Any structural engineers here?

chippie · 22 Oct 2011 at 15:05

I'm trying to figure out what size steel beam to get to hang some speakers from.

I'm sure it's a simple calculation, but I just don't get it!!

So, I need a steel beam that will sit between two existing beams. The gap between the two is 5m, and a set of speakers weighing 48kg will be hung 1m from the end of the new beam.

What size steel do I need?

chippie · 22 Oct 2011 at 15:34

The Craig said:
About tree fiddy.

Excellent. I shall go order some 'tree fiddy' steel.

On a more serious note, we have had a calculation done someone already, but it just seemed way way over the top for the weight we want to hang, so I'm just looking for an independant calculation to see if it is avtually wrong or not. I won't completely trust what anyone on the internet says about this, just want to see if we need to get someone else in to do the calc for us.

chippie · 22 Oct 2011 at 17:26

Ricochet J said:
Do you know how to do a free body force diagram, and simple bending moment calculations if you knew the youngs modulus of steel?

No!

chippie · 22 Oct 2011 at 18:17

DB_SamX said:
Yes, me.

Load is very small; it's the span that's the issue. That's a lot of steel (and cost) for a rather mundane task. Any other way of mounting the speaker, say adapting one of those TV brackets for walls. Otherwise, I can give you a size on Monday when I'm at work.

Yeah, it's the span that is the issue. The speakers are in a church, and they will be at a height of 5.5m from the ground, above peoples heads, and we obviously don't want them falling down.

There are steels supporting the roof running across the room (above a suspended ceiling) and these are 5m apart. The ideal position for the speakers is 1m forward of one of these steels, where there is absolutely nothing until you reach the roof to hang them from, hence spanning the gap between two steels with more steel is the only real solution.

The speakers are big, and heavy. Not sure if any consumer speaker mounts would hold 48kg.

If you had a spare few minutes on monday to do a calculation, that would be really helpful.

chippie · 22 Oct 2011 at 19:11

slambo69 said:
This seems to be getting a bit detailed for something to hang a speakers?

It does seem that way, but they aren't just 'normal' stereo speakers, they are these: http://www.dv247.com/pa-systems-and-live-sound/citronic-cla300-line-array-in-black--64329

Anyway, if people are willing to spend their saturday evening doing calculations I've done a quick drawing of what it is we are trying to achieve. My intention was for the knowledge of the internet to get a quick 2nd opinion. Looks far more complicated than that though!!! I'm certainly out of my depth with all of this. (Don't worry, I'm not making the decision on what to get, as I don't want the responsibility of speakers falling from the ceiling onto people's heads!!)

A = the existing steel beams going across the room
B = The new beams we will put in to hang the speakers from (one on each side of the room, one speaker hung from each, about 3.5m apart)
C = The speaker!!
D = Distance from existing beam to speaker 1m
E = Distance between existing beams = 5m

Not sure if the new beams will be fastened in any way, just sat on top of existing steel with a bit of overhang. Frame will probably be attached with chain.

For reference, the calculation we got that we don't quite believe was to use a beam 300x120mm at 36kg/m, which seemed quite large for what we are doing.

Thanks everyone!!

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chippie · 22 Oct 2011 at 19:58

panyan said:
civil engineer in training, ~500N on each side on a steel beam is almost insignificant, if you are talking about a steel beam like i think your talking about, the beam will weigh a lot more that the sum of the loads anyway - it sounds like your using a cannon to kill a fly to me - but perhaps i misunderstand what you mean by "steel beam"

Well, what do you suggest I hang the speakers from then? Above where the speakers go is a suspended ceiling, then air, then the roof (tiled) then the sky. I don't think any of them are capable of having speakers hung from them.

chippie · 22 Oct 2011 at 20:21

DB_SamX said:
OP, what cross section shape are the 2 existing steel beams? As already said, connections could be difficult, both to the existing beams and to the speakers, especially a 40mm wide box.

Something like a UB 127 x 76 x 13 S275 would be heavier but easier to fix to. You could even connect it to the existing beams without bolts or welding but by using clamps - minimal interference (can't remember the name of the type we've used before though). The heavier mass would help with the vibrations as well.

Can't remember what shape the existing beams are. Pretty sure they have a flat top to them, so are either box, or I shape. Anyhow, fixing the new stuff to them isn't a problem, we have a welder (both the equipment and the person) so will probably weld, although clamps would also be an option.

chippie · 22 Oct 2011 at 20:23

P.S - Can I just thank everyone who has posted in this thread. Didn't really expect much response, let alone detailed calculations and stuff!!

Been really helpful.

chippie · 22 Oct 2011 at 20:25

KaHn said:
Chippie,

Your problem will be how you connect the speakers to the new beams, the rest will be pretty simple. Can you provide details of the speakers mounts? Are they bolted, welded, clamped or hung using cables?

KaHn

Two chains will be attached to the speakers frame, and slung over the new beam (whatever it is). I believe the plan was to weld some additional material on top of the beam to stop forward and backward movement of the chains. I think it was intended to attach without drilling through the beams.