MEMBRANE                                                               

MEMBRANES/COVER/SKIN/FABRIC

In order to take  full advantage 
of the potential of geodesic dome construction -
a membrane of thin film 
should be used on a large diameter FTB Dome (or similar).

Membrane Design is probably the least discussed 
of all aspects of Large Scale dome design,
though RBF built many domes of this type.

The most common material for thin film covers at this time 
is Low Density Polyethylene.
This material has several severe drawbacks, 
the most notable of which is solar degradation. 
After 2 or 3 months of summer solar exposure 
polyethylene becomes brittle  & breaks under slight pressure.
However for some development purposes 
(such as geometry & fastening testing) 
this material is useful due to its availability & low cost.

A second type of this material is now (6/11/88) being tested. 
It is a woven high density polyethylene 
with a low density polyethylene coating. 
It is reported that its solar exposure life may be 
up to 5 years in northern climates & 2 years in West Texas. 
Its cost is close to the low density material 
& it is much stronger tensily.
This material was in place till 7/21/90 
& the majority of it was in good shape. 
Wear was the primary cause of failure. 
This material weathered very well, 
with solar degradation being a minor consideration. 
Fatigue due to flapping in the wind 
was the apparent cause of breaks in the fabric along the strut lines 
& at creases formed within flapping areas. 
A Fabric Tensioning System is being devised 
to reduce wind induced flapping & thereby increase fabric life.

Other possible membrane materials include:
		           
Dupont TEFZEL
		           
Suntek HEAT MIRROR & CLOUD GEL


Membrane Design Criteria


The Membrane design objective is :

	1) a simply and easily applied 
	2) that is environmentally inert (stabile), 
	3) tensily strong, 
	4) will remain attached to the frame
	5) exhibit minimal flapping in the wind
	6) will allow or reflect light, heat & other radiation (valve) as required
	7) will retain moisture
	8) is cost effective
	9) will not blow open in the wind (at seams or elsewhere)
  	10) Aesthetic Pleasing (smooth  fabric - no loose flaps or bunching)
  	11) Puncture Resistant
  	12) Long Life
  	13) Readily Available

Current Elements of Membrane Design:  
   
	1) External application of Plastic for stress distribution throughout plastic 
		  (for inwardly directed forces)
	2) Smooth Vertices on Outward side of dome
	3) Triangular Panels running Parallel to grid lines
	4) Tension applied at panel corners by spring elements attached to adjacent vertices
	5) Top Most Sections Applied During Construction
	6) High Density woven Polyethylene Fabric
	7) Positive Closure at Edges
	8) Overlapping at Edges to effect a good seal & prevent blowing open 
		in strong winds
	9) Edge Tensioning by slits and spring action
	


BCF ENCLOSURE/Fabric tension systems

The BCF retention system has been developed 
to enclose an FTB dome 
using thin plastic films.
BCF stands for Binder Clip Fabric. 
It is a simple system 
that works effectively 
to hold the fabric to the structure 
and 
maintain tension in the fabric.
Working with a 1/2" or 3/4" diameter strut,
a continuous lines of 2" binder clips 
are used to hold the fabric to the struts.
The fabric seams correspond with the clipped struts.
The fabric is tightened and smoothed as it is applied to the dome.
The clipps in turn act a spring tensioners to maintain fabric tension 
over time and differing environmental conditions.


Interior Space

In order to make use of the interiors
I propose fabric be hung from the vertices.
These tarps can provide privacy
and interior design accents 
That are readily relocated on the fly.

For large public spaces,
in pleasant climates
the enclosure of the outside of the dome can be forgone
and the hung fabric can be used 
to shield form the sun and occational showers.




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