Informative Material 6 - Intermodal load distribution

 The text for this informative material has been drawn from three academic papers and is reproduced with the permission of the author.^{footnote footnote footnote}

1 Introduction

1.1 Construction of load distribution diagrams requires fulfilling not only the technical characteristics of maritime containers, wagons and vehicles but also various requirements defined by legislative measures, guidelines and standards. The informative material focuses on the 40-foot general purpose container as an example of the load distribution diagram generation.

1.2 Cargo centre of gravity is important to know when packing containers. The standard ISO 830, section 8.1.3, defines eccentricity of centre of gravity as follows: "longitudinal and/or lateral horizontal differences between the centre of gravity of any container (empty or loaded, with or without fittings and appliances) and the geometric centre of the diagonals of the centres of the four bottom corner fittings".

1.3 The container payload – P is defined according to section 5.3.3 of the standard ISO 830 as "maximum permitted mass of payload, including such cargo securement arrangements and/or dunnage as are not associated with the container in its normal operating condition". It can be calculated by subtracting the tare mass from the maximum permissible gross mass of the container.

P = R - T

P = Payload

R = Maximum permitted gross mass

T = Tare mass

 2 Load distribution diagrams

2.1 Load distribution diagram for 40–foot container

2.1.1 The container payload, tare and gross mass as well as maximum allowed eccentricity of container centre of gravity are necessary to construct container load distribution diagram. The diagram limits the position of cargo centre of gravity (CoG) of certain mass to not exceed container gross mass, payload and to meet load distribution requirements. A 40–foot container with a gross mass of 30,480 kg, tare of 4,000 kg and payload of 26,480 kg is used as an example. The container load distribution diagram (LDD(C)) specifies boundaries of cargo CoG when eccentricity of container CoG is 5% and 10%.

2.1.2 The result in figure 6.1 shows that these boundaries are smaller (within the maximum container payload) and the centre of gravity of any cargo should be located inside border lines around container centre.

2.2 Load distribution diagram of two–axle container wagon

2.2.1 This two-axle wagon is a suitable example because it is possible to load 40-foot container only and wagon payload is lower than container gross mass.

2.2.2 Load distribution diagram for a two axle container wagon is influenced by following parameters:

• wagon tare (mW);

• wagon gross mass for different route category (A, B, C, D), train speed (S, SS) and selected rail operators (A – 32t, B – 36t, C – 40t);

• wagon payload for different route category (A, B, C, D), train speed (S, SS) and selected rail operators (mC+L);

• maximum authorized axle mass per route category (A – 16t, B – 18t, C – 20t, D – 22.5t) R1max, R2max curves as figure below;

• maximum uneven axle load 2:1 according to UIC Loading guidelines;

• axle tare mass (R1W, R2W);

• wagon wheel base (b);

• distance from the end of the loading platform to neighbouring axle (a);

• length of the loading platform (L); and

• position of wagon container locks.

2.2.3 Load distribution diagram for the sample rail wagon are defined as:

• maximum axle mass (R1max, R2max) per route category (A, B and C curves for R1max and R2 max);

• maximum payload for route category (mC+L); and

• R1 : R2 < 2 : 1 and R2 : R1 < 2 : 1 curves.

2.2.4 Load distribution diagrams of a typical container wagon show area where cargo centre of gravity for different cargo mass should be located. This area is bounded by maximum axle mass per different route category and by maximum uneven axle load 2:1. The axle curves meet in one point which presents disadvantage because if it is intended to load the cargo with the highest possible mass its centre of gravity should be right in the middle of the wagon. For example if CoG of load of 29.2 tonnes is 6.7 m (6.39 m is central axis) from wagon floor end this creates axle mass R₁ = 18.9 tonnes and R₂ = 21.1 tonnes which is higher than 20 tonnes permitted per route category C.

2.3 Load distribution diagram of semi-trailer container trailer

2.3.1 Technical characteristics of typical gooseneck 45-foot extendable trailer are used in this section. Load distribution diagram of semi-trailer is influenced by following parameters:

• container trailer tare (mT);

• maximum kingpin load technical suitable for three–axle tractor (R1 max(3))) and kingpin load influenced by two–axle tractor (R1 max(2)) – R1max curves in figure below;

• maximum gross combination mass (mGCM) or semi–trailer gross mass (mGTM);

• kingpin and triple axle tare (R1T, R2T);

• maximum triple axle load (R2max) – R2max curve;

• length of loading platform (L);

• position of container twist-locks for 40-foot container;

• distance from trailer platform front end to king pin axis (a);

• distance kingpin to triple axle axis (b); and

• minimum kingpin and triple axle load (25% / 25% of maximum semi-trailer mass is chosen) – R1min, R2min.

