I'm installing a tile wall for the new shower in a remodeling. The alcove walls have two interior walls. And one exterior wall. The exterior wall has stud spacing on 16" centers. The interior walls have 24" centers. I plan to install 1/2" Hardibacker for the 12"x12" tile. The Hardibacker installation instructions specify no more than 16" center stud spacing. Two of my three walls will not meet this spacing. My questions are:
1) What is the reason for the 16" limit? Is it structural support for the cement board, to prevent "bowing" of the backer board, or is it to keep the wall stiff to reduce flexure?
2) If it's for structural reasons, is my only option to put additional studs in place, effectively then having studs with 12" centerline spacing. This would not be easy, as the existing studs are "notched" to accept a horizontal 2x4 that's in place about a foot off the floor, and runs the length of the wall. Also, the air conditioning duct slightly overlaps the upper side, which would require the new stud to slide behind it. Not sure how this could be done with all the existing things in place.
3) If it's for rigidity reasons, are there other options, such as using 1"x2" nailed horizontally between studs to tie them together?
4) If it's to prevent bowing of the material, would the same idea as number 3 above accomplish the same goal?

We're in the Phoenix, AZ area, and most houses here were built relatively recently, and the construction is likely uniform. I can't imagine everyone who wants to put new tile in place has to re-frame to accommodate this minimum spacing requirement. Thanks for any help you can provide.

Yes, its because of the weight of the cement board, thinset and tile. Option 2 looks good and I'd cut out that 2x4 since I can not picture any reason for it to be there to begin with. Do you know why it's there?

I've done some more checking since originally posting, and here's what I've been told:
Looking up the weight/square foot of 1/2" drywall and the Hardibacker board, there's very little difference in the weight. Acccording to the Hardi website, their weight is 2.6 lbs/square foot, which is close to normal 1/2" drywall weight. The original wall was 1/2" sheetrock with tile on it, so the existing stud spacing seems adequate for supporting the weight. Talking to some construction people locally, they say that in Phoenix, the 24" spacing is quite standard for interior walls. Their opinion was that the 16" spacing is more for lack of deflection if someone were to fall against the walls than for structural load. As mentioned, the load would be very close to what was there originally. I really don't know the reason for the horizontal 2x4. It's lower than the original tub lip was, so couldn't have been for that. General consensus is that some horizontal support pieces between the studs would accomplish that same purpose, rather than trying to put new studs in place.

You are getting a lot of info and some may be correct but much is wrong. Lets start with what James Hardie says about his product. 16" on center and weight is 2.6 lbs/ft while standard drywall in a home is 24" on center and 1.6 lbs/ft. If your old wall was 24" on center with drywall it was under built. Hardoeboard is 50% heavier than 1/2" drywall. Drywall should never be used in a wet area, greenboard was once used but no longer. If Hardie says 16" on center I'd follow his directions. Anytime you cut corners on a manufacturers installation instructions you are on thin ice.

I just bought a home in AZ and wanted tp make bathroom handicap assessable , inside wall beside the stoolhas no where to mount a hand grip or rail . And there is a shower door installed on other side and that's loose , because nothing to mount it to . Is that the way they build houses in AZ ? Nonthing in the walls ?

Don't know how they build houses in AZ but rest assured that there is something in the walls, just not where you want it. I agree that grab bars and handrails should be securely mounted. Cut out a strip of drywall at the height you want to mount the rails and install blocking between studs. Repair and finish the drywall and then install handrails.