I have a 50" x 24" 3/4" thick piece of glass that I'm going to use as
a top on a bathroom vanity. The only question is what kind of vanity to
build for it to sit on.

I'm thinking about making a frame and bolting it to the wall and
setting the glass on top of it. Something like the top part in this
pictu
http://www.lineaaqua.net/shop/LineaA...206-c-261.html

My question is how would you make the frame? I don't know what the
glass weighs, but it is pretty heavy. I'm guessing around 100 pounds or
so. The sink is going to sit on top of it, so another 20 pounds or so.

I was thinking about buying some kind of hardwood (cherry maybe?),
making a rectangle out of it a little smaller than the glass, and
bolting it to the studs in the wall. At 50", I should have 3 studs to
bolt it to, correct?

What kind of thickness for the cherry? I'm assuming I'll have to fasten
some boards together to make the frame thick enough to hold that kind
of weight.

Would you countersink bolts through the cherry into the wall studs? Is
there some other kind of fastener that would work better?

How wide should the board against the wall be? How many bolts per stud?

If you see something obvious I'm missing or some reason I shouldn't do
this, please speak up.

Thanks,
--Michael

wrote:

I have a 50" x 24" 3/4" thick piece of glass that I'm going to use as
a top on a bathroom vanity. The only question is what kind of vanity to
build for it to sit on.

I'm thinking about making a frame and bolting it to the wall and
setting the glass on top of it. Something like the top part in this
pictu

http://www.lineaaqua.net/shop/LineaA...206-c-261.html

My question is how would you make the frame? I don't know what the
glass weighs, but it is pretty heavy. I'm guessing around 100 pounds or
so. The sink is going to sit on top of it, so another 20 pounds or so.

I was thinking about buying some kind of hardwood (cherry maybe?),
making a rectangle out of it a little smaller than the glass, and
bolting it to the studs in the wall. At 50", I should have 3 studs to
bolt it to, correct?

What kind of thickness for the cherry? I'm assuming I'll have to fasten
some boards together to make the frame thick enough to hold that kind
of weight.

Would you countersink bolts through the cherry into the wall studs? Is
there some other kind of fastener that would work better?

How wide should the board against the wall be? How many bolts per stud?

If you see something obvious I'm missing or some reason I shouldn't do
this, please speak up.

Thanks,
--Michael

My only question is, what will you be looking through and seeing from the
top of the vanity? A vanity structure? Some kind of backing material? Frame
piece's? What?

RV

--
"You can lead them to LINUX
but you can't make them THINK"

I think from the top, you would just see the surrounding frame along
the outside of the glass, and your feet on the floor. I'm just talking
about a shelf, basically.

I think you'll be limited to two "arms" holding the glass top out from
the wall, given that a third one in the middle would likely line up
with the sink. Also, it seems unlikely that the final placement of the
vanity will just happen to line up exactly with the studs you'd be
screwing into. You may need to temporarily take off a section of
drywall and add in a horizontal 2x10 or 2x12 to screw into, or else add
a cherry backer board to your frame, which can then screw into three
studs, like you said.

If the depth of the glass top is 2 feet, and the weight is roughly 120
lbs, that should be roughly 120 ft-lbs of torque total, or 60 ft-lbs
per arm. Supposing that you screwed the arms into wall studs with only
one screw apiece, each arm would act as a lever trying to pivot on the
fulcrum formed by the bottom corner of the arm where it butts up
against the wall. The mechanical advantage of the lever would then be
1/D, where D is the distance (in feet) from the fulcrum to the screw.
Thus, if your screw entered the wall 6 inches (0.5 feet) from the
bottom of the arm, the mechanical advantage would be 2:1. In other
words, the screw would be experiencing 60 x 2 = 120 lbs of force
pulling it outward. If the distance was only 4 inches, the advantage
would be 3:1 and the screw would "feel" 180 lbs of force.

Obviously you'd want to size your screws to accomodate a lot more force
than just the torque from the weight of glass and sink. If someone was
to lean heavily on it or even sit on it, especially on the very edge,
they could conceivably add another 300 or 400 ft-lbs of torque on each
arm. Bear that in mind when sizing the screws or bolts. Also, keep in
mind that the wall framing, itself, experiences roughly the same torque
and needs to withstand it. Studs shouldn't have a problem, but if you
add horizontal framing to nail into, the taller it is the better (hence
the suggestion of a 2x10 or bigger). Here, the mechanical advantage is
1/D, where D is the distance between the top nail and the bottom nail
holding the wood in place.

Lastly, keep in mind that the wood frame itself needs to withstand all
that torque without deforming at the joints. If there's any way you
can make it look nice with more of a truss design, you'd be much better
off. The picture you linked to had plane old rectangles for the arms.
I'm guessing they're welded steel or aluminum. With wood, even really
hard wood like cherry, there will be a tremendous strain on the joints.
If you added a single diagonal brace to the rectangle, or a V or W or
a solid wood panel to the interior, it would be MUCH stronger.

Wow, that is good information. Thank you very much.

You mentioned a backer board. That is what I was imagining. So if I
were to use boards 8" wide and put about 3 bolts per stud through that
backer board, how would that affect the numbers you gave me? You said
with 1 bolt, "the mechanical advantage of the lever would then be 1/D."
How does 3 bolts change the equation?

