PREVIOUS | TABLE OF CONTENTS | NEXT

Chapter 5 - Structural Strength

500. Definitions. 

"Limit Load" is defined as the maximum load anticipated to be encountered, and is the load to be used in stress calculations.

"Primary Structure" is any structure, the failure of which will impair safety of the vehicle when operating within it's design envelope.

"Proof Factor of Safety" is the Proof Load divided by the Limit Load.

"Proof Load" is the maximum load which may be applied before consequential permanent deformation impairs safe operation.

"Ultimate Factor of Safety" is the Ultimate Load divided by the Limit Load.

"Ultimate Load" is the load under which primary structure will commence to fail.

501. Loading Cases to be considered. 

In considering the strength and stiffness requirements of primary structure, the designer shall take account of the loads arising from:-

1. Dynamic responses to waves when operating in the Design Specification operating envelope with conditions giving rise to the most severe loadings. (N.B. This will not necessarily be at maximum weight in maximum waves - see Appendix to this Chapter.) See also Sections 550 et seq.
   
   
2. Impact loads resulting from total loss of lift power at any speed up to the maximum for which certification is sought;
   
   
3. Hogging and sagging when floating in trochoidal waves of 10:1 length to height ratio at maximum design weight;
   
   
4. The vehicle standing on, or being supported by, 75% of the designed support area at maximum design weight (amphibious vehicles) or at maximum designed lifting weight (non-amphibious vehicles);
   
   
5. Parking, picketting and mooring in winds of 148 km/hr. (80 knots);
   
   
6. The vehicle being towed;
   
   
7. Snow and ice;
   
   
8. Slinging and jacking.

502. Load Factors. 

For structural loadings considered in Section 501 (a) thru' (d), the structure shall have minimum Proof and Ultimate Factors of Safety respectively of 1.0 and 1.5.

For structural loadings considered in Section 501 (e) thru' (g), the structure shall have minimum Proof and Ultimate Factors of Safety respectively of 1.33 and 2.0.

For slinging and jacking, the structure shall have minimum Proof and Ultimate Factors of Safety of 2.0 and 3.0 respectively.

503. Snow and Ice Loads. 

In considering loadings for structural design purposes, it shall be assumed that the vehicle may experience:-

1. Accumulations of 73 kg/m² snow loading on all exposed horizontal surfaces when stationary;
   
   
2. Ice loads of 60 kg/m² on all exposed horizontal surfaces when operating, and ice accretion of 15 kg/m² on projected lateral surfaces above the water.

504. Towing, jacking and slinging loads. 

In designing structure to accommodate towing, slinging and jacking, the designer shall clearly state any assumptions made, together with any resulting limitations of the vehicle or of the load and it's direction.

505. Deck Loading. 

All decks designed for passenger movement and seating, and for carriage of cargo, shall be designed for local and distributed loads with minimum Proof and Ultimate Factors of Safety of 1.0 and 1.5 respectively. The maximum loadings and loading densities to which decks may be subject in service are to be declared.

506. Collision Accelerations. 

Primary structure design shall take into account all combinations of inertia forces resulting from the following accelerations:-

4g downwards to 3g upwards

zero to 6g forwards

zero to 3g backwards

zero to 3g sideways

up to a resultant of 6g.

507. Primary structure and mountings for main machinery, equipment and seats shall be designed such that any failure arising from the accelerations quoted in Section 506 is constrained so far as is practical from causing injury to occupants or progressive damage to the vehicle.

508. Seats. 

In designing seats and seat support structure and anchor points for seat belts or harnesses, it shall be assumed for the purposes of Section 506 that the combined weight of occupant and seat is 90 kg (200 lb.).

509. Cargo Restraints. 

In designing structural provision for cargo restraint, the limiting values of applied loads and their direction shall be declared. Restraint shall be such that any cargo movement resulting from the accelerations quoted in Section 506 shall not cause injury to occupants in their normal seating; shall not cause damage to primary structure, and shall not adversely affect the safe operation of the vehicle.

510. Flexible Structure. 

All flexible structure and it's attachments and fastenings shall have adequate strength to withstand anticipated loadings resulting from operation within the design operating envelope. In designing flexible structure intended to support and stabilize the vehicle in cushion-borne operation, the attachments shall be designed to withstand without failure any loads associated with lifting the vehicle from a floating condition in the water.

511. Fluctuating Loads. 

All structure shall be designed so as to minimise fatigue or distortion damage arising from vibration, flutter or cyclic loadings.

