The Mote Method

Piperack Design and Commonsense

January 20, 2010

  I have been designing and checking piperack designs for over 25 years and I feel I am often re-inventing the wheel on every project. Most of  the perplexing questions I get in piperack design are related to friction and temperature loading, where there should be none. I will reserve wind loading on piperacks for another update. Application of friction, temperature and wind loads keep slipping beyond the bounds of commonsense. This preamble is intended as a friendly word to engineers looking to understand the critical conditions for piperack design. Most piperack designs rarely includes friction and temperature in the most critical combination.

Friction arises between the surfaces of the pipe shoe trying to move and the support underside. For the sake of clarity I am going to assumes a steel beam and column piperack. The resistance against sliding is a function of the beam lateral stiffness. If the stiffness is so high the forces are transferred to the beam supports until such time pipe slide or the beam rotates, releasing the friction force. Otherwise the top flange will move with the pipe support until a resistance is achieved. 

If there are a number of pipes then strictly the largest friction force acts as an umbrella for other friction forces from other pipes, at the same level, and if they were exerting friction in the same direction. Friction is not cumulative. And I do not take friction to the foundation if the piperack is elevated. Friction  designs the beam for lateral stiffness not the foundation. 

Some corporate design policy will say design for all friction to be concurrent on all levels in the same direction. Wow. Out of politics, I will let 25% of friction loads go to the foundations but I will let the unity code check go close to the limit. Many foundation designs limit the allowable unity to 75% for unforeseen loads but I do not include friction in this. 

Consider the pipe stress engineer, he will look at his piping model for each pipe in the piperack and analyze it purely without respect for the real world of group action. Do not ask this person about friction! You will hear mostly a worse case scenario so far removed from reality. Can you imagine layers and a variety of diameters of pipes with different temperature ranges, insulations, wall thicknesses and quantities of fluids or gases all existing to create friction concurrently at every location and require the foundations to constrain it? 

In a second pass in order to fine tune the pipe stress response to the actual pipe support conditions, consider this; not all supports are equal in vertical and horizontal response. A transverse beam in one location may be a stiffened 600 mm deep section to columns on supports, and another location is only a 300 mm deep section spanning into longitudinal beams. I will focus my final design on the deeper and stiffer section and ensure it can takes everything the weaker beam is supposed to carry. It is a redistribution exercise. Check with your pipe stress engineer if there is an allowance for vertical and horizontal spring support stiffness. this will profoundly affect behavior. 

In a third pass, identify all the anchor supports and look at expansion. Sometimes thermal expansion of pipes occur within the same location to render it as an internal force. The net result of friction is often zero. 

Tying transverse beams together for the anchor forces will often remove the need for friction loads as the top flanges of the transverse are tied against rotation. Also be careful with the pipe stress engineers in their convention, they also differ individually! Thermal loads give rise to anchors but sometimes is friction load. Not all anchor loads are the same as friction loads. The anchor loads for a variety of pipes are usually  found in the same place location and this is taken concurrently, so the design is already over-designed as each anchor force is assumed cumulative. Do not add friction to the anchor loads, it is already included. Friction is a small percentage and should never fail a structure! 

In the days before Structural analysis programs, we did local checks on transverse beams for friction load and verified 50% of the section modulus was available to resist the load. That was it…… 

I do add friction loads in the preliminary design of the piperack and remove them later as we receive the anchor loads. I apply this in the longitudinal direction only.  Friction on longitudinal beams due to pipe turnouts will be reported by pipe stress engineers. Do not add friction to wind loads. If you accept wind applied longitudinally is not required then apply the same logic to friction transversely. 

Whether the piperack is in Kazakhstan, Saudi Arabia or Canada, temperature load designs are not required in piperack design unless you have clients with deep pockets.  Temperature loads are assumed as the difference between a structure at 30 degrees Celsius and the same structure at -50 degrees Celsius. So you will see temperature loads of 80 degrees Celsius applied! Time to step away from the computer! I saw this issue become profoundly out of control on one project, we had to blow the whistle. I found all members were thermally loaded except the braces , which worked so hard to restrain the movement it was set to explode. Apply the thermal load as well to the bracing and the forces of restraint disappeared.  If the piperacks system is bolted up then the temperature loads are released by the internal stresses. and the long term diurnal response of the structure as a flexing member.  No-one needs to apply such temperature difference to a piperack design. If you have to apply it, then apply -40 or -50 and +40 or +30 everywhere, including braces but use this to design the expansion joints locations.  

The reality is no open piperack will ever achieve a sustainable and singular temperature; there is a sunny side, solar gain, shadows, wind, rain, snow. The design average will closer to 50% of these values, so apply -20 or +20 degrees Centigrade and see what the response looks like.  Bolts slip in connections, relieving forces. 

 Please don’t even think about adding the envelope temperature load  to the friction loads as well! Designing piperacks is an art form in commonsense.

Filed under: General Rant, Uncategorized

Tags: friction, piperack design, structural engineer