The Connection Between Stress Analysis and Shimming
Pipe stress analysis is the engineering process that determines how piping systems behave under operating conditions including pressure, temperature, weight, and external forces. The output of a stress analysis directly defines the support requirements for each pipe support point, including the loads, movements, and gaps that field crews must address with shims.
Understanding how stress analysis output translates into shimming requirements helps field crews install shims correctly and ensures the piping system performs as designed.
What Stress Analysis Tells Us About Shimming
Support Loads
The stress analysis calculates the vertical, horizontal, and axial loads at each pipe support for multiple operating cases. These loads determine the required shim size and bearing area. A support carrying heavy loads needs shims that cover sufficient area to distribute the force without exceeding the allowable bearing stress on the support steel.
Thermal Displacement
The analysis calculates how much the pipe moves at each support due to thermal expansion. This displacement data tells field crews whether the pipe will slide on the support, how far it will move, and whether the shim must accommodate this movement.
Cold-to-Hot Gap Changes
At some supports, the vertical position of the pipe changes between the cold installation condition and the hot operating condition. The stress analysis provides the cold set position for spring hangers and identifies supports where the pipe lifts off or where the load increases significantly during operation.
Key Stress Analysis Output for Shimming
- Support reactions: Vertical and horizontal forces at each support point (determines shim bearing requirements)
- Thermal displacements: X, Y, Z movement at each support (determines slide plate requirements and shim positioning)
- Spring hanger data: Cold load, hot load, and travel for each spring hanger (determines shim height at spring attachment points)
- Support type: Rest, guide, anchor, or spring (determines whether shims can slide or must be fixed)
- Operating cases: Design, operating, hydrotest, upset conditions (each may have different support requirements)
From Design Office to Field
The gap between the engineering design and field reality is where shimming becomes critical. The stress analysis assumes that supports are at exact theoretical positions. In the field, structural steel tolerances, concrete foundation levels, and fabrication variations create gaps that must be filled with shims.
The Typical Workflow
- Stress engineer completes the analysis and issues support data sheets with loads, movements, and required positions
- Piping designer creates pipe support detail drawings showing the shim location and dimensions
- Construction surveyor measures the actual steel elevation and compares it to the design
- Field crew calculates the required shim thickness and installs shims to bring the pipe to the correct elevation
- QC inspector verifies the final alignment and documents the installed shim dimensions
Common Stress Analysis Software
The most widely used pipe stress analysis programs in the oil and gas industry include CAESAR II, AutoPIPE, and Rohr2. These programs output support data in various formats that must be interpreted by field engineers to determine shimming requirements.
Regardless of which software generated the analysis, the field shimming data ultimately needs to be captured and documented at the point of installation. ShimSheet serves as the final link in this chain, recording the actual installed shim dimensions that prove the piping system was built to the design intent.
When Stress Re-Analysis Is Required
If the actual field shim height significantly exceeds what was assumed in the stress analysis, a re-analysis may be required. Large shim packs can change the effective support stiffness and may affect the distribution of loads across adjacent supports. Most project specifications define a threshold (typically 50mm or 100mm of shim height) above which engineering review is required.
Bridge the Gap Between Design and Field
Use ShimSheet to capture actual shim dimensions at every pipe support, creating the as-built record that validates your stress analysis assumptions.
Try ShimSheet FreeFrequently Asked Questions
Does pipe stress analysis specify exact shim thicknesses?
No. The stress analysis provides support loads, positions, and movements. The actual shim thickness is determined in the field by measuring the gap between the as-built support steel and the required pipe elevation.
What happens if shims are too thick?
Excessively thick shim packs can change the support stiffness and load distribution. Most specifications require engineering review when shim height exceeds 50mm to 100mm at any single support.
How do I get the design elevation for shimming?
The design elevation comes from the pipe support detail drawing, which is based on the stress analysis output. The piping designer converts the stress analysis support data into construction-ready drawings that include the required pipe elevation at each support.