15' isn't bad, though you may want to put blocking between the i-joists just for any vibration.

The biggest issue with second floor laundries is how handle any overflow from the washer.
Watt's make some turn of equipment, along with others.
Make sure you have something.
No drain pain can keep up with a broken supply.

I am required by the local plumbing code to build the area under the washer the same as a shower floor--complete with rubber membrane, drain, curb and tiles.

Raam - Stupid question... how do you block an I-Joist? Blocking solid lumber is pretty straightforward.. but an I Joist? Do you block just the top flange? The webs?

Brickeye - Hey! Don't know if you remember, but I used to post here as "Saltbox-NH" a few years back.. I owner built a reproduction Saltbox colonial. Sold that, now I'm building again.

The washer and dryer will be sitting on a pan connected to a dedicated drain. It's a high efficiency washer so the total water it would ever have at one point is like 13 gallons... I think it also has some sort of overflow sensor but I'm not sure.. I'll have to RTFM. There will also be easy access shutoffs for hot and cold water supply in the laundry room itself and we are doing Pex/manifold so there's an additional shutoff in the basement.

Worthy-

No rubber membrane, curb or tiles... seems a bit overkill vs. what a pan w/drain can provide in the rare occurance of an issue.

Blocking is easy and many ways to do it. Cut of lengths of I-Joists to fit between the 16" OC and toe nail them, cross bridging with solid wood, metal straps, etc, etc.

Check out www.apawood.org

Vibration is a killer on any construction; particularly that which can get a harmonic resonation that builds and builds like what a washer is capable of. That is why it is good to vary the layout of supports. In other words add your blocking in a slightly irregular pattern. Also, make sure your platform is tied into the walls and down through the floor into the I-joists otherwise it will develop small shear stresses which could cause finished materials to crack.

As I've stated in other threads there are lots of water leak systems out there. Watt's was mentioned above, others include FloLogic and WaterCop. Manual shut-off valves do little good when you are not home or unaware of the leak - get an active one instead.

I like Worthy's idea of building a shower floor pan under the washer. A cheap-o little drain pan will not handle overflow. You need something that can really channel the water away in a hurry.....a 2" drain line at least.

I'd be careful. There's more to consider than just the machine loads. If it's above the foyer there are also load points from the roof but you can actually benefit from that load. Try to design a short balcony into the front porch and you'll benefit from a cantilever over the front exterior wall.