Any structural engineers here?

chippie

chippie

chippie

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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?
 
The Craig said:
About tree fiddy.
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Excellent. I shall go order some 'tree fiddy' steel. :D

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.
 
48kg doesn't sound much to me tbh, a chin-up bar will hold 2x that easy, i would guess maybe 1/2" box would hold 48kg, maybe 1" box

What size were you quoted before.

Ps. no structural knowledge here whatsoever!
 
Yep 48kg isn't much to be honest. Any beam would suffice I would say.

I'm not a sructucral engineer but i've done a bit of this sort of thing and I think most beams available will be adequate. Just think they are designed to hold much higher weight than 48kg, plus you are only hanging the weight 1m form the end and not in the middle which would cause the maximum bending moment.

Put it this way, you can hang yourself on a 1/8" rivet if you could actually tie yourself on it properly. A couple of speakers isn't really much
 
Wouldn't he also need the second moment of area and the centroid for the section to determine the deflection?

All seems terribly overboard for what is effectively a speaker mount:)
 
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.
 
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.
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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.
 
I'll have a look in a min :)

KaHn

/edit:- could you sketch it, just goin to get a beer then I'll have a look
 
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Surely whatever you calculate you're going to then overrate by several times anyhow? Probably best just going for something which you know objectively will support them, I wouldn't try to go as flimsy as possible on something like this.
 
M/I = sigma/y for bending moment M, second moment of area I and distance from the neutral axis y.

Sigma = My/I. M in your case is 400Nm, and y will be half the height of the beam. Try 25mm square section, this gives 400*0.0125/(2*10^-8) = 250MPa. That's pretty much bang on the yield stress, and not a great idea.

So, try 40mm square section, you get 68MPa, well under yield. I haven't worked out how much it will deflect downwards by. I suspect that hanging speakers on a single steel strut will lead to the strut vibrating up and down in response to the music, which is probably a bad thing. Still, I'm pretty sure 40mm square would be fine in terms of the speakers not breaking the beam.

Referances:
i1d5wo.png

http://www.roymech.co.uk/Useful_Tables/Sections/SHS_hf.html

^That's very much rough and ready, but I'm mechanical rather than civil. Are you planning on drilling holes through the beams to mount the speakers? That would change matters considerably
 
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I don't know how he's planning to connect it to the other beams, so went for simply supported on pin joints. Unless I've got that wrong, which is entirely possible (please do correct me if so, it's a bad thing to forget how to do)

edit: I think built in ends would decrease the bending moment, as it would certainly decrease the deflection
 
JonJ678 said:
I don't know how he's planning to connect it to the other beams, so went for simply supported on pin joints. Unless I've got that wrong, which is entirely possible (please do correct me if so, it's a bad thing to forget how to do)

edit: I think built in ends would decrease the bending moment, as it would certainly decrease the deflection
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No you're built in end thing vs simple is correct you should be assuming simple supported at this stage.

You moment calculations (48kg*1m) is wrong for a beam supported between two points, at unequal distances its M=Pab/L which is 48kg*g*1m*4m/5m as per standard beam theory, im just writing up the calc based on AISC code and will post it in a min :)

KaHn
 
Ah, I went for drawing a picture, and let 48g = 500N. Point load at 1m in from one side, total shear force = 0 as it's in equilibrium.

As an afterthought, I think a 40x5mm square section will deflect by about 3cm, greatest deflection nearish the middle. That's taking young's modulus as 200 GPa, and letting this site do the integration instead of scribbling down Macaulay's notation myself.

Worst case stress concentration factor for a hole is something like x3 iirc, but I think that only applies if the diameter of the hole is small compared with the width of the plate. 80mm by 40mm or a similar ratio might be a better idea if drilling holes.

edit: 60x40x3mm comes out at around 35MPa maximum stress, and about half the deflection of 40mm square. Taking the stress concentration factor as 4, I think there's a safety factor of about 2 before yield. I don't believe 48kg is enough to pull a bolt sideways through mild steel. Interested to see what KaHn concludes.
 
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KaHn said:
No you're built in end thing vs simple is correct you should be assuming simple supported at this stage.

You moment calculations (48kg*1m) is wrong for a beam supported between two points, at unequal distances its M=Pab/L which is 48kg*g*1m*4m/5m as per standard beam theory, im just writing up the calc based on AISC code and will post it in a min :)

KaHn
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This seems to be getting a bit detailed for something to hang a speakers, get an invoice off the OP! Try and get a site survey out of it as well...

You still up for a pint sometime?
 
My working.

photo-11.jpg


I'd suggest using a 40x40x5 SHS for the simple reason it gives a 20mm deflection at the 1m point.

If you want any more description let me know.

Also Jon, not sure what you are going on about SCF's for as that comes depending on if hes bolting through or not, if you could actually do the equations from first principles instead of relying on websites to do them for you. Also the 200GPa of steel is a constant, you need to consider if he is using S235/S275/S355 based on both LRFD and ASD design codes for buildings.

I don't really want to draw this out an afwul lot but this is a simple calc to be doing for a simply supported beam. If he wants help with connection deatils on each side then I don't mind helping with that either but it will need more input from the OP as to how he wants to fasten them etc.

KaHn
 
slambo69 said:
This seems to be getting a bit detailed for something to hang a speakers, get an invoice off the OP! Try and get a site survey out of it as well...

You still up for a pint sometime?
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Yeah mate, ill grab adam during the week and we can go for a beer.

KaHn
 
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