Minimum Posi Joist Sizing for services

[SBMS]

We're looking at specifying our posi joists for our intermediate floors.  For a number of reasons )mainly head height) we want to specify the minimum depth joist we can.  The posi joists we're looking at have the following minimum clearance depths between the flanges:

 

108mm

131mm

159mm

 

We'll be using flexible MVHR ductwork which should be fine with any of these.  I think our biggest service will be any 4" soil pipes with bends etc.  What would be the minimum recommended depth for this?

[crispy_wafer]

if your soil pipes run perpendicular to the joists work out the maximum length of soil pipe into your equation as you’ll need to account for fall.

[Mr Punter]

131mm but make sure there are no strong backs in the way.

[garrymartin]

1 hour ago, crispy_wafer said:

if your soil pipes run perpendicular to the joists work out the maximum length of soil pipe into your equation as you’ll need to account for fall.

That's a great observation to make and something that might have been forgotten. 👍

[JohnMo]

2 hours ago, SBMS said:

We're looking at specifying our posi joists for our intermediate floors.  For a number of reasons )mainly head height) we want to specify the minimum depth joist we can.  The posi joists we're looking at have the following minimum clearance depths between the flanges:

 

108mm

131mm

159mm

 

We'll be using flexible MVHR ductwork which should be fine with any of these.  I think our biggest service will be any 4" soil pipes with bends etc.  What would be the minimum recommended depth for this?

Your structural requirement comes first then bigger if required for services. So get them designed, not a random guess.

[SBMS]

20 minutes ago, JohnMo said:

Your structural requirement comes first then bigger if required for services. So get them designed, not a random guess.

Thanks @JohnMo, a fair point. I'll ask the floor engineer to design to minimum depth, based on the SE arrangement and then increase as needed.  My question was really, if the floor manufacturer can do a 202mm does this even work for services.

[torre]

1 hour ago, SBMS said:

if the floor manufacturer can do a 202mm does this even work for services

Hard to say beyond "It depends" as so much can vary. If you can keep spans small and don't have to run a soil pipe far or perpendicular to joists that will help, and be wary of the amount of deflection.

 

I remember seeing in this thread that a 195mm easi joist may work a bit better than a 202mm posi. 225mm is generally easier though as it works with blocks.