Wind Load on 1" pipe that is mounted low to the ground?

[Redacted]

Hi there,

I have a project where a client has a single 1”(28mm OD) pipe that they would like to mount along with a 80mm high cable tray on top. See below image:

The client would like to use a precast solution to hold the pipe down and were initially looking at using some parking bumpers to anchor into and use as dead weight. These bumpers are concrete and are 30” (0.762m) long by 8” (0.203m) wide and 5”(0.127m) deep. They wanted to have these spaced at 2m and for this to be checked to see if it is adequate for hurricane force winds (150mph).

According to the ASCE 7-10 factors, the 150mph translates to 2kPa of wind pressure.

I used the “Wind Loads for Petrochemical and other Industrial Facilities” ASCE document as guidance for this.

The document recommends a simple formula of F=qz*G*Cf*A

Where A is the tributary area of the length multiplied by the height of the pipe or cable tray.
qz is the velocity pressure
G is the gust factor (taken as 1)
Cf is the force coefficient which the document specifies should be 0.7 for pipes and 2 for cable tray.

I would like to make sure that I am not being too conservative here as the pipe will only be around .25m above the foundation and the cable tray only around 0.5m above the foundation.

Some quick hand calcs are showing me that for a 2m spacing.

Wind force acting on the pipe = 2x1x0.7x0.56 = 0.0784kN
Wind force acting on the cable tray = 2x1x2x0.16 = 0.64kN

So total sliding force is 0.7184 kN.

Total moment acting on base is ~0.454kNm.

According to these loads, the original parking bumper dimensions are not adequate to resist sliding.

A 36” long, 8” deep and 16” wide concrete block seems to be sufficient instead.

So :

Question 1. For a structure like this where they primarily will just be using it as a block of deadweight to stop the pipe system from sliding or uplifting, does it need to have minimum reinforcement? Or can it be seen as a mass concrete block?

Question 2. If the 150mph wind is the design wind load, would I then need to go ahead and apply the standard sliding safety factor of 1.5 as well? Or would it be reasonable to use the 150mph design wind speed and have a safety factor of 1?

Question 3. It appears to me that the sliding will govern but is there anything else that I should be checking for in a system like this?

 

[bugbus]

I would not have thought wind loading would be too important. 0.7 kN is very small. Aren't there any other accidental/impact loads that need to be considered?

 

[TLHS]

I often use plain precast concrete blocks for this kind of situation. I think there are plain conrete provisions in the aci standard, I know there are in our CSA concrete standard up here.

I've done lots of this type of thing, and I wouldn't stress too much about sharpening your pencil to the absolute bone. Unless you're doing kilometers of these things, a couple of extra inches on a precast unit isn't going to run you much compared to the install effort.

However, can you run these completely at grade? You can generally buy precast sleepers that have unistrut or similar embedded in the top. You can then mount the tray and pipe maybe eight inches from grade directly on top of the sleeper using standard unistrut fasteners. The height minimized the overturning moment. You also save the cost of the steel support.

Is that windspeed the newer factored style windspeeds? If it is, you can presumably use a load factor of less than one to bring it to working stress level, if you're using working stress on sliding (which it sounds like you are)

 

[Redacted]

@gusmurr thanks for the response and that is a good point. I guess this is quite low to the ground, so there could be the potential accidental load from someone tripping over this. My local code states that handrails/guardrails should be able to resist a minimum of 200lb applied, so I will use that.

@TLHS thanks for your response and you are correct a few extra inches shouldn't be an issue. I guess another question is, is a 3' long concrete element too long to be plain concrete? I presume it will need to be lifted off the ground and moved into position by a few men.

Yes these can be run completely at grade. Unfortunately, we don't have precast sleepers where I am. The precast parking bumpers they were trying to use were the only precast elements we could source. For this project, they will need to make the form themselves and cast these elements.

 

[Redacted]

Ok, so I took another look at this earlier today and I have a question about the accidental loading and wanted to get opinions.

Although this is not a handrail or a guardrail, I thought it may be useful to consider the load case where someone may lean against it or pull the pipe.

In all literature for handrail loads they are stating that this should be a 200lb concentrated load applied anywhere in any direction.

My main question is for an application like mine (or any for that matter), does this load need to be increased by the live load safety factor of 1.5 (making the load 300lb)? Or is the 200lb requirement meant to be taken as just that... 200lb?

@TLHS, yes the wind speed I was using was the new ASCE 7-10 factored style wind speed.

 

[IFRs]

I'm surprised the precast parking bumpers were not sufficient for either sleepers or your bracket. Did you try them parallel or perpendicular to the pipe run? Or alternating?

 

[Redacted]

The precast parking bumpers were only around 105lb (0.47kN). They are perpendicular to the pipe run in the majority of cases but have to be parallel in one specific area.

At the spacing that the client requested of 2.5m they would fail in sliding and overturning.

I would like to know if the 200lb force is reasonable or if it needs to be factored or not, as this would impact the sizing.

 

[IFRs]

Those are light ones...

 

[Redacted]

Yes, unfortunately, that is all that is available here. There is a longer one at 72" (6') but due to site restrictions, only the depth and width can be altered, not the length.

 

[LittleInch]

I know this isn't your question, but 500mm tall is a terrible height for a set of things above the ground. Not big enough to stop someone going across it, but too high to easily step over it. It's quite tall to walk over, but not tall enough to see if you drive anything close to it, people will trip over it and injure them selves and also stand on it.

Could you bury the cables and pipe instead?

Or build a lower box type structure with two vertical legs and then just bolt the cable tray to top?

Also whilst you can calculate wind force, what about everything else being blown about like trees, small tanks etc?

We can't see what you can see but the fact you're building this implies there is nothing else close to fix these pipes and cable tray to?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Redacted]

@Littleinch

I must admit the 500mm height isn't ideal but that height is driven from the client side. Unfortunately, there is nothing close by that I would trust to tie into.

Realistically people shouldn't be tripping over it or walking over it because it is going to be about a foot off a fence line.

It's a fair point about debris flying. Although if that was to happen it is more likely that the 1" pipe or cable tray will get damaged before the foundations do.