
Tie Down Load Restraint Method
Tie-down restraint is the most common form of load restraint and involves the use of lashings.
The load is prevented from moving by friction between the load and the vehicle. The friction force prevents the load moving forward, rearward and sideways. The lashings are tensioned to clamp the load to the vehicle and to prevent the load from moving upwards.
The friction force comes from both the weight of the load and the clamping force of the lashings. When the surfaces are slippery, the friction forces can be very low. Lashings that clamp the load onto the vehicle are called ‘tie-down lashings’.
Friction cannot be taken into account unless the tensioned lashings provide adequate clamping of the load on the deck. Unrestrained loads, even on high friction surfaces, can bounce when travelling over uneven road surfaces and then shift during changes in speed, direction or slope.
If the load does not shift, it is not the strength of the lashing that determines the holding ability of a tie-down lashing. It is determined by the amount of tension in the lashing from initially tightening the knot, or operating the ratchet, winch or dog, in conjunction with the amount of friction present.
Tie-down should not be used on slippery loads because too many lashings are needed.
The lashings must be correctly pre-tensioned. If they loosen below the minimum required pre-tension during a journey, the friction forces are reduced and the load could shift. If a load is not tied down, friction cannot be considered as part of the load restraint system. Unrestrained loads, even on high friction surfaces, can bounce when travelling over uneven road surfaces and then shift during changes in speed, direction or slope. If the load is crushable or could be damaged by the lashing during tensioning, tie-down is not a suitable restraint method.
Tie-down lashings used on offset loads can loosen if the load shifts sideways. Such movement can be sudden and without warning. Offset loads should be blocked by dunnage or directly restrained to prevent sideways movement.
(Dunnage comprises packing, often wooden blocks placed under or to one side of the load. Note that when placed under the load, dunnage can substantially increase the friction that develops between the load and the vehicle.)
Tie-down lashings are most effective if they are vertical and tight. The more a lashing is angled from the vertical, the less is the clamping force. The clamping force is very small when the lashing is near horizontal. The lower the lashing angle, the more lashings are required to give the same clamping force.
For example, a strap tensioned to 500 kg and angled at 15 degrees to the horizontal, will provide a clamping force of only 125 kg (25%) on one side of the load. A vertical strap, would therefore provide four times the clamping force (the full 500 kg tension) on that side of the load. One strap at 90 degrees is therefore equivalent to four straps at 15 degrees!
Many loads are not high enough for tie-down lashings to be used effectively.
Dunnage can be used to increase the lashing angles, by lifting the load.
Alternatively, the load can be separated.
To maintain the friction force during normal driving, the load must always remain in contact with the vehicle including during bumps and vibration from rough road surfaces. To achieve this, the tie-down lashings must be correctly tensioned at all times.
The lashing tension is greater on the side of the load where it is tensioned. The lashings lose tension where they catch or stick on sharp corners or rough surfaces on the load. The tension on the other side of the load, can be more than 50% lower. To prevent the lashing losing tension, smooth rounded corner protectors should be used.
To ensure even load restraint, it is recommended that every second tensioner should be placed on the opposite side of the vehicle. Alternatively, two tensioners can be used on each lashing, one on each side of the load and this can increase the clamping force by approximately 20%.
Rubber load mat is cheaper than most lashings and is the most cost-effective method to reduce the number of lashings needed. This photograph shows tilt test of the friction between an unrestrained pack of steel tube and rubber load mat on timber dunnage. (The pack has a loose belly strap to control any sliding sideways). In this case, the weight of the load and the increased friction from the rubber provide 75% of the required restraint force.
Summary: DOs AND DON’Ts
DO make sure you have enough lashings and that they are in good condition and strong enough to secure your load.
DO make sure that tie-down lashings are as near to vertical as possible.
DO attach lashings at tie rail support points.
DO check and re-tighten the lashings or other restraining devices as required.
DO use lashing protectors on sharp edges.
DO make sure that loose bulk loads cannot fall or be blown off your vehicle.
DO use a vehicle that is built strong enough for the job.
DO take extreme care when releasing a fixed lever dog or an elastic strap.
DON’T tie down loads onto greasy or dirty steel decks.