Oval shape earthbag, need buttressing?

For an oval shaped earthbag, measuring 15ft x 30ft, would I need buttresses on the long wall?
I was looking this plan by Owen Geiger, and it doesn't seem like there's a buttress, even though the floor plan size is much larger. Thanks!

http://www.earthbagbuilding.com/plans/englishcottage.htm

hello rahmi hidayat,

if you'll notice, the plan isn't for a dome roof made out of earth bags.....the roof is made of wood and other comparatively light weight materials.   the roof is also symmetrically holding itself laterally together and not putting outward force on the wall but rather downward force on the wall

no buttress needed if the wooden roof doesn't push laterally/horizontally on the tops of the walls

Thanks Orin!

The cottage in the link only has about 16' of straight wall length, so the absence of buttresses or cross walls does not necessarily translate to a plan based on a 15' x 30' rectangular foundation. Curving in the corners so the top is oval might be enough; you would have about 15' of straight wall there.

Thanks Glenn, so to be clear, if an oval structure with 16ft straight walls doesn't need a buttress, then our oval structure with 15ft straight wall also doesn't need a buttress, correct?

Probably not, depending on the thickness and height of the walls. I would make sure yours are at least as strong as the ones in that plan. (As wide and no taller.)

By the way, the common practice around here is to have one long thread for a given project, rather than starting a new thread for each question that comes up. This keeps the information together for easier reference to future readers, and allows responders to know the background for a question, for instance, that you have a 15' by 30' rectangular foundation that you want to build on.

Got it, thanks!