Anchoring of Inflatable Buildings

[EuanT]

We need to anchor an 8m dia. hemispherical inflatable radome used as a temporary erection. We intend to use Platipus or similar ground anchors. How do we estimate pull out forces in a range of soil types & conditions. We need to do this to get safety clearances etc. Since estimates such as these will inevitably be liable to large errors, how should we give useful information while protecting ourselves legally against possible claims if the wind should blow the thing down (English law)?

 

[ishvaaag]

Since spanish I am not familiar with th UK codes. In any case they will state loads for buildings; if temporary maybe safety factors will be downplayed a bit. For the whole frame, on projected area but for ordinary buildings, the spanish code accepts for hemispherical building 4 m tall 0.4·50=20 kgf/m2 of projection in non exposed areas to 100·0.4=40 kgf/m2 of projection in exposed ones. More detailed procedures allow calculation element per element. Plus safety factors to be later applied, of course. The only way of protecting you somewhat is making a thorough check by the standing law of your country, and if to be sold, including leaflet with thorough instruction about how to anchor with what sold. I suggest you making checks in different kinds of soils for the typical pulls that stand from the calculations, at the typical inclinations of the cables.

 

[EuanT]

What I need to know is:

1) how to do the actual pull-out force calculations

2) how reliable the answers are likely to be - factor of 2, factor of 10, etc.

3) how I define the soil condition for the calculation e.g. composition, porosity, water content etc.

The problem is that a soil is a poorly-characterized material with uncertain and variable mechanical properties. For a building that can be deployed anywhere, how can I condense the potentially infinite variety of cases into a few (e.g. 3?) "typical" cases.

 

[ishvaaag]

The detailed calculation since membrane and cable, and pneumatic without doubt requires from some engineer expert in large deflections membrane and pneumatic calculations; I am not.

If you have no access to such expertise you need to cover it by bigger safety factors, no doubt...even so the calculations may prove not be safe enough for a reviewing party... what brings us to the pertinent question on what authority is the reviewing party, for it could provide you with criteria on how to proceed.

I will describe how I would proceed in Spain if no one other more able than me (the required skills above and equipment not at hand) was coming, assuming the inflatable thing is stable enough in shape under snow and wind that no significant change in shape is required to be accounted for.

Code NBE AE-88 then defines inwards pressure and outwards pressures for the shape, for situations with both door open and closed (being pneumatic might help to forfeit the open door cases. Put then in FEM model, assume a hemispherical membrane for the total of those standing at the skin, your attachment points and intended anchoring points. This will give you envelopes of the tensile forces in the cables, even for a variety of anchoring setups (distances from center). If you have it, use P-Delta design and Tension-only members for the cables, lest them show support the thing in compression!

I would add then the on projected area loadcase.

And since forfeiting all that I don't know about the thing, I would use for the general forces a safety factor of say, 2.5 to 3.5 for the insertion points to the membrane and the membrane itself...and bigger for the cables. It was not unusual in the past the cables safety factor going even up to 12, 6 to 8 being more typical, and this being for fixed structures. Your nylon or so cables will suffer abrasion, so I would stand at no less than 5 or 6 safety factor on the tension (specify maintenance for the cables).

You can test your setup for fatigue cycles and UV aging, this way you and one certifying party (another interesting possibility for tour peace of mind) could be satisfied with the level of safety provided.

Your soils can be a) poor b) middle c) good d) hard

I understand that excepty you want to provide with at least 2 sizes of beak anchors the 3 first classes should be dealt with the same size. For hard to rocky soils a set of nail-like anchors may be an alternative.

Thie beak design needs then only be tested for the soft soils, and in strength for its ability to deepen in the middle and hard. The nails should only be used for near rocky conditions. With FEM you can also model the response of the insert in the soil, to see if it detaches from it. Even simpler lever calculations will make you aware on if you are within the passive state capacities to be hoped from the data soils. The problem is not much dissimilar to the cantilever trench wall, only that the push is your cable at the standing tip. Here you may not go higher than 2.5 to 3.5 safety factor, lest your beak anchors become unsightly big...but that the calculations will show.
Ensure also in the calculation that friction with the soil is enough in the sides to stand the upwards or along the insert anchor component.