512. Transmission Loads. 

All components within systems transmitting power from engines to lift fans or propulsive devices shall be designed with Proof and Ultimate Factors of Safety of at least 1.0 and 1.5 respectively when considering the most adverse combination of torque, vibration, inertia and gyroscopic loads.

513. All transmission component mountings shall restrain components from breaking loose when subjected to the loadings quoted in Section 506.

514. Transmission components shall be designed to accept the stresses from torque variations by using the following torque values:-

1. Spark Ignition Engines.
   
   4 x mean maximum torque for 2 cylinder engines) Firing 1
   
   2 x mean maximum torque for 4 cylinder engines) cylinder
   
   1.5 x mean maximum torque for engines with more) at a time than 4 cylinders )
   
   
2. Compression Ignition Engines.
   
   4 x mean maximum torque for 2 cylinder engines)Firing 1
   
   3 x mean maximum torque for 4 cylinder engines)cylinder
   
   2.25 x mean maximum torque for engines with more )at a time than 4 cylinders )
   
   
3. Gas Turbine Engines.
   
   1.33 x maximum operating torque.

515. It shall be the designer's responsibility to ensure that the output characteristics of any engine, in terms of power, speed and torque, and the vibratory characteristics of all components driven by it, and of their mountings, do not result in any resonant vibrations or fluctuations which would lead to premature component failure or structural damage, throughout the engine's operating envelope.

516. Design Calculations. 

The designer shall submit calculations resulting from consideration of the loadings required by this Chapter to demonstrate compliance with safety factor requirements under the most adverse operating conditions within the design operating envelope. Calculations shall take due account of any effects upon material strength resulting from fabrication techniques, using normally recognized practices.

550. Hydrodynamic Loading. 

In submitting design data for approval, the designer shall provide estimates of the weight and c.g. position of all major vehicle structural, machinery and equipment components, and of the vehicle and it's maximum design weight, together with the radius of gyration in pitch both at maximum design weight and at a representative minimum operating weight.

These data shall be used to prepare and submit for approval a structural analysis resulting from wave impacts giving rise to maximum accelerations. An acceptable development of the loadings to be considered is outlined in the Appendix to this Chapter.

551. The hull structural strength in bending shall be analysed for at least 3 wave impact cases:-

1. Symmetrical bow impact;
2. Impact at the c.g.;
3. Symmetrical stern impact.

552. Combined torsional and bending strength of the hull structure shall be analysed by investigating an assymmetrical bow impact with conditions giving rise to the most severe loadings within the design operating envelope.

553. For design analysis purposes, the distributed pressure on the bottom skin plating may be assumed to be 44% of the peak impact pressure determined from paragraph 3 of the Appendix.

Appendix to Chapter 5
Hydrodynamic Loading structural analysis. ^

1. Vertical acceleration of vehicle c.g. in response to wave impact

The vertical acceleration of the c.g. may be obtained from:-

Where N_w is the vertical acceleration of the c.g., in g

k₁ is a hull station weighting factor, reducing linearly from a value of 1.5 at the bow to a value of 1.0 at the station of the longitudinal c.g., thereafter remaining constant.

V is the vehicle velocity relative to the water V_v is the vehicle vertical relative velocity, given by:-

where h is wave height;

l is wave length, and

W is vehicle weight

x is the ratio of the length between the

impact point considered and the c.g. to the

radius of gyration in pitch.

It will be noted that the maximum value of N_w will not necessarily result from consideration of maximum values of W or of V.

2. Rigid Vehicle Response.

The complete response of the vehicle, assumed to be rigid, to a wave impact, shall be determined in order to provide a longitudinal spectrum of acceleration to be applied in performing the required structural analysis. The accelerations shall be determined by:-

Where:-

N_w is the c.g. vertical acceleration previously determined

a is the acceleration at the station considered

L₁ is the distance between the wave impact and the c.g.

L₂ is the distance between the station considered and the c.g.

K is the vehicle radius of gyration in pitch

Since the vehicle is rigid, the relationship of acceleration to longitudinal station is linear and only 2 values of L₂ need be considered; the full spectrum may be clearly presented graphically.

3. Local Impact Pressures.

In designing hull structure underside plating, the designer shall take account of local pressures due to wave impacts, derived from:-

P = 0.0162 K₂ V.V_v where

K₂ is a length-related weighting factor, varying linearly from a value of 2.0 at the bow to 1.0 at 22% of the hull length, thence continuing at 1.0.

V, V_v are as in paragraph 1 above.

PREVIOUS | TABLE OF CONTENTS | NEXT