Floor Hardness vs. Strength
Every year, a few hundred waterbeds, large aquariums, and overgrown rock collections fall through upper story floors because people fall prey to a common misconception: They believe hard floors are synonymous with strong floors. The truth is, the two are not very well correlated. Strength and hardness are mutually exclusive materials concepts in general. This is especially important in flooring.
Most local building codes require floors in residences be designed to support an average of 40 lbs. / sq. ft. Regardless of whether a floor is constructed of a hard or flexible material, it is likely designed to the minimum specification that meets code.
Concrete Floors
To illustrate the point on hardness vs. strength, consider a 1” thick slab of concrete. It is very, very hard. In fact, it’s just as hard as an 8” thick slab. But the 1″ slab isn’t nearly as strong. In fact, it’s less than 5% as strong as it’s 8” counterpart.
So, when you’re standing on a concrete floor in, say, an apartment, you don’t have any assurance that the floor is stronger than a similarly built wood floor in another apartment. The concrete floor will certainly be harder, and therefore feel stronger. Unfortunately, it isn’t stronger.
Steel I-Beams
The same principle holds true for steel I-Beams. A 4” I-beam is far less strong then an 8” I-beam. Of course, both are equally hard, and a floor sitting on top of them will feel the same underfoot.
Wood Joist & Plywood/OSB Floors
Perhaps the most deceiving of all floors are plywood/OSB over wood joist floors. When weight is first placed on a wood joist structure, the structure bends (e.g. if flexes, or feels a little weaker). As additional weight is put on the structure, the floor actually bends less as it reaches the maximum stress point. Wood floors that are near failure hardly flex at all (as a result, they feel harder).
What do you think? Did this surprise you? Have you ever seen a floor that failed due to being overweight?
Photo by Paul Schultz
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9 Responses to Floor Hardness vs. Strength
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April 10th, 2008 9:42 pm
You just reminded me that I forgot to put “no water beds” in my lease for tenants.
April 10th, 2008 10:18 pm
Joanne, We have a lease with tenants and specify no “extra heavy items” including waterbeds, large aquariums, weight sets, etc. Hopefully your tenants will abide by your wishes even without it in the lease. Thanks for stopping by!
April 10th, 2008 11:34 pm
Part of the reason I’m redoing the basement family room is because the PO left a huge 125-gallon aquarium that I’m just itching to set up. But there’s no way in heaven or hell that thing is coming anywhere near my first or second floors. Basement or bust!
I’m semi-wary of having the 38-gallon aquarium up here in the office with me. But I’m pretty sure that’s not a huge problem, plus it’ll be moved once the family room is done too.
April 11th, 2008 6:36 am
Andy, I’m going to write a post soon about how I learned this lesson the hard way. Essentially, I almost dropped an aquarium through the third floor in my first apartment after college. Fortunately we caught it before it went through (the floor had separated from the wall). I have that same aquarium on the first floor in my current home now, but I sistered new joists to the existing ones to support it. It’s now spanning 11 2×10s. I have some good aquarium tips if you want an awesome freshwater tank.
April 11th, 2008 12:04 pm
Well this is something that we’d never really thought about until recently. You see we have this billiards table that we would really, really like to put in our upstairs gameroom…
But we don’t know how long it would STAY in the upstairs gameroom before it unceremoniously lands in the living room below.
All I know at this point is that our second-level floor consists of a lot of metal beams in an X pattern. Know anything about this?
April 11th, 2008 6:11 pm
Anita,
The pool table is the equivilant of a waterbed.
While it may not be as heavy as a waterbed there is a problem of location. Most waterbeds are adjacent to a wall. This makes the span supporting the bed shorter.
The pool table, though it may weigh less, is most likely located in the center of the floor joist span; that is, centered between the walls, its’ weakest point.
The 40lb per foot dead load mentioned by Fred is accurate.
Take the weight of the table divided by the sq ft area it takes up to see where you land. (Small pun).
Pool tables, like waterbeds, work best in basements!
Good luck,
Bill
April 12th, 2008 10:12 am
Wow…..where to start…first off I’m a structural engineer….so I’d like to say some of what you said isn’t exactly technically correct. Your comparison of hardness versus strength is right on. However, your statement about concrete is actually a bit misleading. Reinforced concrete is actually quite elastic at failure. Now if there’s no rebar in it then it is brittle. Also, whether it’s steel, concrete or wood, the strength of a beam is not proportional to it’s depth. In fact, beams tend to be stronger by a power of 3 with respect to depth. When in doubt…consult an engineer! This post was a great reminder for everyone about structural integrity in houses!
April 12th, 2008 11:27 am
Todd,
Thanks for the point about the elasticity of concrete with rebar in it. I will correct the post to remove the statement about brittle floors.
I actually knew about the x^3 strength ratio when adding height to the beam; I just didn’t write about it here. (Your comment sounded like you were correcting an error in my post that isn’t there).
Thanks for weighing in (and setting me straight on the concrete).
Fred
April 12th, 2008 3:04 pm
Fred,
I didn’t mean to say you were in error….just thought the post was not clear. Actually I applaud your post…most DIY folks get them selves in trouble when they play around with structure. I only wanted to point out the importance of consulting with someone on structural problems.