Restraint Loads: Include friction and/or wind

Hi,

Our structural department insist that we supply them loads on pipe supports including friction and some times wind loading.

I run analysis with and without friction to see the stresses and loading on equipment. The question is that should I supply the loads on pipe supports including friction. What I understand that lateral loads due to friction can be used to design structure as they are not continously acting loads anddisplacement should also be taken into account.

May be some experienced members through light on this that what is the normal practice.

Thanks

Sorry for typo: It should read as (Can not be)

What I understand that lateral loads due to friction can NOT be used to design structure as they are not continously acting loads anddisplacement should also be taken into account.

Hello M Waheed,

The structure must be designed to accommodate the highest loading combination that will be applied to it - even if that loading is transient. You should include reasonable friction in the analysis and transmit the calculated loadings to the structural engineer. Note that ALL maximum loadings must be accommodated by the structure design and that includes loadings transferred to the structure due to dead loads and live loads on the piping (wind, snow and large birds).

Regards, John

Hi John,

Not really CAESAR II issue but an interesting issue you raised. Snow and birds. Many years ago my lecturer told how he designed a lamp post when he was young engineer. Did all the wind, snow etc loads he could think of. Gave the design and everybody was happy. One winter day he was looking out and saw a bird sitting on top of a snow covered lamp. He realized that he never thought of such loading. His solution. Next time he would go around and would measure every lamp post standing he could find and would base his design on those values.

I thought it was a good lesson. Have used it since then.

The design conditions are not the same during the day. The ambient temperature varies, and the design conditions may too. I think you should give loads including friction and not as an occasional –transitional condition.
Sometimes, structural people need to combine the loads in different way as we do on CAESAR, so maybe that is way they need different types of loads.

Regards,

Depends. For at least one of our clients, the direction to structural engineering is something like

"all supports to be designed for a coefficient of friction of XXXX".

I frequently put a not on the results pointing to this (ie design to frictional loads per client requirements not stress analysis).

-SLH

I always include friction forces for line-stops and guides by adding them to the operating loads that I transmit to Structural. You don't always have to do a computer analysis, many times you can calculate the friction by hand and add it to the operating loads. For sliding supports, Structural derives the friction forces on their own from the vertical loads at the support.

I give wind and seismic loads separately from operating and usally give the contribution from wind and/or seismic and let structural add them to their other loads.

Other loads that may be required on some projects that haven't been mentioned are hydrotest loads, transport acceleration loads, blast loads, wave slam, acceleration on floating installations, etc.

Do not ignore these other loads when designing supports or pipe attachments.

Thanks for your replies.

In some instances there may be high lateral loads due to friction but movement of pipe is very small, say 1mm. After moving 1mm that frictional force causing lateral load is not there. The structure guys will take this as continuous load and design a support base accordingly which some times comes out to be quite big.
Is that big support base really needed?

Often they complain these loads are very high.

Any more thoughts on this?

M Waheed,

If this is your problem, my suggestion is to break out and list the Friction contribution separate from the frictionless operating loads and inform Structural that you have done this.

Structural can then design using the loads per their requirements.

You mention base supports, make sure you consider the friction and all loadings when designing your trunnion and calculating local stresses at the pipe/trunnion juncture.

Several points:

1. ASME B31.3-2006, Para 319.4.3: "The significance of all parts of the line and of all restraints introduced for the purpose of reducing moments and forces on equipment or small branch lines, and also the restraint induced by support friction, shall be recognized."

2. Friction is a very tricky thing. A force vs. displacement curve can be thought of as a sawtooth sort of curve, where the pitch of the peaks depends on many incalculable things. In general, it is NOT correct to reason that "the displacement here is X, and that is enough to relieve the friction." Once the displacement becomes sufficient to cause the line to move, the load also does not go to zero, the friction force merely drops from the static friction value (0.4 for steel to steel) to the sliding friction value (usually less than half of the static friction value). So the amplitude of the sawtooth curve is not 100% of the static friction force value, but somewhat less.

3. If you rely on friction to help you, you will be disappointed. If you ignore friction, you will also be disappointed.

4. If you install an intermediate anchor between loops, your analysis will show that the friction forces on opposite sides cancelling each other out to some extent. This is a very dicey assumption - each suppport near the intermediate anchor has a separate sawtooth curve, and in general the pitch of the individual sawtooth curves will not be the same. So, even if the supports are spaced very regularly, the first support on one side of the anchor may be at the peak of one of the sawteeth (movement is just about to occur) while the first support on the other side may well have just moved and be at the valley of its curve.

5. One can make a good argument that friction is only relevant for the first N supports near a piece of equipment or intermediate anchor. The reasoning is that the force on the (N+1)th support acts on the Nth support, but the deflection that it causes is small and thus the force from the (N+1)th support has no mechanism to pass itself through to the (N-1)th support other than the infinitesimal compression of the pipe as predicted by Young's modulus. Despite this, it is common practice on a pipe rack to design anchor towers for friction loads calculated from the weight of the pipe between anchors. This sounds huge, but unless you are in a seismically inactive part of the world, it's still way below what one would normally include for seismic loads. And then there is wind... (Don't ask me how to calculate N. I have no idea.)

NozzleTwister's suggestion is a good one, though - break out friction, wind, seismic, and other design loads separately for your structural guys. They so love to do arithmetic. :-) Seriously, the building Code they are using may require, or at least allow, them to treat the various categories of loads differently. And they may have some leeway to use less than the direct sum of the individual loads for their effective design load.