New seismic restraints should be designed to be “stiff”, which in practice means that they should not deform more than 1/8” under seismic load. In these cases, the supports can be modeled as rigid in the direction of action. For restraints that are not as rigid, the exact seismic analysis solution would require the support stiffness to be included in the analysis model. This could however cause unnecessary iterations when the installed support (and therefore its stiffness) does not exactly match the design. To avoid the complications and costs of an iterative reconciliation process, restraint stiffness should be modeled with approximate, rounded values. For example, supports may be grouped into three categories: Very stiff (K > 1E6 lb/in), stiff (K = 1E5 to 1E6 lb/in), and soft (K < 1E5 lb/in). Restraints within each category would then be assigned a nominal stiffness, with the very stiff supports modeled as rigid.
At the same time, all supports should be designed to a minimum seismic load, for example 100 times the pipe size. For example, a seismic restraint on a 6” line would be sized for the calculated seismic load, but no less than 600 lb. This would avoid future iterations on lightly loaded supports if the support stiffness or location changed, causing a change in seismic load.
Restraint Gap
It is common practice to provide a small gap between pipe and structural support steel to avoid binding during normal operation. Such a gap represents a rattle point during a seismic event. As a result there will be a local impact between the pipe and the support during the earthquake. The exact solution to this impact problem depends on several factors: the gap size, the pipe and support local and global stiffness, the pipe velocity at impact, the pipe and support mass, the elasticity of pipe and support [Kumar]. The study of earthquake damage indicates that this type of local impact through support gaps is mostly of little consequence, but needs to be considered in the following cases:
(a) The pipe span contains impact sensitive components (instruments, valve actuator controllers, etc.).
(b) A gas pipeline operating at high pressure (hoop stress close to 72% of yield), where a surface dent or gouge could cause the pipe to fail.
(c) For large gaps, in the order of the pipe radius for 2” NPS and smaller pipe, and 2” gap for larger pipe, the restraint load calculated on the basis of zero gap may be amplified by an impact factor of 2 to account for impact.
Seismic Design and Retrofit of Piping Systems
July 2002
AmericanLifelinesAlliance
Working With Nonlinear Restraints The engineer’s interpretation of piping system boundary conditions (e.g., supports) and how to input them in CAESAR II is the greatest source of model variation. In this set of videos you will explore how CAESAR II accommodates nonlinear boundary conditions in piping systems. A pipe lifting off a resting support or a pipe that closes a gap on a guide present modeling and analysis complications in piping system evaluation. These nonlinear conditions and their CAESAR II model counterparts are introduced and their impact on pipng code stress evaluation are reviewed. This presentation will also provide guidance in resolving situations where the program is unable to resolve the interaction of nonlinear conditions in the program’s F=KX solution.