I’ve always heard that the center of a floor is the strongest part in the house as opposed to the edge of the floor that holds up walls.
I’m talking about a room 20x20, like say…a large living room. Is this true for the most part? I know I could get really technical and maybe my question is even a little vague, which is why I say, is the center of a floor “usually” the strongest.
I’m assuming you mean strongest as in withstanding the most downward pressure/weight? So the rules out anything not supported by a joist. So is the question, Where along a joist is the floor strongest?
the (first) floor of my house is cement. So I’d expect it to be equally strong everywhere.
Doing a quick mechanics 101 sketch on some scratch paper shows that the most torqing on any support would be in the middle. Less at the points nearest to the verticle supports for a given weight bearing down.
The edges would be strongest, but the middle would be designed to support the maximum anticipated load, plus a safety factor.
I talk with lots of guys who set up large aquaria. The consistent conclusion is that they should be placed perpindicular to the joists against a load bearing wall.
After thinking about that, I thought, “of course”.
Common sense would lead us to think that the closer you are to the main beam(s) and the more joists the load extends across - the more stable the load will be.
And I think common sense wins here.
Dingdingding!
butler1850, you’re also correct, but you’re wording is a tad bit vague. The middle of the floor will be able to support the design load, and the edges will be able to support more than that.
For somebody wondering what these guys are talking about, think of a diving board that’s supported on both ends (or a 2x4, or anything else that’s cross section is small compared to its length). If you stand on it where it’s supported, it won’t deform much. However, if you stand right in the middle, it will deform quite a bit. This is because you have more distance (and hence, more material to deform) between the support and the load.
And Uncommon Sense loses! 
The same rule applies if you have lots of bookcases. Align them along the walls if possible. All those gorgeous picturesque libraries in old homes obeyed this rule, although other factors such as the ease of building them into the walls rather than making them free-standing of course played a major role.
“Load bearing wall” is probably the operative word here. In older houses (like, sigh, mine), they frequently spanned fairly large areas with undersized joists (or huge unsupported timber joists). These joists would, over the years, sag. The least sag would be at the foundations and, at the edge of the room farthest along the joist, you’d have the most sag. Drop a marble in my dining room and it will be going pretty fast by the time it hits the north wall.
In modern construction, they will have pillars supporting the joists every x number of feet and the joists will be sized to allow only a very small amount of deflection between pillars. These joists are sized for normal loading – if you’re going to be putting in a hot tub or anything else of unusual weight, you’ll have to increase the size of the joists. If you’re talking about a second-story floor or higher, the weight has to be transferred through a load-bearing wall on a lower floor. In older houses, people were often cavalier about removing these load-bearing walls, so you’ll often see some significant sagging in the ceilings.
That said, while a floor will be strongest at the foundation wall or along a row of supporting pillars, in modern construction, I don’t know how meaningful the word really is. It would be rare for a floor to fail under any reasonable loading.
Many years ago, my wife and I were looking at houses to buy. In one, I noticed that there seemed to be a lot of small holes in the floor, possible caused by some sor of insect. So I jumped up and down on what looked like the worst spot – and managed to go through the floor. So floors can fail under reasonable loadings. 
It depends on your definition of fail. Failure in structural design is normally where deflection exceeds x% of span. Absolute failure is barely even a design consideration. Floors are designed so that the deflection of the joists under the design loads will not be so much as to cause cracking in the finishes (e.g. plasterwork ceiling below). I’ve seen some pretty ‘bouncy’ floors in old buildings that you could actually feel deflect underneath you as you walked, but even they would have supported a hell of a lot more weight before they actually fell.
Regards the OP, yeah close to a loadbearing wall (below*) across the directions of the joists (i.e. in the same direction as the floor boards) is as best a rule of thumb as you are going to get. But it is only a rule of thumb, it all depends on the specifics of the construction.
Note that not all walls/floors are created equal! What is ok against a room in one wall may not be ok in another room (even if it’s the back of the same wall). Note also that for short spans the centre may actually be the best bet although it would need to be quite a short span for this to be true.
*this may seem obvious but the rule of thumb in the OP has some merit where the floor is actually bearing the load of the wall above rather than the opposite way round. Not sure if this was just a wording thing but it certainly can be the case that a wall is built onto a floor with nothing below. The pont is, what the layout on your floor is is irrelevant, it’s the layout of loadbearing walls below that counts.
I assume what you mean is the floor boards? It’s a good point actually, when i think of floor i normally think of joists, but the strength of the the flooring should also be considered. It’s normally not an issue in healthy timber as most very heavy loads are also sizable and hence span several joists. However if it was a very heavy item with a small footprint (or three or four small feet) you should be careful to position the feet as directly over a joist as possible in addition to the other considerations.
Yes, I could see from the nails where the joists were, and I was jumping between the joists. If I’d broken a joist, that would have meant a much more serious problem.
For something like this large, it’d make sense to look under the floor to see where the support is. In my house, the living/dining room is about 13 by 26, but there’s a steel I-beam in the basement splitting that in two. Right above that I-beam can support a lot of weight, whether it’s near a wall or in the middle of the room. A 20 by 20 room may have (probably has?) a sturdy beam below it somewhere, to avoid having joists spanning 20 feet.
Can you give me an example to help clarify this? From the (rediculous amount of) shear-moment diagrams I had to make in statics, the shear will be the same regardless of the length, and the moment will increase proportionate to the length. Are you saying that if, in a short beam, the load is placed near the edge, the connection at the end in question may have to support more than its share of the shear, even though the shear in the actual beam is uniform? Yet if placed in the center, both connections will share the load equally, and be able to support the load?
If this is the case, we’re getting dangerously close to a factor of safety of 1!
Yes, that’s it exactly. I’m not sure what you mean by shear in the beam being uniform. If a load is placed off-centre then the shear will be greater at the end nearest the load.
It’s unlikely to ever be a problem unless you were putting on a huge load and without doing the maths i’ve no idea of what span vs beam section could possibly fail under shear with an end point load but not due to bending moment of the same load centre span, but i think it’s possible.