Anchoring of Inflatable Buildings
Anchoring of Inflatable Buildings
(OP)
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)?





RE: Anchoring of Inflatable Buildings
RE: Anchoring of Inflatable Buildings
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.
RE: Anchoring of Inflatable Buildings
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.
RE: Anchoring of Inflatable Buildings
Carl Bauer
www.bauerconsultbotswana.com
RE: Anchoring of Inflatable Buildings
RE: Anchoring of Inflatable Buildings
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?
RE: Anchoring of Inflatable Buildings
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.
RE: Anchoring of Inflatable Buildings
Good luck!