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Giant spider webs in fiction always seem very thin compared to the scale of the spider itself. (See: Hobbit 2) I would expect giant spider webs to be like cables, but that might not be the case.
My question is: Does the thickness and consistency of web scale linearly with the scale of the spider?
There must be some actual spiders that we could draw reasonable comparisons from.
Assume:
Well the largest known web-spinning spider is the Nephila komaci. It gets to be about 5 inches and can spin webs up to about 3 ft. Otherwise you might notice most other large spiders don't spin webs. The Goliath Bird eater is the largest known living spider and it maxes out at about 12 in. and it's a hunter
Likely what is going on is that the silk needed to catch larger prey for the larger spiders becomes more easily visible to it's prey and thus is avoided. So REALLY large spiders with webs would be unlikely unless they learn to herd their prey or use them like fishing nets casting them out into the path. Most of the webbing you see is for the creepy factor and really doesn't have much based in reality. It appeals (claws) at our nature, to be caught in such a terrible helpless position.
Most likely a Giant spiders silk would be reserved for wrapping and storing it's prey if it would even have kept that ability as it evolved larger.
I am going to compare a 50kg and a 1g spider with a silk radius of .003mm. The breaking stess would be about 1000MPa for an average spider. Lets say the giant spider manages to get this up to 2000MPa which is the strongest in the world.
Quick calculation shows that the 1g spider can hold about 3 times its body weight of a single thread of silk. How much would that need to be for a 50kg spider? Only .5mm
This calculation is enough to show that in theory a spider with thin strands (say 2-3 mm) would be possible.
In practice a spider often uses several strands of silk together. I have my doubts about the accuracy of the .003mm radius. Basically, however, this shows that a giant spider could still possibly use silk like a small one iff it were worth it. I don't believe that it is an evolutionarily favorable path which is why we don't see it. In your world, however, you could possibly make it work.
Please note that there are likely many nuances that would make this more difficult for a larger spider. I don't want to lift myself by such a small part of my body. The thin diameter likely increases mechanical strength more than bulk properties would imply. There is likely evaporation which occurs in the thin strands which would not occur in a thicker one. There is potential to focus stress on a small region of the silk. Nevertheless, spider silk's properties are incredible and, while they would need to be thicker for a giant spider, they would not really need to be THAT thick.
Spider silk has a tensile strength of 1.3 GPa, or 132,563 g/mm^2. A "giant" spider could be described as any spider larger than 175 g, which is the record for the world's largest real spider. A 175 g spider would need silk with a cross-sectional surface area of greater than 1,320 μm^2 (0.001 mm^2), or a diameter of 41 μm (0.041 mm). For a 62 kg human sized spider, the silk would have to be greater than 771.68 μm (0.77 mm). Shelob from The Hobbit was 2.44 m long and 5.18 m across. This would make her 700-1,500 kg, and would require silk 2.6-3.8 mm in diameter.
