RSJ spans and steeldeck

[TomHoward]

I have a situation where I'm looking at costing spanning a width with RSJs, to affix steeldeck on top (or Ali etc to reduce self weight), and need someone / something to spec this if possible.

It's for an regular event in the same place where there is a body of water needs clearing.

 

Are there any off the shelf reference guides for this, or do you usually get a structural engineer to design each situation separately?

[kerry davies]

It depends (doesn't it always?)

There are guides and tables you can use LIKE THIS and there are umpteen builder forums (fora) which have topics but unless it gets complex I found the place you buy your RSJs pretty good at having the basic info.

 

Of course most loads are static and performers are worst kind of dynamic so your safety factors might be greater and of course if it is complex or the spans are longer then a structural engineer is the way to go.

 

You could possibly keep down weight, as we used to, by using beams with captive 2X2/4X2 timber then screwing down ply tops rather than Steeldeck units. I can't advise on that not knowing detailed specifics.

 

E2A as it is water have you thought of using one of the many proprietary pontoon stage systems LIKE THIS?

[kitlane]

If I was planning this I would be looking for a structural engineer straight away. From what you have told us it sounds to me that this very much falls within CDM regs so you need to work out who is responsible for what.

[TomHoward]

It's clearing the water rather than floating on it, it's not a monster span but it's around 8m span with a single 8ft deep row of deck.

It's more if there's an entertainment guide or any docs from Steeldeck that might specify, but ground conditions etc and spreader beams might need looking at too.

 

I'll consult a structural engineer even if we refer to anything ourselves but I wondered if there were any docs as a starting point. I think the self weight of the deck is likely to make it a big beam.

 

At the moment a scaffolding company are used to cover the water, then sheeted on top of the scaffold boards for that span. I'd rather not get into making an alternative top to steeldeck as it adds even more liability, although we might look at Ali or metro deck if the self weight of the deck is a big problem.

(The next problem we have also is transporting two 9m+ beams used to span the width and may have to bolt and plate in the middle as well so I think we are well into getting it designed.)

 

Just for a tiny bit of reassurance it's a small stage with about 4 acoustic performers on it. It's just the placement with the natural landscape that makes it difficult.

[empyfree]

Sounds like a job for the existing scaffolding company! RSJ is surely a permanent option?

[kerry davies]

Guessing again but I presume the scaffs are using ladder racks as tube is only 6M long and I would probably stick with them rather than get into heavy lifting and big beams.

 

You could reduce weight by losing the planks and using minge clips holding 2X2 or 3X2 as suggested with ply screwed directly into that. You could also Google "Temporary Event Footbridges" or "The HAKI System" for ideas.

 

It might help to try a different approach and think not bridge but stage roof, most of which are more than strong enough but for your needs that scaff company knows the site, knows the requirements and can access structural engineers if required. Theirs is then the liability and the reassurance you seek.

[the kid]

We did this for an install in a fountain. The idea is it can be built in a few hours and is a segment contrtuction so half or whole or some weird design also. Prolyte did the work, I think it was a mix of custom and off the shelf items.

 

Haki is really big here. Haki or layher will "talk" with stagedex if I recall.

[adamharman]

Even for normal domestic floor loadings you'd be looking at fairly large beams to span 8m.

With a heavy deck and more dynamic loading, even bigger.

If you don't provide lateral restraint to the compression flange (i.e. brace the two beams together to prevent them flexing sideways) even bigger again.

You also need to consider the possibility of them tipping over if you have a load of people on the deck suddenly step forward at the same time.

 

It's achievable, but it's not going to be easy to transport, store or assemble.

 

I'm qualified (BSc Civil engineering) but don't have indemnity insurance so I'm not quoting figures but I've got beams holding a similar span of floor here above my head and trust me they were bloody hard work to move!

 

If you have the budget for a custom system then by all means go for it, but for a once a year job I'd stick with the scaffold crew who've done it in the past.

[ImagineerTom]

Like adam I'm not going to give specific figures on a public forum but most circus's have a beam that spans 9m, has to take 1ton evenly distributed and is 50cm deep requiring a FLT to move it in to position. Your application as described would sound like it needs something even stronger than that.

 

Spanning is very complicated and resource consuming - a much smaller/lighter weight solution comes from just using one or two legs to reduce the overall unsupported span and if you go down this route the job would fall very comfortably in to the abilities of the better scaffolding companies or staging hire co in-house engineers abilities to design and certify.

[TomHoward]

In terms of tipping etc, the stage is at ground level (rsj on the ground on both sides of the pond, it’s just spanning the gap)

 

A leg unfortunately would mean landing in the pond which is a bit of a nightmare as we’re not certain what the footings on the bottom are like or if there’s any liner / footings to damage.

 

We might be able to make a pillar up height to a bigger truss system but it’s getting complicated. I will consult an SE to spec a beam and see how financially viable it is. (The scaff costs a fair bit each time)

 

I’m a bit worried about the handling as well - the bigger the rsj the harder it’s going to be to handle and transport and it may not be worth it.

[andy_s]

You mention looking for documents from steeldeck - have you consulted them directly? They don't just sell the stuff, they will design and build. they are pretty experienced in putting up strange structures in strange locations. Probably expensive, but your going to spend quite a big pot of money anyway...

[adamharman]

In terms of tipping etc, the stage is at ground level (rsj on the ground on both sides of the pond, it’s just spanning the gap)

 

I meant the beam itself tipping over, as it is likely to be somewhat taller than it is wide. The only way to keep the height down is to use something with much thicker flanges e.g. a column section, but that will work out significantly heavier for the same load and span.

 

You may not be planning anything on the stage which might cause that to happen, but that doesn't mean it never will so you need to allow for it. Not difficult to overcome but will require a few extra bits fabricating and more to store, transport and assemble.

 

Steel sizes are specified as "depth mm" x "width mm" x "weight kg/m" so you can easily work out the total weight once somebody's designed it for you. UB are beam sections, UC are column sections.

 

To give you an idea, the beam ImagineerTom mentioned being around 50cm deep - the nearest standard size is 533mm x 210mm which range from 82 to 122 kg/m

 

8m instead of 9m does make a big difference as you're looking at something that's proportional to the square of the span, and loadings in a circus tent are complicated, so you may well not need anything that big but I'd expect something approaching half a ton per beam

[TomHoward]

I imagine there'd have to be some sort of support bolting the front & rear RSJs together to keep the spacing, spreader beams etc, rather than single beams on the ground which would be free to move.

 

I'd just seen it enough times that I'd wondered whether there was any available documentation or mention of the subject that I could take to an engineer as a starting point.