When the space shuttle is sitting on the launch pad, what holds it
upright? I know that it is attached to that big structure in places,
but what part of the shuttle is supporting all that weight from below?
From TV you can see that it doesn't just sit on its engines so
something must be holding it (suspended?) on the launch pad. Thanks!

Here's a newsgroup thread on the subject:

http://groups-beta.google.com/group/alt.fan.cecil-adams/browse_thread/thread/43bb10c13bc8fd1d/7c7c69aa0694b3be?lnk=st

Of particular interest is this post from the thread:

http://groups-beta.google.com/group/alt.fan.cecil-adams/msg/40b4688c21f1b9e9?dmode=print

OK, so the shuttle is attached to the SRBs and the SRBs are attached
to the launcher platform by 4 bolts.  But surely those 4 bolts don't
hold up all 4.5 million pounds.  I was thinking that something holds
the whole thing up off the launcher platform so when the engines fire,
the exhaust has somewhere to go other than straight into the platform.

"I was thinking that something holds
the whole thing up off the launcher platform so when the engines fire,
the exhaust has somewhere to go other than straight into the platform."

   Ah, that's true. Beneath the full stack is a large pit, and at
ignition the pit is flooded with thousands of tons of water. The water
acts as a vibration dampener. Most of the cloud which bellows out of
the pad is actually hot water vapor.

search Google Images with   discovery launch    and you can see lots of photos.
"Discovery and its auxiliary rocket and tanks sit on an immense
platform that moves on caterpiller-like tracks.

From the images, it really looks like it's the SRBs that support
everything, but if there's a pit beneath the SRBs then what keeps them
from falling through? It still seems like something should be
suspending the SRBs and everything else over the pad/pit. I'm
wondering what that "something" is that can hold all that weight.

Here's the scoop. The full stack is locked onto the mobile launcher
platform at the bases of the two solid rocket boosters with four hold
down posts per booster. (The launcher platform, with the shuttle
stack, is delivered to the launch pad atop the crawler transporter,
which has the caterpillar tracks on it. The crawler leaves the
platform at the pad adn retreats to a safe distance.)

   The launcher platform has three large exhaust holes in it, one for
each of the solid boosters, one for the trio of shuttle main engines.
The rocket exhaust passes largely through these rectangular holes and
down into the flame trench. The flame trench is a 300,000 gallon
concrete box, but with one wall at an angle such that it deflects the
rocket exhaust sideways. The flame trench at liftoff is flooded with
water at the rate of 900,000 gallons per minute, which of course gets
turned rapidly to steam within the trench and billows out the side as
white water vapor as it quickly cools to below the temperature of
steam.

   So the gist of it is that the supports for the stack are firmly
fastened to the launcher platform around the perimeters of the large
exhaust holes in the floor.