2 Terms and Symbols
2.1 Terms
2.1.1 Permanent loads
Load of which the value does not vary with time during the structure service period，or of which the variation may be neglected compared with the average value，or of which the variation is monotonous and trends to limit.This load exists in each case in the buildings and special structures in petrochemical industry.
2.1.2 Variable loads
Load of which the value varies with time during the structure service period，and of which the variation cannot be neglected compared with the average value.
2.1.3 Accidental loads
Load which does not always occur within the design working life of the structure，but its quantity value is very large once occurred and its duration is very short.
2.1.4 Operating load/working loads
Load which is a variable load，such as the weight of contents contained in equipment and piping，equipment temperature action and vibration load generated from equipment and piping for normal operation case during the structure service period.
2.1.5 Maintenance loads
Load which is a variable load，such as equipment bundle pull load and maintenance crane load for the maintenance case during the structure service period.
2.1.6 Testing loads
Load which is a variable load，such as the weight of water for hydrotest，the weight of liquid for hydraulic test and pneumatic action from pneumatic test for the equipment and piping testing case during the structure service period.
2.1.7 Representative value of a load
Value of loads to be adopted in the verification of limit states in the design，such as characteristic value，combination value，frequent value and quasi-permanent value.
2.1.8 Design reference period
Time parameter which is selected to determine the representative value of variable load.
2.1.9 Characteristic value/nominal value
Basic representative value of load and characteristic value of maximum load statistical distribution within the design reference period（such as mean，mode median or some tantile）.
2.1.10 Combination value
For variable load，load value which can make the exceedance probability of the combined load effect within the design reference period consistent with the corresponding probability when this load occurs separately；or load value which can make the combined structure have a unified reliability index.
2.1.11 Frequent value
For variable load，load value that the total exceedance time is the specified smaller ratio or the exceedance frequency is the specified frequency within the design reference period.
2.1.12 Quasi-permanent value
For variable load，load value that the total exceedance time is about half of the design reference period within the design reference period.
2.1.13 Design value of a load
Product of representative value and partial factor of a load.
2.1.14 Load effect
Reaction of structure or structural member caused by loads，such as internal force，deformation and cracking.
2.1.15 Load combination
Provisions for various design values of loads emerging simultaneously in order to ensure structure reliability in the limit state design.
2.1.16 Fundamental combination
Combination of permanent load and variable load for verification of limit state of bearing capacity.
2.1.17 Accidental combination
Combination of permanent load，variable load and one accidental load for verification of limit state of bearing capacity.
2.1.18 Characteristic/nominal combination
Combination that adopts characteristic value or combination value as the representative value of a load for verification of limit state of normal use.
2.1.19 Frequent combination
For variable load，combination that adopts frequent value or quasi-permanent value as the representative value of a load for verification of limit state of normal use.
2.1.20 Quasi-permanent combination
For variable load，combination that adopts quasi-permanent value as the representative value of a load for verification of limit state of normal use.
2.1.21 Dynamic load
An external action that varies over time，under which inertia force generated by the structure may not be neglected compared with static action.
2.1.22 Dynamic coefficient
Equivalent coefficient adopted for structure or member subject to dynamic load in static design，of which the value is the ratio of maximum dynamic effect to corresponding static effect of structure or member.