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The Black Widow’s Habitat: a Comparison with Contemporary Design & Construction

The misunderstood Latrodectus, or female black widow is shy and nocturnal in habit. She does not leave her hidden web voluntarily and is completely out of her element when away from it. These arachnids spin three-dimensional tangle webs of coarse silk in dark places, usually outdoors. Contrary to popular belief, the female only eats the male in extreme situations and it is not a regular occurrence.

The Web Material & Method of Construction

The structure is purely tensile because that is the nature of the materials in use. The breaking strength and elastic modulus of spider dragline silk (up to 2 GN m2 and 30 GN m2, respectively) exceed the values for highly drawn nylon (0.7 GN m2 and 2.4 GN m2, respectively). Black widow spider silk happens to be on the stronger side of the scale. Thus, an industrial product that is spun at elevated temperatures for optimum tensile properties is matched by a natural material spun into fibers under ambient conditions.

It is remarkable to note that silk, while light and elastic, is stronger than steel yet easily recyclable, potentially replacing plastics for many uses. Spiders also have several silk glands, producing different silks for different purposes.

One such example is its scaffolding silk: The spider builds a small platform at the hub, from which temporary spiral scaffolding works out to the edge of the web. Once the scaffolding is in place, the spider walks on the non-sticky scaffolding, from the outer edge to the centre, attaching sticky threads that form the trap. The spider cuts away the non-sticky scaffolding as it progresses toward the centre.

The thread is produced from a group of spinnerets located on the underside tip of the abdomen. Here’s another instance were our own construction principles mimic that of nature: The spider uses its rear legs to pull taut the excess coagulated silk from the spinnerets. As in concrete technology, by pre-stressing, the silk is strengthened.

The Form

Both Eastern and Western Black Widows’ webs are particular because they seem to possess no visual geometry. However such “Spatial” webs are distinguishable because of their three-dimensionality. Their so-called “tangle webs” lack form and are erratic in appearance. By wind-gliding, this particular spider swings from support to support (leaves, twigs, outdoor furniture) without pattern.

This is in sharp contrast to that woven by the common garden spider. The latter has a distinct pattern and is said to simulate illusions of flora when in harmonisation with the rays of the sun.

For example, the pattern and complexity of orb webs varies from species to species and some scientists have suggested that the very pattern of the spider web is designed to attract insects. These webs are thought to produce patterns that resemble patterns reflected by many flowers in UV light. Thus insects who are searching for their favorite type of flower see the decorated web in the UV light and fly into the trap.

Inevitably, the tangle web has an opposite effect on humans, who generally are made very wary by the visual effect of such a web.

Are such erratic methods so alien to architectural design methods today? Principles of Open Architecture describe design methods based solely on feeling. Drawing becomes a one-man journey through an escapist’s realm.

Created from a design drawn explosively, quickly, with closed eyes. Unbroken concentration, the hand as the seismograph of feeling, which calls the constructed room to life.

Coop Hmmelblau, from an essay on “The End of Architecture” 1992

Similarly, Deconstructivist buildings too may seem to have no rational visual logic: They may appear to be made up of unrelated, disharmonious abstract forms. And yet there is sense to it. Reason can be seen in the tangle web when we examine its structural properties.

The Structure

The impact of a common fly on the web may be likened to a punching shear action. This action would cause all members within the structure act in tension. How does this mechanism translate in our every-day use of this phenomenon, particularly when the Black Spider’s web is so non-geometrical?

I referred to a study on fibre-reinforced glass concrete slabs whose findings related to my question. It was very interesting to see that within their results, the sample which was reinforced with short randomly distributed fibres had the highest ultimate punching shear load and ductility, without changing the structure's failure mode. The explanation given is that short fibres which are uniformly distributed yet randomly oriented have a better chance of bridging the structure diagonally wherever impact occurs thus increasing the shear strength.

The paper also documents the importance of irregularity in structures since, in say, a concrete slab. Irregularly shaped glass aggregate would increase the bond between the aggregate and the reinforcement matrix. In fact, the punching shear strength of glass aggregate concrete slabs are much higher. It was also noted, that a glass concrete slab was more easily improved by arch action than an ordinary concrete slab... a result which may nevertheless be translated in our case since the web can be considered to be an upside down arch.

In fact, any spider web is even capable of stopping a bumble bee at full speed. To calculate the shear stress impact causes, all you need to know is the weight of the insect and the speed at which it travels. Lets use the common fly as an example:

For punching shear failure:


shear stress =       	                 (failure load )
				      __________________	
    (structure depth) x (average circumference of failure surface)


where the failure load = mass x accelaration



Therefore,
shear stress =       	              mass x acceleration
				      __________________	
    (structure depth) x (average circumference of failure surface)


Where,

mass =			mass of a common house fly = 12mg    (0.000012kg)

acceleration = 		20 000m per hr
(since it would take circa 1 second to decelerate this is 
approximately equal to the speed which can be up to 20 000m per hour)  

 
structure depth = 	v. small since it is related to the diameter of a Black Spider silk thread
	i.e. approximately 0.00000015 m

average circumference of the failure surface = 
	approximately double the circumference of the fly = 0.00122m

Therefore, a typical punching shear stress = 1.3115 GN m2 (less than the afore-mentioned 2 GN m2). Since the impact is less than the strength of the web, the web does not fail under collision, thereby stopping the common house fly dead in its track. Bzz bzz?