Floor live load for church storage loft 1

[BridgeSmith]

This should be a simple question, but I'm not very familiar with the building code. The loft is just under 200 square feet, if that matters.

 

[JAE]

Light storage per the IBC is 125 psf live load.

If this is an existing floor then it can be back-checked for actual capacity.

Based on what is discovered, I've seen cases where the owners post signs all around stating the maximum live load capacity.

 

[BridgeSmith]

It's not existing; I'm designing it, and will probably end up building it. I need to keep the depth to a minimum to have adequate clearance above and below.

Is the 125psf the ultimate load? Using that load, what strength would I use for Douglas Fir 2x joists?

 

[JAE]

125 psf is a service-level code mandated live load for rooms purposed for light storage...whatever that term means. There is also a "heavy storage" category at 250 psf - more for large warehouse floors.

For a church (a non-residential building) and the room is specified for light storage then the 125 psf capacity might be mandated by the local building official.
Conversely, as I said above, you could design for what you calculate/determine is the most likely maximum weight on the floor and then post the live load limits on the door and in the room.

We've done similar things with those pesky not-really-a-storage-room spaces as 125 psf is fairly robust if the church is storing boxes of Christmas decorations. If they are storing boxes of paper files three or four boxes high I might be inclined to go 125 psf.

Is the 125psf the ultimate load? Using that load, what strength would I use for Douglas Fir 2x joists?

Not sure what you are asking here. The IBC and NDS do have span tables for DF 2x joists of various spans, spacing and loads.

If it's just a plywood floor and ceiling below I might use 15 psf dead load with whatever live load you end up with.
Joists for floors are usually at 16" o.c.

 

[BridgeSmith]

Conversely, as I said above, you could design for what you calculate/determine is the most likely maximum weight on the floor and then post the live load limits on the door and in the room.

We've done similar things with those pesky not-really-a-storage-room spaces as 125 psf is fairly robust if the church is storing boxes of Christmas decorations.

You nailed it - it's for Christmas decorations (mostly trees), so I will probably stick with the 100psf I originally assumed when I laid out the framing plan, and post it, if I can.

Not sure what you are asking here. The IBC and NDS do have span tables for DF 2x joists of various spans, spacing and loads.

I might have to get a copy of one of those, then. I was expecting to calculate the applied stress and compare to a bending stress limit. The joist span varies across the loft (from 10' to almost 13'), so I was going to change the joist spacing make the 2x8 joists work for the different spans. I just wasn't sure if the design had shifted from the 'ASD' approach of the 1999 NDS I have, to an ultimate strength or LRFD design approach, with higher stress limits.

 

[JAE]

I wouldn't alter the spacing as that tends to make the floor sheathing layout a nightmare. 12" o.c. or 16" o.c. or 19.2" o.c. or 24" o.c. allow for the 4' x 8' sheets to work out with minimal cutting and waste.

Usually for floors it's 16" o.c.

For design - try this:

https://awc.org/calculators/span-options-calculator-for-wood-joists-and-rafters/

We typically specify Spruce Pine Fir No. 2 in our area as it is generally available and a more conservative, minimum material vs. Douglas Fir or Southern Pine.

Try the 15 psf DL with your 100 psf LL.

 

[BridgeSmith]

I wouldn't alter the spacing as that tends to make the floor sheathing layout a nightmare. 12" o.c. or 16" o.c. or 19.2" o.c. or 24" o.c. allow for the 4' x 8' sheets to work out with minimal cutting and waste.

I was only planning on using 12" and 16" oc. The sheathing layout (and the framing plan, for that matter) are already a nightmare The 3 walls are all at odd angles. Relative to the opening across the front, the 2 ends are at 7o degrees and change, and the back wall is skewed 12 degrees and change.

Douglas Fir is the most common lumber here for everything bigger than a 2x4x8'.

Since I'm building it, I'm going to calculate the actual dead load to use in the calcs.

Thanks for the link; I'll check that out.

 

[JAE]

There's probably all sorts of 2x joist tables and online quickie software out there.
We typically use our own in-house spreadsheets and/or model in RISA3D/RISAFloor to design wood framing like that.

 

[BridgeSmith]

I'm sure I could use joist tables, but I'm used to calculating capacities using section properties and stress limits, which I'd prefer to do, so I know what's going into the design. At both ends of the spectrum - span tables or FEA, it's a bit of a 'black box' solution.

 

[JAE]

This would give you what you need if you want to hand calculate things:

https://awc.org/publications/2018-nds/

 

[BridgeSmith]

Thanks for the link, JAE. With that and the supplement, I think I have what I need. as I looked at what's locally available, I'm leaning towards using the #1 Southern yellow pine available at one of the local big box stores, unless I can find #1 Douglas Fir at a lumber yard. (Big box stores only have #2 Doug Fir).

I might still have to double up a few of the joists, or go with smaller spacing for some. Is there an advantage to one option over the other? I'll probably be notching them 1/2" to bear on the flange of the W8x15 steel beam that supports them on the open side, and I'll likely use a ledger with a 2x cleat on the other end, since I don't think I can make hangers work with the 78 degree angle at the (concrete block) wall.

 

[BridgeSmith]

Here's the framing plan I had worked out before. With the slightly lower stress limits of the joists available, and using the 100 psf live load, which is higher than I originally designed for, looks like I'll have to adjust the joist spacing down some.

[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1696955792/tips/loft_framing_plan_rameeh.pdf[/url]

 

[JAE]

Have you checked the W8x15 for deflection? Usually we try to keep to about 45% to 50% of the span for minimum steel floor beam depth.
In other words, with your 19 ft. spa, we'd want to see at least a 10" beam...especially for 100 psf live load...keeping the deflection to L/360 for LL only.

From a quick hand calculation I'm getting a 2.3" live load deflection vs. an L/360 limit of 0.63".

 

[BridgeSmith]

From a quick hand calculation I'm getting a 2.3" live load deflection vs. an L/360 limit of 0.63".

Using my estimated clear span of 18.5', and tributary live load to the beam of 660 lb/ft, I'm getting 1.25" of deflection, which is about twice the L/360.

Now that you've got me thinking about it, with some other changes I've made, I think I could make a deeper beam work. I just have some other possible clearance issues to check on.