RSJ strength estimates

[KnappGate]

A search through the archives turns up numerous questions where people have asked for help sizing RSJs for a building project, and the usual answer seems to be that it isn't possible to answer without making dangerous assumptions and the OP needs to consult a qualified structural engineer. And that seems fair enough.

My situation is slightly different. I'm thinking about making a vehicle lift consisting of four chain blocks suspended from a framework of RSJs. This would be for my personal use on private property with no planning permission, building inspection or liability issues. My main concern is to ensure the result is easily strong enough to support the intended weight, but on the other hand the heavier it is the greater the risk of hurting myself when I put it up so I don't want it to be needlessly over-engineered. I can calculate the expected loads easily enough, and apply a nice large safety factor to tell me what loads the RSJs need to support. Essentially I'm looking for a beam 12' long that can support a point load of 4400 lb in the middle of the span, and a beam 16' long that can support a point load of 4400 lb in the middle of the span. The total weight of the vehicle I plan to lift is 2200 lbs and the weight would normally be shared over two beams, so these 'safe working' figures above already include a 100% margin to allow for dynamic loads and a further 100% so that it would be safe to lift the entire vehicle from any single lifting point, even though I don't plan to do that. And despite all the safety factors and over-specced components I'm going to work on the assumption that everything safety critical will fail at the worst possible moment; I've no intention of getting squashed.

I haven't described the design of this assembly but I'm happy to take responsibility for the stability and strength of the other elements myself - it's the bending strength of the RSJs which I'm uncertain about.

I had hoped to be able to find manufacturers' figures for the maximum point loads that their RSJs can support for a given length, but I haven't found anything yet. As a starting point I have guestimated the maximum bending moment of some W section RSJs listed at http://www.engineersedge.com/standard_material/Steel_ibeam_properties.htm based on the published section modulus and material yield strength. My estimates suggest that a 16' length of W section 8"x4" at 13 lb/ft would support about 19000 lbs (giving me a safety factor of 4) and a 12' length of W section 6"x4" at 13 lb/ft would support about 14000 lbs (giving me a safety factor of 3).

Those numbers look reasonable to me but I'm not used to working with RSJs so I could be miles out. Anyone with any practical experience able to tell me whether these figures are in the right ballpark?

 

[CambridgeBrummie]

I'm happy to help you with this.

Although you have quoted everything is lb and feet, I assume you are based in the UK, in which case you will be needing in SI units (as most published information is based on this).

A quick sketch would be useful to support your written description above.

We don't have W sections in the UK, this is only in America.

So a 8" x 4" I am assuming is a 203 x 102 UB. This has a moment capacity of 64kNm. The moment is calculated PL/4. However, the moment capacity will be reduced if the beam is free to move laterally and torsionally (hence the sketch requested).

A 6" x 4" beam, I am assuming is a 152 x 89 UB. This has a moment capacity of 33kNm. Again, this will be reduced if the beam is unrestrained along its length.

Also what is supporting the whole frame in the air?

Guidance can also be found from the Steel Construction Institute.

 

[KnappGate]

I hope this drawing gives you an idea of the general layout.

The lifting frame will be built inside a rectangular garage roughly 12'x16' and will fill the garage. It will be supported by a post at each corner. A pair of 16' I-beams used as sliding rails will run along the two long walls. A pair of 12' lifting beams will rest on top of these and span the gap between them. The lifting beams can be positioned at any point along the sliding rails but would typically be at 1/3 and 2/3 of the length. Chain blocks will hang from the lifting beams.

The supporting posts will be strapped to the garage walls. The long sliding rails will be bolted to plates on top of the supporting posts. The short lifting beams will overhang the sliding beams slightly and be clamped to them. When unloaded, the lifting beams can be positioned along the sliding rails by loosening the clamps. The clamps and bolts are intended primarily to ensure that the parts can't slide off each other, although they will also tend to resist twisting.

The vehicle weighs 1000kg. I am sizing the lift to have a safe working limit of 2000kg. Since this will be a dynamic load, I'm treating this as 40kN. In practice this will be shared between the two lifting beams and the two support rails and will be applied fairly close to the ends. The worst case for the lifting beams will be to place the whole load in the middle of a single beam. The worst case for the sliding rail will be to place the whole load at the end of a lifting beam which is in the middle of the sliding rail. In other words I will assume the whole load is applied to the mid point of each rail.

I think this corresponds to a maximum bending moment of about 50kNm on the long rails and about 38kNm on the short rails. Your corrected figures for the RSJs suggest that the sections I was considering would be extremely marginal for these loads and do not provide any safety margin. I think a safety factor of 3 - 5 on the bending moment would be sensible. Do I need to allow an additional margin to allow for the beams being free to twist, and free to flex sideways?

 

[oldbutnotdead]

I'm not going to dig out my Strength of Materials textbooks- it was too long ago and too late at night. But that structure, unless the verticals are restrained by something not shown, is always going to be unstable. And I suspect that your moveable lifting beams will be way too heavy to move safely.

Why not buy a 2 post lift?

Ahh, just seen where the verticals will be strapped to the walls. What construction are the walls? You've got lots of loads on or near the centre of spans, will all make for very chunky bits of steel. Which will increase the weight of the structure so increasing the size of the beams required.

 

[tony1851]

A problem here is deciding on the degree of lateral restraint of the 16ft (4.9m) beams, as this dictates the effective length.
Allow - say - Lx1.2, = 5.9m. A 50Kn load mid-span would give a b.m. of 74Knm, way above what an 8x4 would take.

Something like an 8x8x47 column section would be needed for that; maybe review your safety factors?

 

[CambridgeBrummie]

A 203 x 102 x 23 UB (Grade S355), spanning 3.6m, will take a 2 tonne (20kN) load at it's centre, without any additional lateral restraints.

The 16' beam (4.8m long), is a little more tricky. If we say it is taking 2 No 20kN loads at third span points.
Unrestrained you would need a 254 x 146 x 31 UB (Grade S355).
If you can get some restraint back to existing structure or cross bracing at third points and can get this lateral load transferred down to ground floor level then a 203 x 133 x 30 UB (Grade S355) would be acceptable.

Not sure what size posts you have but probably 152 x 152 x 23 UC would be needed.