I have a question on how to model U-bolts in Autopipe. If the U-bolts holding a pipe are not clamped down tight to the pipe I have been modelling them as a guide. If the U-bolts are clamped tight can anyone tell me the proper way to model these supports? What friction factor should I use? Thank you!
A friction factor of 0.3 to 0.4 is commonly used for guides unless low friction pads are added in which case 0.1 can be used. If the U-bolt is tight, then a non-metallic padding is usually added, otherwise the pipe will dent due to hoop radial expansion or the support will break due to axial growth.
Most of the friction will come from horizontal surface where the weight is acting and so the above mentioned factors should be used. If the tightness is excessive, it can be expressed as an equivalent normal radial force (contact pressure*contact area) and that can be used to evaluate the friction factor by dividing the force by the gravity reaction.
Request for Clarification:
Loose U-Bolts should be modeled as frictionless guides
Tight U-bolts should be modeled as guides with friction.
1) If no low-friction pad is present, a friction factor of 0.3 or 0.4 should be applied.
2) If a friction pad is present, use a friction factor of 0.1
Is there a reference for this methodology and is this the "only" realistic method that should be applied to modeling u-bolts?
An approach has been used in the past to model loose vs. tight u-bolts was to install a small gap (~1/16" or so) for three of four directions (zero gap in the gravity direction since the support is applied to normally carry the deadweight). Is this approach an alternate acceptable method for analysis?
Can you please send an example on how to model tight u bolt in autopipe? For example u bolt for 4" pipe. What are the common value?
A u-bolt is typically modeled as a guide with gaps settings as needed. A tight u bolt may have a gap left & right but gap up / down have no gaps. So in this case you can set the guide gaps up / down = 0.00 and left / right set to a small value (ex. 0.25"). Otherwise, you can assume that all guide gaps are set to 0.00 for all directions.
Mike DattilioBentley Systems Design AnalystDesign Engineering Analysis group===================================================
If the u-bolt is applied to a vertical pipe, and the u-bolt has a bolt tension of say 10kN, then how are you able to model the frictional restraint applied by the u-bolt to the vertical pipe?
You can apply a concentrated force of 10KN at the support to simulate the u-bolt tension pre-load. Apply the load under GR case as it will be there all the time. Based on the friction factor, the friction force can be calculated as mu*10KN. Make sure the support stiffness is rigid, otherwise there can be excessive deflections and stresses due to this force at the support.
This approach is limited to rigid supports.