D1.L.5. ASME NF 3000 Service Level Conditions
Service Level Conditions are basically the loading conditions for which the plant structure and its components are to be designed. The same primary load can be multiplied by different factors to signify the different service levels. Also the load combinations for various service levels are different and pre-defined by the code.
D1.L.5.1 Service Levels
The following is a short overview of each of the service levels specified by the code:
| Condition | Description |
|---|---|
| A. Normal Working | |
| B. Upset | This situation can be termed as a short term failure or a local failure, and the repairing or modification of the structure can be done without shutting the entire plant. |
| C. Emergency | This situation can be termed as a major failure, and the repairing of the structure can be done only after shutting down the entire plant. |
| D. Faulted | This situation can be termed as devastation, and the main objective of this level is to have sufficient time for safe relocation of human life and valuable properties, and to initiate the controlled failure of the plant structure. The plant is already at an unusable state and a rare chance to repair it back into the operation. |
These Service Levels are the attribute of the whole structure or the structural system. So, the existence of different Service Levels to the different parts of the structure at the same point of time is totally ruled out.
The Service Level Factors are basically few multiplying factor by which the Allowable Stress values are to be multiplied based on the Service Level. The different actions (e.g., Tension, Compression, Bending, Shear etc.) have different Service Level Factors.
However, this is to be noted, the stipulated multiplying factors for creating load combinations for Service Level B, C, and D are to be user defined in this case. The facility of creating Auto Load Combination for different Service Levels is out of the scope of this implementation. The user has to take care of this.
D1.L.5.2 Stress Level Factors
For the Member Design, as per Clause NF-3321.1, the Allowable Stresses may be increased by the Factors as per Table NF-3523(b)-1 and NF-3623(b)-1. Table NF-3523(b)-1 is applicable to Component Support Structures and Table NF-3623(b)-1 is applicable to Piping Support Structures. However, as the values are the same for the service level factors in each table, STAAD.Pro does not make any differentiation between component and piping supports.
The values used for the stress level factors in STAAD.Pro are as follows:
| Service Level | Ks | Kv | Kbk |
|---|---|---|---|
| A | 1.0 | 1.0 | 1.0 |
| B | 1.33 | 1.33 | 1.33 |
| C | 1.50 | 1.50 | 1.50 |
| D* | KS | KV | KBK |
* It is evident from the Table NF-3523(b)-1, that there are no predefined Stress Limit Factors for Service Level D. So, for Service Level D, the Factors Ks, Kv and Kbk are to be user defined. Refer to Appendix F in the code for guidance on values to specify in the design parameters.
where
- Ks
- Stress Limit Factor applicable to the Design Allowable Tensile and Bending Stresses.
- Kv
- Stress Limit Factor applicable to the Design Allowable Shear Stresses
- Kbk
- Stress Limit Factor applicable to the Design Allowable Compressive Axial and Bending stresses to determine the Buckling Limit.
The program uses the service level factors —either those specified for levels A through C or the user defined values in level D— as follows:
- The Allowable Axial Tensile Stress is to be multiplied by Ks
- The Allowable Axial Compressive Stress is to be multiplied by Kbk
- The Allowable Bending Stress is to be multiplied by Ks
- The Allowable Shear Stress is to be multiplied by Kv
- As per NF-3322.1.(e), for checking Combined Stresses as per equation 20, the value of F'ey and F'ez — the Euler Stress divided by the factor of safety, may also be multiplied by the appropriate Stress Limit Factor. This is also implemented. F'e is to be multiplied by Kbk.