### Nomenclature

*a*: General coefficient

_{mn}*b*: Breadth of a rectangular grillage plate

*E*: Elastic modulus

*I*: Moment of inertia of longitudinal stiffeners

_{r}*I*: Moment of inertia of transverse stiffeners

_{s}*l*: Length of a rectangular grillage plate

*m, n*: Wave numbers

*P*: Load per unit area on a rectangular grillage plate

*r*: Number of longitudianl stiffeners

*s*: Number of transverse stiffeners

*w*: Deflection of a rectangular grillage plate

*ν*: Poisson’s ratio

*σ*: Allowable stress

_{allow}### 1. Introduction

### 2. Material Property Variation of Hull Steel due to the Neutron-Irradiation Phenomenon

^{3}(Korean Register, 2020; RMRS, 2018), and MPC_1 to MPC_6 show the changes in material properties at void volume fractions ranging from 1% to 3%. The reduction rates of the elastic modulus were 2%, 4%, and 6%, and those of Poisson’s ratio were 0.4%, 0.7%, and 1%, respectively. The changes in the allowable stress of neutron-irradiated DH36 steel were only considered for MPC_4 to MPC_6; these were increased by 25%, 50%, and 100%, respectively.

### 3. Stiffened-Plate Structures and Their Analytical and Numerical Solutions

### 3.1 Analytical Solutions for Laterally Loaded Flat Grillage Plates

*s*evenly spaced stiffeners in the length direction

*l*, and

*r*evenly spaced stiffeners in the width direction

*b*. The moment of inertia for the longitudinal stiffeners is

*I*, and that for the transverse stiffeners is

_{r}*I*. If the deflection in the

_{s}*x, y*plane of the stiffened-plate structure caused by the bending load is denoted by

*w*, the deflection curve of the stiffened-plate structure can be obtained using the following equation.

*y*corresponds to a specific stiffener, and the corresponding equation for the

*p*-th stiffener is expressed as Eq. (3).

*P*, the work done by this load is expressed as Eq. (8).

*a*can be found by equating the general terms of Eq. (7) and Eq. (8) as in Eq. (9).

_{mn}*P*, the coefficient

*a*obtained from Eq. (9) can be used in any distributed load acting on a simply supported symmetric stiffened-plate structure. As most load distributions of interest are uniformly distributed loads, if a stiffened plate with a symmetrical distribution load is considered, the coefficient

_{mn}*a*can be obtained using Eq. (10).

_{mn}*p*-th longitudinal stiffener and the

*q*-th transverse stiffener can be obtained as follows.

*p*-th longitudinal stiffener,

*q*-th transverse stiffener,

*p*-th longitudinal stiffener and the

*q*-th transverse stiffener can be determined individually (Clarkson, 1965).

### 3.2 Finite-Element Solutions for Laterally Loaded Stiffened Plates

*x-, y-,*and

*z*-axis directions (

*UX, UY, UZ*) along the sides of the stiffened-plate structure.