Looking for threads relating to best practice/calcs for building temporary supports for mobile struc

[SandyBill]

Apo0logies if this is an old question answered many times. My search came up with not a lot...

I am looking for best practice, old wisdom or preferably calculations relating to the construction of temporary supports for containers, offices, toilets etc on construction sites. Height to width ratios, best materials and the relative advantage and disadvantage of these (e.g. wood block vs concrete slab...) etc. It may be my inaccurate search technique, but i can't seem to find much on the subject. Which is why I have just joined Eng-Tips!

 

[1503-44]

Something other than a skid mount sitting on gravel?
I can't even recall ever building a concrete slab for these, although you often find them set on existing slabs, or actually an existing parking lot pavement.
Gravel, oyster shell, etc. padding is the best I've ever used. Set on wood blocks, or sleepers, only if there was potential to get standing rainwater in a poor drainage patch. Skid, or wooden sleepers placed on gravel pad placed on rudimentary grading to drain to a temporary water retention pond is about as fancy as it gets.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SandyBill]

Thanks for your reply.

I work in the world of temporary demountable structures for large scale events. I have been doing it for a quite a while now. Recently i realised that there was a certain blindness on my part about how to level toilet units, containers, toilets etc etc. It is just done by eye or with a spirit level to get an even floor. Give n that event sites are often annoyingly uneven, this has resulted in some interesting solutions to the levelling problem.

There is an inbuilt 'that just looks weird' instinct in almost everybody and thus the temporary supports are normally pretty good. However. I am working in a desert setting on uneven ground and some of the local contractor/suppliers have a less attuned 'that just looks weird' instinct.

What I am looking for is some ready reckoners on width to height ratios for very temporary supports for mobile units up to say 20 tonnes. also any guidance or interesting articles on the failure point of piles made of different materials, i.e. wooded scaff boards, versus marine play vs concrete slabs versus cinder block for example.

I hope that an engineering forum is a good place to start looking

 

[1503-44]

I've seen lots of strange looking things in the deserts. Seldom can you trust that anything will be built to what you are accustomed to seeing and level floors are not high priority. Generally, if the bathrooms have running water and lights come on, you won the battle.

Most of us here tend to think of these kinds of things.

Again, usually on 12" of compact shell, or gravel pads.
The odd concrete slab here and there for more permanent, or extra heavy equipment.

Piles depend so much on soil at the site and probably would be too expensive for a temporary gig, but then how long is temporary?



Try asking specific questions.
Make a general arrangement view of whater your "mobile unit" looks like.
Show weights, locations and heights.

Flooring depends on span length, so show plan dimensions.

Engineers dont say anything much without details, so give us whatever you know.

Maybe someone else can be more helpful.






--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[SandyBill]

Hey 1503-44

Thanks for your guidance, much appreciated - having another go!

I have attached a very simple drawing of an example of what I am talking about. My industry spends a huge amount of time building or placing temporary demountable structures in sometimes unhelpful spaces. For the most part something like a stage structure or a large scaff structure will be designed and loads calculated by the supplier or a third party structural engineer. This is all good. Below are some examples of the kind of things we build.

When it comes to dropping and levelling smaller things like a 25t welfare unit, office or ablutions it is very often left to the site crew to manage that process, depending on expertise of the supplier.

The one in the first image needed a strap attached tot he ballast block. The one in the second image was redesigned after a short discussion with the crew.

What I am after is the mathematics behind the safety of a pile of wood supporting one corner of a 25t (or whatever) tonne object.

Is there a standard ratio for width to height of an unsecured column of wooden planks, concrete blocks etc?

What does an eccentric loading do to that calculation?

Are there formulae to apply that will account for slippage between the blocks (of whichever material) when a horizontal (e.g. wind or vehicle impact) force is applied to that suspended unit.

I hope that at least gives some more detail.

I am just an intrepid explorer looking for answers. None of this is currently mission critical.

As a by the way, I am in the middle of an EIT course - Structural Engineering for Non Structural Engineers, and very much enjoying it. We just haven't covered enough subjects yet for me to answer my own questions!

 

[1503-44]

There I can tell you a little something anyway.
Use some simple static calculations for a billboard at height.
Say you have a wind load at a center of pressure 5m above your supporting points.

Wind Overturning Moment = Wind Force x height
4000N x 5m =20,000 Nm

Weight = 2000N

If resisted by a two point bearings 4m apart

Weight at each bearing point = 2000/2 = 1000N
OTM load = 20,000Nm/4m = +/- 5000m

At the wind side, total bearing load = 1000N -5000N = -4000N
You need a 4000N hold down counterweight here.

At the leigh side bearing load = 1000N +5000N = 6000N
Your pile of blocks needs to have a strength of 6000N before crushing.

Of course the wind can blow from both directions, so each bearing point must be designed for both the maximum uplift load and the max crushing load.

Friction, assume half the wind goes to each bearing point. Wind Force /2 = 2000N/ 2 = 1000N at each pile of blocks.

Apply appropriate safety factors suitable for your materials, etc. Maybe 2?

Check the safety against overturning.
If the OTM is resisted only by the 2000N weight at a distance from one bearing point of 5m/2 = 2.5m you have 2000N x 2.5m = 5000Nm of resisting moment RM and the Safety factor against Overturning is RM/OTM = 5000/20000 = 0.25
Generally it should be greater than 1.5, so the above is not a very good design.
Some weight should be added to the structure, or the distance from centerline of the structure to each bearing point should be increased.

Now you could do the same kind of analysis on whatever is holding your bearing points in place.


--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[bimr]

As a rule of thumb, the soil is generally able to support loadings of approximately 1,500 lbs per square feet. You would want the supports to have enough area to spread out the loading so that number is not exceeded.

There have been many instances of stages collapsing. For that reason, and with the attendance, as well as the stage sizes that your posting, you should have a structural engineer review the application.