Sep 17, 2014

cantilever loft

All photos taken post-completion
My current room is a bit smaller than the freshman double I had last year, so when I moved my glass desk here, I had trouble fitting a bed in the space. The conventional loft-bed setup calls for posts at each corner, but this setup would either conflict with a desk or have a post right in front of the door. Not ideal.
I had a few ideas for what I wanted in a loft:
  • 6'+ height: I didn't want to hit my head on it, nor did I want my tall-ish friends to hit their heads on it either.
  • minimal diagonal bracing: I wanted my loft out of the way and not be the only thing people see when they walk in my room. Diagonal bracing takes too much space from an aesthetic point of view.
  • open space: there will be no posts blocking my desk or the center of my room. All posts will be as close to the walls as possible.
  • stability: I should be able to hang from all points of my loft. I should be able to climb on and off my loft without creaking or other sad noises. 
East Campus rush provided all the lumber I needed (we use up a lot of wood) and even several 3/4" bolts, which proved useful (though overkill) in assembly. I made the bedbox using 2x8s and 2x4 slats; posts are 8' 4x4s; I had one long 2x6 crossbeam and some 2x4 braces.

Tools used: chopsaw, circ saw, drill w/ 3/4" auger bit.

Drawings from CAD: my working drawings are much more chicken-scratch

CAD model of the planned loft
2x8 lumber make great lofts. They are wide enough that you can screw a 2x4 beam to them, lay 2x4 slats on top, and still have room to insert a mattress. Plus, they're strong and unlikely to have been cut into sub-8' sections at the end of rush.

The bedbox was the first thing I built. Outside, I cut and dry-fitted all beams and slats, leaving room for posts, then drilled pilot holes for each screw to prevent splitting. Slats were spaced 12" apart. I then clamped 4x4 posts in place to drill bolt holes through both the bedbox beams and posts. This ensured that the bolt holes would line up. I stuck bolts through the posts and leaned them against the wall. Then friends helped lift the bedbox to 6'-1/4" as I secured the bolts.

I chose 6'-1/4" to be the height of the loft because I have a 6' tall bookcase. I gave myself a 1/4" error to ensure that the loft would rest on its legs and not the bookcase (1. dorms get upset when their institutional furniture gets locked in wooden structures, and 2. I have no idea how structural that bookcase is over time.)

1/2" plywood added after loft was completed. Here you can also see the 2x4 screwed into 2x8 for slats to rest on.
At this stage of construction, the loft was only supported on three legs. The bookshelf served as a temporary support while I attached the crossbeam and the 4th leg. After I learned an unhappy lesson about torque with falling wooden beams (luckily nothing broke!) I had more friends hold the 4x4 post steady while I screwed the crossbrace to the loft.

Once the loft was secure, it was a simple matter to climb up and screw in bracing. I did grudgingly add one diagonal brace, but it's not evident from the ground.

Crossbrace added
Sideview: One horizontal support further attaches the corner to the post.
A bookshelf nook underneath stores my mess!
(There's a few outlets that prevent shelves from sitting flush against the wall)

Though fun, using the bookcases and slats to climb on my loft wasn't going to be feasible with a mattress, so I quickly attached a ladder to a post. The 2x4 leg of the ladder is slightly offset from the edge of the bookcase so I still have grip room.

The one diagonal brace. I also wedged a piece of wood between the crosspiece and the wall to prevent lateral movement.

I'm really happy with the results. There are no ominous creaking noises when climbing/moving around my loft, and I don't notice it when I'm working at my desk. I've even hung from the middle of the crossbeam without seeing noticeable deflection. Running up and swinging from it, though, does produce sad creaking noises (not sure what my friend was thinking there)

This cantilevered loft design is the first of its kind in East Campus, so I was a little worried about how well it would hold up with use. In particular, I wanted to know how much force I could put on the cantilevered end before strain would break it or before it fell over (there's glass under that!) If it did well, maybe I could hang a swing from it in the future.

Well, I have a CAD model in Solidworks. Might as well put weight on it in FEA and look at the pretty colors, right? 

In Solidworks, I assumed my dimensional lumber to be red pine and Douglas fir and my bolts/screws to be generic steel alloy. (There a few varieties of pine common to Massachusetts, so I just picked one that had material info available.) I then put 300lbs of force on the farthest slat (right next to the diagonal brace) and ran the simulation for static displacement and strain. Below are the results.

The loft moves about the pivot leg
Crosspiece and pivoting post experience the most strain, as expected
Close up picture of strain around bolts

The simulation says that with 300lbs, the free corner of the loft deflects ~1cm (~0.4 in) This number seems reasonable, so I asked Ben and Wesley to stupid-check my results. They said "Oh, that's easy to do" and hanged from the corner.Their combined weight (almost 300lbs) displaced the corner around 0.25".  

So, the simulation is pretty close. Let's try 500 lbs.

More sad colors appear when 500lbs are placed on the last slat. It also appears that even at 500lbs, the loft moves as a whole and the screws/bolts don't fail. There's some scary displacement going on at the last post.

What happens when you put a literal ton of force on your loft? Solidworks complains and refuses to finish static analysis. (it outputs the message "Large Displacements Detected" before failing, so I assume the loft falls over.) The following is its output of "33% load", so with 667lbs.

I see posts lifting off the ground
That's a bright blue.
Solidworks says the entire thing will tip over (sorry desk), but also says the stress on the pine crossbeam exceeds red pine's fiber stress limit (not shown). So, 667 lbs at the corner is the limit before the loft tips over or breaks.

(no, I don't have a physical ton to test this out empirically)