SPREAD THE LOAD

Submitted by:Orly A. Alcarioto Polaris 2-A

Submitted to:2/M Moises Teňosa

OBJECTIVES:

Specific:

To define what is point load and distributive load.

General:

To know how the load was spread and the important of distribution of load to the loading system on-board the ship.

What is a point load?

-In the field of engineering, a point load is a load applied to a single, specific point on a structural member. It is also known as a concentrated load, and an example of it would be a hammer hitting a single nail into a beam.

What is an uniformly distributed load?

-A uniformly distributed load has a constant value, for example, 1kN/m; hence the "uniform" distribution of the load. Each uniformly distributed load can be changed to a simple point force that can be used to determine the stresses in an object.

The alternative to a point load is a distributed load. As the term implies, the force in a distributed load is applied across a given area rather than at a single point. An example would be water in a river that comes up against a dam; the pressure or force of the water is distributed fairly evenly over the entire width of the dam.

The differences between point loads and distributed loads become important in the analysis of a structure. In simple construction, the joist is a beam that distributes weight across the length of the beam.

. An unsupported beam-to-beam connection is a point load and a potential weakness in the structure. If too much weight or force is concentrated on a specific point, it can cause a beam to snap, and there can be other catastrophic damage to the overall structure.

In construction, a point load is reserved only for lighter structures or other cases in which the load or force is relatively light and not subject to undue stresses. To maintain structural integrity, designs and blueprints are engineered to avoid the use of point loads in other situations.

Point-loading and uneven distribution of cargo weight can, and frequently does, cause unnecessary damage to decks and hatch covers. Unless the weather-deck has been specially strengthened, it is unlikely to have a maximum permissible weight-loading of more than 3tonnes/m².

Similarly, unless hatch covers have been specially strengthened, it is unlikely they will have a maximum permissible weight-loading of more than 1.8tonnes/m². The ship’s capacity plan/or general arrangement plan should always be consulted. If the information is not there, try the ship’s stability booklet.

In the event that specific values are not available onboard the ship, allow no more than 2.5tonnes/m² for weather-deck areas; and no more than 0.75tonnes/m² for hatch covers in small vessels; 1.30tonnes/m² in vessels over 100m in length. (The word tonnefused later in this article means tonnes force.)

•The adverse effects of point-loading are not always fully appreciated. On the other hand, a 6 tonne machine with a flat-bed area of 3m² will exert a down-load of 2tonnes/m².

When exceptionally heavy weights are to be carried, it may be necessary to shore-up the weather-deck from below ; but, again, care must be taken to spread the load on the tween decks so as not to overload that plating.

In the not so dense range of cargoes, units of 20 to 40 tonnes weight are common today, and stacking of unit weights is widespread. If a piece of machinery weighing , say, 30 tonnes with a base area of 6m² is placed direct on the weather-deck the point-loading will be 30/6=5tonnes/m².

If, however, the deck plating has a maximum permissible loading of 2.5 tonnes/m² then the minimum area over which that 30 tonnes load must be spread is 30/2.5=12m².

Good dunnage must be used to spread the load, and it is always good practice to add 5% to the weight to be loaded before working out the dunnage area. For the 30 tonne weight, for instance, 31.5 tonnes would be used and the dunnage area would go from 12m² to 12.6 m².

Dunnage timber is often no more than 6”×1” (150×25mm) rough planking; but where weighty cargo items are involved dunnageshould not be less than 50mm(2”) thickness ×150mm(6”) width and preferably 75mm(3”) ×225mm(9”). It is acceptable, however, to used two dunnage planks nailed together securely to make up the thickness.

A dunnage width greater than 150mm is always acceptable-225mm(9”) to 305mm(12”), for instance; but where the thickness goes to 75mm(3”) care must be taken to choose straight-grained timbers of as great a width as possible, and to ensure that they are laid with the grain horizontal and parallel with the deck.

There have been incidents in the past where what appeared to have been a soundly dunnaged and well-secured item of deck cargo broke adrift and was lost overboard due to a sequence of events commencing with the collapse of 3”×3” dunnage timbers along the curved grain used on its edge, followed by consequential slackness in otherwise adequate lashing arrangements, followed by increasingly accelerated cargo movement and finally breakage of the lashings.

Lashing used in containers to distribute cargo weight

Because of the random nature of grain configurations in the thicker dunnage timbers it is acceptable to achieve thicknesses by nailing planks together. A 2” thick dunnagetimber can be made up using 1” thick planks, and a 3” thick dunnage timber can be made up using 2” and 1” thick timber planks, all securely nailed together.

To a large degree, this will correct the tendency for separation in timber with a bably-alligned grain. And remember, it will be as important to install good lower-level foot lashing as it will be to install downward-leading lashings if load-spreading dunnage is to remain fully effective.

A lashing is an arrangement of rope wire or webbing with linking device used to secure and fasten two or more items together in a somewhat rigid manner. Lashings are most commonly applied to timber poles, and are commonly associated with the cargo, containerisation, the Scouting movement, and with sailors.

Lashing with board nailed together to support corner castings.

SUMMARY:Proper loading and distribution of cargo weight can maximize the strength of the ship. Point loading is necessary to avoid damages to deck and hatchcovers and it also prevent accidents. When exceptionally heavy weights are to be load, it may be effective to spread the weight of the load on the decks as to not to overload that plating. In spreading the load, we can used lashing to secured the load for the equal distribution, effective and safe loading of cargoes on-board the vessel.

QUESTIONS

1. What is point loading?

2. What is its importance to the loading of cargoes?

3. Define uneven distribution of cargo weight?

4. How does lashing secure the load?

ASSIGNMENT

1. Give other examples on how to avoid damage on decks due to uneven distribution of cargo weight.

2. Site some incidents that was caused by uneven loading of cargoes.