We cut a 28' span in half, had a second floor wall over the center, and attic joists on top of that. The width like yours was 16' (maybe a little less, but ours had much more loading than you do);
The PE spec'ed a 6x8 3/8" I beam. f'ing heavy SOB, and the beam weighed a lot too!
Casey

Without adequate footers/load spreading the posts can actually crack the concrete floor they are resting on (concentrated point loads).

Brickeyee,

If I use the steel I beam, I plan on cutting a square out of the floor, digging it out, and pouring it a foot thick. The ground around here is solid clay.

I wouldn't consider this a critical application since worst case for an undersize beam would be what I have right now - a bouncy floor. I was thinking that maybe a 6 inch or 8 inch beam with a post at the wall and another maybe a foot or two from the other end giving me a 14 or 15 foot span. It seems like even an undersize beam would be better than nothing at all.

Do you have any thoughts on the size of I beam and the number of posts needed?

Why steel? The cheapest and easiest would be wood. You can go with (2) 2x12" at this span. Put a post mid span and you could get by with (2) 2x6"

I don't have a lot of headroom in my basement. Bottom of joists are at most 7 feet. I thought a smaller (6" maybe) steel beam would be less intrusive.

Should I scrap the beam idea and just glue/screw 2X4's to bottom of joists as I have seen Brickeyee recommend? (I don't like the idea of messing with steel straps)

Make it a flush beam so you don't lose the headroom.
Casey

What do you have for floor joists and a sub floor? 2x8's with diagonal boards? Is there any bridging in between the joists? Aside from span there are other conditions that could make your floor springy.

How involved is the remodel?

If you are going to glue and screw, plywood to the side would be a better choice. Just lift any sag before you secure it. 12' is readily available and should suffice but if you call an engineered floor manufacturer you can probably order some OSB/waferboard from them at the exact length and width which will work equally as well. It would be like doing a flush beam for each joist. Whatever you do if there is no cross bracing you will want to add it.

Jey, thanks for the reply.

The joists are 90 year old 2X8's. Dimensional lumber but probably stronger than today's 2X8's. There is cross bracing between them. The floor isn't all that bad, I just hear the pans in the bottom of my stove rattle a lot when walking hard across the middle of the floor. Sistering or adding OSB to sides is not an option, since I have electrical running perpendicular through the joists (conservatively done so as to not contribute to my issue).

The remodel is not overly involved. The subfloor is 1X6's with 1X4's hardwood on top that I plan on refinishing. New kitchen cabinets with, hopefully, concrete countertops. The concrete countertops are what is making me think I should do something about the floor.

If I thought 2X4's glued and nailed to bottoms of joists would do the trick, I would go that route. Otherwise, a beam would be easy but would leave me with a post in the middle of the basement floor. I checked into a basic 6X4 ibeam. It was about $120. I don't know how much more an 8X6 would be, but wouldn't think it would be more than double.

You can "notch" the ply/OSB and still make a huge difference. Still stronger than nominal lumber. It does not have to go the whole 13-6 but I wouldn't go any less than the twelve. Unfortunately ever since the hit and run builders and big orange boxes came on the scene in full force it is harder to find people who stock it. Call someone like Eagle and find out who they sell it to in your area and see if they will sell you a sheet or two. May even get it for free as it is a "waste product" for commercial and especially government jobs.

Electrical and plumbing are not an issue as much as existing cross bracing. The best way is to pre- drill the hole at the appropriate location and make a saw cut to the hole to get it over the wiring. The other thing that would make a noticeable difference is scrapping the subfloor.and replacing it with plywood but that doesn't really seem to be a viable option for you. Chances are good it is a clear vertical grain fir, cedar or white pine so whatever you do should you decide to remove and relay the hardwood on a new plywood subfloor don't send it to the dumpster.

While it can be done by one person two is better three is best. Pull one side of the cut towards you push the other away to get the wire in. Hopefully when they replaced the knob and tube they at least went higher than I have shown. Get all pieces in place and lift the floor until everything is level upstair before applying adhesive (to the top edge too) and pinning (Screwing or nailing). The majority can be nailed but the area around the cut should be screwed or pinned with ring shanked, resin coated nails applied with an air,powder, gas or electric actuated nailer. The resin is worthless when hand nailed. The squash blocks are any solid material that will fill the void. You can also use an OTC flitch pate (Simpson or eq) if you uncomfortable with the slice.

The other and often easier way is to just disconnect the wires from the closest junction box, install the pieces and then reconnect the wiring.

Either way you will need to do some plumbing and electric and to remove it from your way would be well worth the hour± to remove and replace.

Kitchens are not cheap and wether you are doing a custom kitchen or assembling it with OTC products it is a considerable investment and a major part of the value of your home. A Ferrari on flat tires performs worse than a Trabant on the same. 2x8's and 1X subfloor wasn't a wild guess and the reason you want to use a quality adhesive on the top edge as well. I am certain from experience that that (1x subfloor) and the lack of cross bracing are a big part of the springiness. Appliances are much lighter today than they were in the past.

Make certain you always shut off the electric to those lines before doing anything.

{{!gwi}}

Hope you project comes out well.

Most large lumber yards have an engineering staff that will make recommendations for your fix.
A detailed drawing of existing conditions and your objective should be enough for a few qualified solutions.This service is usually free when you purchase through them.
Remember,lumber yard not box store or home improvement center.

I didn't read the whole thread but if you are reinforcing a joist to decrease deflection (bounce) it is only necessary to add strength at the bottom portion of the joist so notching full height reinforcement is unnecessary. Add a 2x4 along the bottom of one side of the joist or add a steel strap along the bottom edge with more nails/screws as you move out from the middle toward the ends.

While that is correct we are all giving advice based upon assumptions. I'm assuming Steve doesn't weigh in at 500+ pounds so any movement of the joist is lateral not vertical and he can tell by the difference in the patina on the underside of his floating "subfloor" immediately next to the joists. There is no doubt in my mind that even if the subfloor was nailed to the joist every inch the nails have rusted and the holes have enlarged and the floor is floating. The house hasn't had a floor system for probably about 100 years. He has one pile of lumber supporting another pile and that is the reason why the joists have sagged in the first place. They do not spread the load and instead each joist supports whatever live or dead load is applied on it's own because the subfloor just slides over the neighboring joists.. That is one of the main reasons that they began to sag in the first place. He cannot secure the subfloor to the joists because a finish floor is already in place. Plywood has more structural strength than 2x's and by notching the "flitch plate" (or better yet just remove the wires) to make it full height to the side with an adhesive to the top edge will secure the subfloor and will solve both problems. The "springiness" and deflection is between the joists not on top of them. Unless of course Steve [is] weighing in a 1/4 ton. His other option is to pocket screw the joists to the subfloor but that is a tedious and time consuming prospect that won't correct the sag at the same time.

Well, first of all, I only weigh about 165 lbs. I wanted to set the record straight!

I am not sure that the springiness is just between the joists. If I walk "heavily" across the kitchen, 3 or 4 joists from the range, the pizza pans stored in the bottom of the stove rattle pretty badly.

I might try temporarily supporting several of the joists with 2x4's from the floor and see what effect this has.

I had hoped to hear from someone who has nailed and glued 2x4's to the bottom of the joists (so it looks like an inverted T). I have heard of this before. I would rather do this if it would work instead of the steel strap idea (the straps seem like a pain since there is so little tolerance and so many screws are needed. I would worry this would split my joists). If I went the 2x4 route, I would use polyurethane construction glue (that doesn't come off my skin for a week if I get it on me!) along with nails or screws.