Box-beam or I-beam question

[Aussiebushman]

I'm working on my own so lifting and bolting a steel beam into place would be difficult and potentially dangerous, also welding in this shed is not an option due to the fire hazard. Construction of an engineered timber beam will be simple because the lower section will be bolted into place first, then the vertical web(s) and upper beam added.

I'd appreciate it if one of the engineering experts here can advise whether a timber box beam or I-beam would be the better choice.

Dimensions are overall length 5 metres (15') depth 300 mm (see below) and the beam would be used to support a chain block lifting approximately 300 Kg (660 lb) shackled onto a welded steel "trolley" moving on wheels along the upper beam surface.

The box-beam option would have internal frames made of 75 X 120mm (3" X 5") Douglas fir with a 9mm ply skin either side.​

The I-beam alternative would be easier to build with a single 9mm ply strut rebated into the same sized top and bottom frames as above. It would be possible to increase the depth of the ply strut to say, 450mm as there is plenty of space to do this. I would epoxy the ply into the rebates.​

As said above, comments from engineers about the relative component sizes would be much appreciated

Alan

 

[Baby Grand]

"Better" in what sense?
Easier to make, easier to install, highest strength/weight, lowest cost? Easier to run electrical/plumbing/HVAC service around?

When you ask a group of engineers a question, be prepared for a lot of annoying questions in reponse.

Based on your description, both beams are relying on the plywood as a shear panel for bending strength. It's important that this panel is continuous (one piece) over it's entire length for it to function as a shear panel. From what you have stated, I'd go with the box section. It has twice as much shear panel and would less apt to buckle.

You have decided to use wood/wood composite instead of steel. Why not also consider a combination of these materials? Considered using a flitched beam - this seems like a good situation to use one.
-Jim

 

[Egon]

The box beam will have better torsional resistance.

I'm not an engineer.

 

[Argosy]

One thing I would be really concerned about is the tall narrow section with the trolley on top. If your lift isn't perfectly vertical each and every time it will twist the beam.

 

[PILOON]

I would laminate 3/4" plywood strips (12" wide), joints staggered glued and screwed from both sides.
to make up a 4" x 12" beam. (maybe 6" wide for stability)
Top the beam with a metal plate for wear.

This approach is similar to a commercially available laminated beam only 'made in place'.
I know that 4 x 12 spanning about 12 ft is sufficient to support a house.
We did a 40" long house and specs were engineered that 2 posts were more than adaquate for the task. ie foundation at ends + 2 jack posts.

 

[john4153]

You may want to reconsider the grain direction for the web pieces, if you want maximal strength. The loads (tension and compression) are carried by the top and bottom members. The purpose of the web is to hold those two structures together. OSB is often used. Sometimes, vertical grain is used. Longitudinally laminated as shown in your drawing is probably not the strongest design. For reference, look at wood spar design for aircraft.

John

 

[evo803]

I would recommend using a Micro-Lam built up beam they 1 3/4" wide x whatever depth needed starting from 7 1/4" and you can bolt together multiple beams " in place " so you are only lifting the weight of one beam at a time. These are very common for structural ridge beams for cathedral ceilings they are very strong and easy to work with and secure in place. The biggest one I had to deal with was 6 pieces 36" tall x 40' long.

 

[Travelover]

One thing I would be really concerned about is the tall narrow section with the trolley on top. If your lift isn't perfectly vertical each and every time it will twist the beam.

+1 . When you use these type of beams in the construction of a house, you are assured that the load is purely vertical. When used for a trolley, there can be a lateral component prior to the load being free hanging. Making the top and bottom elements wider would increase the lateral strength, as well, assuming the trolley could span them.

 

[LD1]

I would seriously reconsider the use of a steel beam.

Not that it wont work, but wood is not an Ideal method for overhead lifting.

If I am reading your post correctly, you are wanting to span 15 feet and lift 660lbs. Is this correct?

If so, you can get by with one of the lightest wide flange beams. A W6x9 would be more than enough. That beam means 6" deep and 9 pounds per foot. So you are only talking 135lb beam. I am sure You could figure a way to set it by yourself or even one other person.

With the span you have, you are allowed a deflection of 1/2". A 660lb load on that beam would only deflect it about 3/16". A 1000lb load would be .25" and a 2000lb load would be the limit @ .5". So it will be more than strong enough for the loads you intend. And I am sure you can come up with a way to support/anchor/bolt it without welding.

And since I-beams are ususally priced by the lb @ anywhere from .50-$1.00 per pound (depending on where you go), that is not going to cost you a lot of money either. End result will probabally be cheaper, easier, stronger, and lighter overall. This is assuming there is no other load on the beam like overhead storage/loft, etc.

If you can manage the 135lbs, that is the route I'd go.

 

[Argosy]

You could tie a couple of trusses or rafters together and use a chain fall or come a long to lift 135 lbs. That would be easier than building a beam (in place?) out of wood.