From: gherbert@gw.retro.com (George William Herbert)
Newsgroups: sci.space.policy
Subject: Re: SPS Questions....
Date: 25 Sep 2000 00:01:02 -0700

Scott Lowther  <lexcorp@ix.netcom.com> wrote:
>Thomas Kalbfus wrote:
>> Office buildings 10 miles high are possible to build today, the required
>> materials exist to build such a structure.
>
>Nope. If you wanted to build a mountain out of composites, you could
>probably do so... I believe GD studies such a concept some years ago as
>a rather lame way of improving launch vehicle performance. What you
>basically wind up with is a pyramid of foam... not a buiulding as such.

No, you could do really, really large buildings if you wanted.
The problem is that they get exponentially more expensive.
The weight of the floors proper dominates most building designs,
the slab of concrete, underlying steel sheet, the I beams that
sheet is supported on, etc.  Only the largest buildings in the
world approach equal weight in vertical supports as in the floors.

For really large buildings you have to cut back on the floor weights,
using foamed fiber-reinforced concrete instead of poured solid stuff,
aluminum or titanium sheets under that, high strength steel beams,
the use of ablative fire resistance coatings rather than air entrained
spray on cement (less weight for same fire protection), etc.

For a real building, fire resistance ends up being the killer.
If you could ignore what fires would do to it, you could build
a really pretty damn tall building using carbon fiber composites.


-george william herbert
gherbert@retro.com

From: gherbert@gw.retro.com (George William Herbert)
Newsgroups: sci.space.policy
Subject: Re: SPS Questions....
Date: 25 Sep 2000 11:42:07 -0700

Scott Lowther  <lexcorp@ix.netcom.com> wrote:
>George William Herbert wrote:
>> For a real building, fire resistance ends up being the killer.
>> If you could ignore what fires would do to it, you could build
>> a really pretty damn tall building using carbon fiber composites.
>
>Maybe... but a 10-mile high office building? That pretty much determines
>that the base of the building takes up no more than one city block. This
>would more resemble a wire than a building.

The one-city-block (roughly 1-200m square) requirement makes it very
hard to make huge buildings within a city.  However, you can do tricks
like cable-stay the building to spread out the loads; the building
ground print is only a block, but the loads come in over a wider area.
Also, building large buildings on the edge of a major city, say at or
next to a transit node area, is more practical than urban cores in
several ways.

Also, once you start to seriously consider megabuildings, the one block
rule starts to soften... cities regularly take multiblock areas and build
large convention centers, etc, rerouting traffic around, over, under,
or through.

As I said, the hardest part of really really tall buildings is the
fire resistance of the high strength materials.  You can go quite a
ways with more basic materials; HS Steel with 100,000 PSI yield strength
assuming basic deadweight loads are limited to 30% of yield, and assuming
building dead+live loads of 100 lbs/ft^2 can take you up 300 stories
(3,000 feet, about a thousand meters) with one square inch of column
area per square foot of floor area.  If you limit the structural area
fraction of the base to say 5% (7 sq in/sq foot on the average),
that's a height of around 7 km; 10% and you get 14 km, almost 10 miles.
At these heights wind loads, earthquake loads, etc. will predominate
so it either has to be very wide or side braced (cabled) or whatnot.


-george william herbert
gherbert@retro.com