Any structural engineers here?

I'm going on about stress concentration factors on the basis that if the OP buys 40mm square then drills an 8mm hole through it, it's far less certain whether the beam will hole up or not. It's not always intuitive just how much drilling holes weakens a structure.

The youngs modulus of steel doesn't change much with grade.

I can do the equations from first principles, they come from considering equibrium of moments and forces through an arbitrary plane in the beam then repeatedly integrating the result. Or cheating a bit and using Macaulay's notation to hide the integration. Or cheating a lot and letting a website draw the pictures for me.

I'm not sure what I've done to put your back up here. I approximated 48g as 500N (err on the side of caution at every step, if you know your theory is inaccurate), but then you took the bending moment to 7 significant figures. Engineers bending theory just isn't accurate enough to justify that. And we both concluded 40x40x5mm for the steel, if there aren't any holes drilled through it. Where's the issue?
 
slambo69 said:
This seems to be getting a bit detailed for something to hang a speakers?
Click to expand...

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!!



Uploaded with ImageShack.us
 
JonJ678 said:
I'm going on about stress concentration factors on the basis that if the OP buys 40mm square then drills an 8mm hole through it, it's far less certain whether the beam will hole up or not. It's not always intuitive just how much drilling holes weakens a structure.

The youngs modulus of steel doesn't change much with grade.

I can do the equations from first principles, they come from considering equibrium of moments and forces through an arbitrary plane in the beam then repeatedly integrating the result. Or cheating a bit and using Macaulay's notation to hide the integration. Or cheating a lot and letting a website draw the pictures for me.

I'm not sure what I've done to put your back up here. I approximated 48g as 500N (err on the side of caution at every step, if you know your theory is inaccurate), but then you took the bending moment to 7 significant figures. Engineers bending theory just isn't accurate enough to justify that. And we both concluded 40x40x5mm for the steel, if there aren't any holes drilled through it. Where's the issue?
Click to expand...

Not anything to put my back up, just explaining how with the initial bending moment equation was incorrect and giving higher stresses than actually present.

The steel grade is all what matters in this case for allowable stresses, E is pretty much constant but will not effect when the beam fails.

Bending moment to 7SF was just me writing it down, not exactly something question about, the drilling the holes in the tube is a different problem as you will get into edge distances and how the load is transfered which is why its not needed in the beam design, thats the connection design but can be avoided in a number of ways, this is all jsut discussion between engineers but I am trying to point out some flaws in the way you analysed the problem, sorry if this is coming across as bitchy it's not meant to I am just trying to explain how I've done this for a number of years now.

KaHn
 
chippie said:
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!!



Uploaded with ImageShack.us
Click to expand...

The beams will need to be fastened, this is to stop lateral movement, what you have drawn is fine and what I expected, I would suggest either welding/bolting though the structure to stop movement. the people using a 300x120 (assuming thats a rough size and not a british section) is overkill but could be to accomodate the speaker connection into the beam, we've both specified a small beam but if that needs a custom mount to attach to the box then that will be more expensive than using the larger section and drilling holes through the bottom flange.

As I said this is where the details in the design come into play.

KaHn
 
The differences in approach are interesting, I tend to meet beam theory as an approximation for what happens in gearboxes. Consequently I don't have great confidence in the accuracy of "engineers bending theory", and overestimate values to make sure I err on the side of caution. Excessive deflection in a gearbox being very bad indeed.

Steel grade will indeed change allowable stress. I think 250MPa is an approximate figure for yield of mild steel at room temperature, I freely admit no familiaraty with the building codes used by civil engineers. Pretty much everything has to be better than generic mild steel though, so I'm assuming a worst case scenario again.

I've not come across deliberately seperating the model into beam design and connection design, but can see the sense to the approach (especially if considering a number of connection methods).

I think we're basically on the same page KaHn.
 
JonJ678 said:
The differences in approach are interesting, I tend to meet beam theory as an approximation for what happens in gearboxes. Consequently I don't have great confidence in the accuracy of "engineers bending theory", and overestimate values to make sure I err on the side of caution. Excessive deflection in a gearbox being very bad indeed.

Steel grade will indeed change allowable stress. I think 250MPa is an approximate figure for yield of mild steel at room temperature, I freely admit no familiaraty with the building codes used by civil engineers. Pretty much everything has to be better than generic mild steel though, so I'm assuming a worst case scenario again.

I've not come across deliberately seperating the model into beam design and connection design, but can see the sense to the approach (especially if considering a number of connection approaches).

I think we're basically on the same page KaHn.
Click to expand...

Yeah, high grade steel brings a whole different problem in certian circumstances, sorry as I think I've pointed out that I do this on a daily basis hence the nit picking on the process, the problem is always a lot more complex than just the intial sizing :)

I've actually went one geeky step too far and modelled this in STAAD, results below. If you all really want I can model this quite quickly in Ansys and go that way?

Untitled2-2.jpg


one sec going to upload at the correct size.

KaHn
 
Last edited:
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" :p
 
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" :p
Click to expand...

Then you have no idea really, do you?

KaHn
 
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" :p
Click to expand...

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.
 
panyan said:
thank you for explaining my error and helping me to become a better civil engineer
Click to expand...

Well read the OP, he wants to hang ~48kg on a 5m long steel beam.

Think about it :p

You're saying the total load will be less than the weight of 'the beam I'm thinking of', he's not going to need a massive structural I-beam (which is what it seems you're referring to).
 
Dunks said:
You're saying the total load will be less than the weight of 'the beam I'm thinking of', he's not going to need a massive structural I-beam.
Click to expand...

ah - you see - we've only been taught the section of Eurocodes that deals with massive structural I-beams, so far, to us, a "steel beam" is a "massive structural I-beam" :p

(our first project was to design the structure of a 4-storey office block and work out the forces in each member)
 
panyan said:
your just as bad as he is - i nobody prepared to be civil and explain where i am going wrong?
Click to expand...

Read my previous posts in this thread, both me and jon are saying to support the speakers you need a 40x5 box, this weights 5kg/m as such you have 25kg of beam and 50kg of speakers, now think about that logically and the amount of steel self weight does not out weigh the speakers.

If you want me to explain more I will :)

KaHn
 
You must log in or register to reply here.
Back
Top Bottom