2.4 Three alternatives of load distribution diagrams of the example container trailer are shown in the figure above. Permissible load on kingpin determines if it is necessary to use two-axle or three-axle tractor when it is possible to use permissible technical load on kingpin of 15 tonnes. Because of maximum allowed mass of 18 tonnes for two axle tractor limits permissible load on kingpin to 9.8 tonnes (8.2 tonnes tractor tare supposed) when two axle tractor is used. Maximum gross combination masses and maximum semi-trailer gross mass are shown as mGCM and mGTM axis respectively and limits the payload of container trailer.

 3 Intermodal load distribution diagrams

3.1 Intermodal load distribution diagram of a 40-foot container carried on two-axle container wagon

3.1.1 An intermodal load distribution diagram of a 40-foot container loaded on a wagon can be constructed from container and wagon LDDs. Consideration should be given to the container tare because this also represents the cargo for the wagon, therefore the LLD for the wagon is constructed using the container tare as LDD (W + C). The diagram below shows the container GM on right vertical axis and cargo mass on left vertical axis so it is possible to check loading of container as well as wagon with the container simultaneously.

3.1.2 When container LDD (C) and wagon LDD (W + C) are combined then nine areas of position of cargo centre of gravity for different mass are bounded by LDD curves for this type of wagon constructed for route categories A, B, C. LDD (W + C) limits maximum cargo mass and LDD (C) position of CoG around container centre line. Diagram LDD (W + C) for route category C also shows that it is not possible to utilize full container payload.

3.2 Intermodal load distribution diagram of 40-foot container carried on container chassis

3.2.1 An intermodal load distribution diagram of a 40-foot container loaded on a chassis can be constructed from container and chassis LDDs. Consideration should also be given to the container tare mass as LDD (T + C) because this also includes the payload of container chassis. Again container gross mass is on the right vertical axis and cargo mass on the left vertical axis in the diagram below. Therefore, it is possible to check loading of container as well as chassis with the container.

3.2.2 When container LDD (C) and chassis LDD (T+C) are combined then correct load distribution to maximum payload is possible only when three-axle tractor is used. When maximum kingpin load is limited by two-axle tractor than the centre of gravity should be eccentrically towards container doors and almost on the limits of container load distribution. When lighter two-axle tractor is used the loading situation looks more favourably for gross combination weight 40 tonnes but for GCM 44 tonnes there is not a big difference. In case that the cargo centre of gravity is in first container half (close to front wall where loading with container doors towards back is supposed) then the tractor is overloaded (see figure 6.7).

3.3 Intermodal load distribution diagram of a 40-foot container carried on two-axle container wagon and container chassis

3.3.1 Intermodal road-rail-sea load distribution diagram is constructed when LDD (C) of container, container wagon LDD (W+C) and container chassis LDD (T+C) are combined. Here the limitations for loading on wagon and container chassis are again seen.

3.3.2 Final diagram specifies cargo mass up to 15 tonnes, where LDD (W+C)s and LDD (T+C)s are not exceeded. When cargo mass is 16 tonnes route category A of railway wagon is exceeded when cargo CoG eccentricity is 5% and on the boundary of allowable mass on kingpin for two-axle tractor and GCM 40 tonnes. With increasing mass the position of CoG must move towards container doors up to 22 tonnes, for GCM 40 tonnes and maximum cargo mass for route category C at 5% cargo CoG eccentricity. If GCM 44 tonnes is allowed then full container payload of 26 tonnes is utilized but such container is not possible to carry onto railway wagon because cargo mass for route category C is exceeded.

3.3.3 Following example shows how to use previous intermodal load distribution diagram. Such a container will be loaded by 44 pallets with a pallet mass of 480 kg, cargo mass of 21.12 tonnes and container gross mass of 25.12 tonnes. Pallets are loaded in two layers, upper layer incomplete. Bottom full layer consists of 30 pallets and upper layer from 6 pallets loaded at container front end and 8 pallets loaded at the doors. Eccentricity of cargo CoG is 1.024% and container CoG is 1.020% towards doors so the pallets are correctly loaded with regard to container LDD(C)'s and also correctly loaded to GCM 40 tonnes and rail route category C.

3.3.4 The figure above shows all LDD's and we can clearly decide that maximum cargo mass in this case is 22 tonnes limited by container chassis and gross combination mass of 40 tonnes. Maximum eccentricity of the cargo centre of gravity will be maximum 3.6% which is limited by maximum axle load of railway wagon for route category C.