When the ship's energy is resisted through foam-filled or pneumatic fenders, the

resulting force is concentrated in a small area of the ship's hull. In such cases,

the allowable pressure on the ship's hull becomes a critical design issue. Most

fast combatants have a thin hull plating with a low allowable hull pressure. For

more specific information on the ships being berthed, consult NAVSEA. See TR-

6015-OCN, and note that values in Table 7 are based on yielding of the hull

plating and include a 1.5 safety factor. Consequently, when checking for an

accidental condition, the allowable value may be increased by up to 50 percent.

5-4.4.3

Because ship berthing is a short-term impact type of loading, the following

increases over previously published values are permitted. The fender system

may be designed as a Class B structure.

a. Timber. For operating condition, the allowable stress in flexure

(tension and compression) may be taken as 0.67 X modulus of rupture or

the published allowable values increased by a factor of 2.0, whichever is

less. For the accidental condition, the stress-strain curve may be

assumed to be linear up to 0.9 X modulus of rupture, which should be

taken as the limit.

b. Steel. For operating condition, the allowable stress in flexure (tension

and compression) may be taken as 0.8 X yield stress. For the accidental

condition, full yield stress may be used. However, the sections used

should satisfy compactness requirements or the allowable stress reduced

proportionately. Members should be sufficiently braced for development

of the yield strength.

c. Concrete. Design reinforced and prestressed concrete members not

intended for energy absorption with a load factor of 1.7 over forces

developed due to operating condition; they will be satisfactory for the

accidental condition. Do not allow further prestressed members to

develop tensile stresses in excess of 12 f'c (f'c = 28-day compressive

strength) in the precompressed zone. Prestressed concrete members

specifically designed for energy absorption will have a maximum allowable

working energy (Ewc) equal to 0.67 x the applied load (P) x the deflection

at the point of the applied load. Three criteria must be satisfied to

determine the maximum allowable working energy (Ewc) in the pile section

for a given length:

Maximum concrete compressive strain must be equal to or less

than 0.0021 in/in (00021 mm/mm.)

Maximum stress in the prestressing steel at working energy must

be equal to or less than 210,000 psi (1 447 898.9 kPa.)

130

Integrated Publishing, Inc. |