 

[phuduhudu]

As you say soil is a very variable material. I would add a caution to ishvaag's advice that it is not a good idea to add safety factors for what you don't know about. Safety factors are there for the variability which we do know about. So with the soil I would get advice froma geotechnical engineer and ask him to design for a worst likely soil condition. The uplift and sideways forces should not be that complicated. Any good textbook on wind loading will give uplift forces for domes which is what your structure is. Then it's just a case of dividing the total uplift by the total number of anchors to get the vertical pull up force. Of course the cables will be inclined so you can resolve to get the sideways force on the anchors. Carl Bauer

http://www.bauerconsultbotswana.com

 

[ishvaaag]

That is another interesting subject, safety factors. I agree that the intent and method that inspire most of the present codes is to warrant and establish some precise degree of safety, in accord of what the best intent and knowledge of the codemakers, those who make comments included, advises be what convenient. In any case my many lacks of knowledge do not include the one of not knowing everyone else has as well some lack of knowledge. Some fuss about specified moment frame unions in LA failed under earthquake is a good reminder about that. The stochastical approach to safety embedded in codes since the early consideration of limit states' codes has brought some relative consistence between the parts of design that make up for safety, yet all them run to state that the probability of failure can't be taken as any of the numbers evaluated; this happens for limit states, for fatigue etc. The stated probabilities simply don't match with the experience of anyone of us not even on important failures, least if we take unto account the many serviceability problems seen that most structural designers we were badly armed to fight upon till the advent of PCs, and it is very clear that on account of all this, the different safety factors, that is just again my view, in some way throught the paulatine variation of their values, have had since inception (be their effects called allowable stresses) built-in coverage of precisely imperfections in the knowledge of the loads, on the behaviour of the structures, on the statistical variances in general, and of course even human failures through improper design and construction. That is just my view, my philosophical stance says there's much we don't know about, I have seen for myself on me and others, and even if I agree that the intent is being made for the codes be in the best agreement with the sicences in general and the sciences of construction in particular, I feel that the particular devices that tailor what we design to safety and we call safety factors meet both in theoretical considerations and unavoidably in practice anything the codemakers, designers and builders together don't know yet.

 

[thaidavid]

EuanT-

The American standard code, ASCE 7-98, has Table 6-10, which gives wind force coefficients for round structures. If you can find a copy of it then this might give you a simplified way to calculate the gross forces involved.

Also, concerning Ishvaag's statement about safety factors and temporary useages: in American engineering no reduction is allowed for safety factors in "temporary" structures, because mean return periods of damaging winds are just that: estimated means. The design wind could return tomorrow. Who knows?

 

[ishvaaag]

What explains the practice of using reduced safety factors for temporary structures is that the likelihood of the event is lesser. For longer times standing the probability of the event is higher. If those establishing the codes have good data, they may if so want impose even if using different safety factors the same LIKELIHOOD of the structure having a failure, since the diminished factor si compounded with the lesser probability. Furthermore, it needs be also argued in favour of the practice, that the safety factors are ower does not mean they might, by design, be never allowed fall below 1 for the estochastical projected average of the forces. In all, that the practice needs not be unsafe, but use of data and mathematical knowledge to gain some economy for temporary structures, for an accepted -maybe the same than for long term structures!- likelihood of failure, and of course always in the safe side.

Furthermore (this not denying at all what ASCE may be indicating, but commenting about) some american colleagues have said in this forum forms for concrete don't need be calculated against earthquake since the probability is nil. Temporary structures they are, except we want to use labels to be unfair. Following your statement, the earthquake can come tomorrow.

 

[jheidt2543]

Chance Division of Hubbell Power Systems, Inc. manufactures a "Soil Screw" system of anchors that may be of some help. They also have a technical services department (573-682-8414) and a website (

http://www.hubbell.com/abchance)

you might try. They provide design criteria for their anchors in a variety of soil types.

Good luck!