What's Your Opinion Of This?

[cabinetman]

On another forum, a member is planning a cherry trestle table. The top will be 6/4 Cherry, 42" wide, 96" long. The span between both legs will be 60". The question is whether the center should be supported from sagging.

A member replies to this as follows:

"This depends on a lot of factors. I doubt you'll get any sag in the middle of the table but I would support it anyway and maybe even shim it slightly so that the table has a very slight crown. The reason is this: our eyes play tricks on us. A long table can be perfectly flat with a straight edge but our eyes will see a sag. On long tables you actually want to build it with a very slight crown so it will appear flat. The Greeks knew this when they built the Parthenon, the middle of the floor of the Parthenon is actually about 6" higher than the edges to account for our eyes to see straight lines as dipping in the middle."
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[Larryl]

While I have no actual knowledge of this, I have also read of this effect upon the eye. I can say that I have had to look at things from several angles and measure with a straight edge when making large flat projects to convince myself that it was straight and flat. Sixty inches is a long span and a slight bow would probably make the top apprear flat. IMHO.

[leehljp]

Not all people see everything the same. We have a 78 inch table with a 16 inch insert to make it 96. It has right at 1/8 inch sag from end to end and I notice it EVERY TIME we put the extension in. First time I noticed it, I put a tensioned string to it and measured. I would probably notice a 1/8 inch rise too.

I have looked down 8 foot 2X4s and noticed that much off, that much bow, and that much twist.

[docrowan]

The Greeks called it entasis - swelling with strength - similar to the bulging of a muscle as it is strained. Can't say if I've ever noticed the effect myself, though.

[gjat]

I don't see a sag in my dining room table, and it's flat. I do know about the Parthenon's "bow", but that is on a much larger scale.

I think that the scale of a 5' table would not need or give the illusion.

[cgallery]

Boy, I'd hope that I could put any crown up on the top (depending on how both sides of the top look), but I don't think I'd want to shim any crown into the top.

But yes, sagging tops stand out to my eye. Perfectly flat tops, depending on perspective, can appear to sag. But I'm not sure a 5 or 6' long top is long enough to fool my eye.

[drillman88]

My Dad is a brick mason when he builds tall curved brick fences he always put a slight lean in them.If they are built square they look like they are falling over.He also always crowned any long walkways and broken tile porchs.It has to do with drainage as well as appearance.
After someones been doing something 50 years I am not going to doubt him.

[rnelson0]

I believe it. I have a 7' pool table and when you sight down the cue, everything looks a little off - the cue looks warped, the table looks bowed, and I never make that long shot The effect does exist, but I am not sure over 5' whether it needs to be offset and if so, by how much.

[cabinetman]

The Greeks called it entasis - swelling with strength - similar to the bulging of a muscle as it is strained. Can't say if I've ever noticed the effect myself, though.

IMO, it's a supposition that the use of entasis was to correct or create an illusion. It may be a word used for what was thought the results would create. When an architect or a designer comes in to check on their work and wants to lay an 8' straightedge along the length of a table top and if a crown is detected, I can't envision explaining eye trickery to him or her. It's actually comical at times because they don't usually build anything, but they sure know how they want it done.

The idea of creating a crown may or may not induce trickery to the eye, but in table tops IMO they should be fabricated flat and hopefully if the total design and fabrication was done properly, they will stay that way.
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[Mr__Bill]

I just envision that errant pea making it's way to the edge.....

[pelligrini]

I really don't think it would be beneficial to a tabletop, probably quite the opposite. Just think how a big platter would sit.

As far as those greeks go, I bet the foundation contractor screwed up and was using a bad level. All the other subs did their thing on top, just to get it to work, complaining about a bad foundation. The builder made up the entasis concept so the owner wouldn't sue.