Hi, Michael. I reread your post and realize now that you'd likely be
using solid planks for the arms, not rectangular frames like the
picture showed. And, since you'd be wrapping it around four sides,
yes, you can screw directly into the studs, like you said. The force
calculation remains the same, except, as you said, the force on the
wall is spread out over three studs. Also, of course multiple screws
per stud will lighten the load on any one screw, but keep in mind that
the top-most screw will still experience the most force. If you put
two screws per stud, one above the other, that doesn't mean that the
force on the top one will be cut in half.

Thanks again.

BTw, is this basic mechanical engineering, or is there some resource
for information like you're providing?

--Michael

Being too lazy (and busy with other things) to try to derive the
relationship for sure, intuition tells me the force distribution on the
screws is as follows:

Let's say you have three screws per stud and you place them at 3", 5",
and 7" from the bottom. I think the force distribution is 3/(3+5+7) =
20% of the total force on the bottom screw, 5/(3+5+7) = 33% on the
middle screw, and 7/(3+5+7) = 47% on the top screw.

I think (anyone else please tell me if you agree or disagree), the
total force referred to above is still the same as if there was only
the top screw. Adding the other two screws essentially cuts the force
on the top screw in half by absorbing the other half between the two of
them.

By the way, we're referring to the outward (tensile) force on the
screws caused by the torque. There's also a downward (shear) force
divided equally among them which is simply the total weight of the
vanity and anyone sitting one it divided by the total number of screws.
That force shouldn't be much of an issue.

Lastly, regardless of how many studs you screw into, there will still
only be two wood arms that reach out to hold up the vanity. Assuming
that the arms are held onto the backer board with screws from the back
side, the force calculation on those screws is just the same as before,
except that the total force will only be divided between the two arms.
Having three or three hundred studs to screw the backer board into
doesn't help for those screws.

It's mostly stuff you'd learn in Physics 101. If you can get your
hands on a basic physics book, it'll be in the section with all the
statics problems (trusses, pulleys, etc.). I'm sure a good mechanical
engineering text on statics and strengths or the like would have some
good info. I looked briefly for a good online reference but didn't see
one. Perhaps someone else reading this group knows one?

Here's a really brief out-of-my-head description.

When an object is at rest, the sum of all forces on it must be zero.
Some forces want to push or pull an object in one direction, such as
gravity pulling down on an apple hanging from a tree. Given that the
apple is stationary, there must be an equal and opposite force pulling
up on the apple from the stem. Other forces are rotational, such as
the torque experienced by your vanity. If it's not falling off the
wall, then the screws that fix it to the wall must be applying an equal
and opposite torque.

The calculation of torque is pretty simple; it's merely force (usually
expressed in lbs or newtons) times perpendicular distance from the
point about which the rotation occurs.

T = F*D_perpendicular

When I say perpendicular distance, think of it this way. Draw a line
along the direction of the force you're applying (straight up and down
in the case of the gravitational force on your vanity) passing through
the point at which you are applying the force. Then draw another line
perpendicular to that one which passes through the point about which
the rotation occurs. Depending on the direction of the force, this
second line will not necessarily pass through the point where it is
being applied. The perpendicular distance is the distance from your
original line (but not necessarily the point where the force is
applied) to the center of rotation along the perpendicular line. This
becomes important if the force you're applying is directed toward or
away from the pivot-point, rather than tangential to it. Imagine
trying to spin a merry-go-round by pushing straight in toward the
middle. It wouldn't happen. You'd be applying a lot of force several
feet from the center, but the perpendicular distance (and hence the
torque) would be zero.

In the case of your glass-top vanity, the downward force would not be
directed at a single point, but would be spread out evenly from the
wall out to the outer edge. You could find the total torque using
integral calculus, but in this case we can do it more simply. If we
ignore the weight of the sink for simplicity's sake, we can say that
the 100 lbs of glass is spread out evenly over the span from the wall
to a distance 2 feet away (the outer edge of the glass). This is
effectively the same as if the full 100 lbs of force was applied in the
middle of the glass, at a distance of 1 foot from the wall. Granted,
the weight of the sink might not be spread symetrically, but we can
probably ignore that and just say that the two together are applying
~120 lbs of force at a perpendicular distance of about 1 foot, giving
us 120 ft-lbs of torque. If a 200 lb person sat on the edge of the
glass, they would be applying 200 lbs of downward force at a distance
of 2 feet from the pivot point, and hence another 800 ft-lbs of torque.

Meanwhile, assuming your screws hold and the vanity doesn't fall down,
the screws must be applying 120 ft-lbs of force in the opposite
direction (or 920 ft-lbs if the 200 lb guy is still sitting on the
edge). If the screw is 1/2 ft from the pivot point, it must be
applying 240 lbs of force in order to come out to 120 ft-lbs of torque
(or a whopping 1840 lbs of force with the person sitting on the edge).
Of course there will be multiple screws spread out over multiple studs,
so no one screw will have to survive that much force.

Given your three-screw-per-stud design where the top screw is, say, 7
1/2 " from the pivot point at the bottom of the backer board, you'd
probably want to use screws that can support around 300 lbs of tensile
force. I don't think that's a really big deal; you won't have to use
3/4" diameter lag bolts or anything like that.

You can always do a mockup out of cheap lumber, screw it to a wall out
in your garage or somewhere, and sit on it or jump up and down on it or
whatever to convince yourself that it'll hold.

Josh

In my third from last paragraph above, I wrote "the screws must be
applying 120 ft-lbs of force in the opposite direction." I meant to
say "torque", not "force".

Great information. Thanks very much.