C-section beam loadings

[Pheasant Surprise]

Little project for next week; I want to mount a space heater in my workshop suspended from the roof. Plan is to bridge across existing roof beams with two C150 type section, to which the space heater will be bolted to underside with four M10 threaded rods/bolts. The space heater weighs about 150kg and the span from one roof fixing to the other fixing for the two C150s is about 3.6 metres.

Will the C150s be man enough for that load or should I go up to C200s?

I've got some spare lengths of C150 lying around here so would prefer to use them if I can.

 

[Morph]

75kg in the middle of 12ft should be ok.
Heavy heater mind.

 

[Pheasant Surprise]

Thanks @Morph, that's what I'm thinking. Heavy but should be OK.

I googled up the Steadmans load charts last night:

http://www.steadmans.co.uk/wp-content/uploads/2013/03/Purlins-Manual_LoadTables.pdf

Spec weight of heater is 141 kg, lets round up to 150 kg for good measure. That works out to be 1.47 kN apparently. So that divided by 2 beams is 0.735 kN per beam, over a 3.6 metre span.

I need to measure the gauge of the steel in the C-beams, I think its 1.6mm. Also they must actually be 140mm C-section....my bad...but I think I should be OK according to this chart:

 

[Morph]

Those charts refer to evenly spread load over the distance, your heater load is located over the bolt centres in the centre of the span.
Take one of your spare beams but it on couple of blocks set 3.6 m apart and stand on it either with a mate or holding something . Then you have tested deflection over the span with a point load of 120kg just 4 inches off the ground. Blacksmith science.

 

[Pheasant Surprise]

Those charts refer to evenly spread load over the distance, your heater load is located over the bolt centres in the centre of the span.
Take one of your spare beams but it on couple of blocks set 3.6 m apart and stand on it either with a mate or holding something . Then you have tested deflection over the span with a point load of 120kg just 4 inches off the ground. Blacksmith science.

Good thinking about the point loadings, I was thinking about how that would affect (de-rate) the overall load rating. I like the blacksmith science idea; so will rig it all up on the ground with the beams bolted together sat on 2 stacks of pallets at either end and then gradually bring the full weight of the heater onto the beam setup using the threaded bolts and see how much it all deflects. Better a 4 inch drop then a 4 metre drop as you say!

 

[Pheasant Surprise]

Deflection is the key. Probably worth a call in to Steadmans on Monday (or an engineer who owes me a favour!) to double check though.

Looking at the load tables again below - for supporting mezz. type floors (probably closer to what I'm trying to achieve):

Supporting bolt centres on the heater are 576mm (crosswise) x 760mm (lengthwise), so gives the load spread over 0.438 m2, so thats 150kg / 0.438 = 343kg / m2.

Got to measure the gauge of the sections I've got, but if its 1.6mm (14016 section) and the 2 beams are set at 400mm apart, then according to the table below, the working load at 3.5m span is 353kg / m2. So within spec, just.

Deflection limit needs to stay within "Span / 360" so about 10mm, if I'm reading the table the right way.

 

[Morph]

Good plan . I would make up some flat bar washers to fit into the c where the heater bolts through as that could be the weak point the threaded bar deforming the section out of profile and reducing strength.

 

[Nearly]

If I wasn't sure I'd be looking on the re-usable pile for another section to strengthen the C into G or [] or [I or ][ or [o just to make sure?

 

[MickMoor]

Don't forget you really should factor in the known unknown (!) If the space heater has a fan, what torque reaction will there be when it starts up? If you are right on the limits, things like this should be factored in. If you have loads of spare capacity don't worry about them. We all have heard of the straw that broke the camel's back.

 

[Pheasant Surprise]

Good plan . I would make up some flat bar washers to fit into the c where the heater bolts through as that could be the weak point the threaded bar deforming the section out of profile and reducing strength.

Yep that was the plan, the suspension rods would be bolted onto some flat bar, sitting on top of the C-sections rather then drilling through the actual rails themselves.

 

[Pheasant Surprise]

If I wasn't sure I'd be looking on the re-usable pile for another section to strengthen the C into G or [] or [I or ][ or [o just to make sure?

Don't forget you really should factor in the known unknown (!) If the space heater has a fan, what torque reaction will there be when it starts up? If you are right on the limits, things like this should be factored in. If you have loads of spare capacity don't worry about them. We all have heard of the straw that broke the camel's back.

Yes good thinking chaps. I reckon to be on the safe side, its either going to need to move up to a 170mm section, or do as you say Nearly and bolt two existing 140mm C-sections back to back, ][ , to form one stronger "I" beam, and then get 2 more and do the same thing - in effect there would be 4 rails so should be plenty strong and stiff to do the job.

 

[MickMoor]

Sorry, I only gave half an answer. If you are fixing to the C section, (or any structural section). it is normal to drill through the neutral axis, without affecting the strength. The web only serves to keep the flanges apart. As nearly says, if you are uncertain, why not double them up? You are concerned with buckling in the middle, so there is no need to go right to the end if it would interfere with bolting the section to its cleat. I like the blacksmith's test! However, any beam is stronger with fixed ends, so if the test proves satisfactory, you will give yourself a margin once it is fixed properly.

 

[Pheasant Surprise]

Sorry, I only gave half an answer. If you are fixing to the C section, (or any structural section). it is normal to drill through the neutral axis, without affecting the strength. The web only serves to keep the flanges apart. As nearly says, if you are uncertain, why not double them up? You are concerned with buckling in the middle, so there is no need to go right to the end if it would interfere with bolting the section to its cleat. I like the blacksmith's test! However, any beam is stronger with fixed ends, so if the test proves satisfactory, you will give yourself a margin once it is fixed properly.

I'll fix either ends of the span using 2 bolt plain cleats to the C-sections and these will be mounted back-to/back to form an I-beam. There will be 2 of these I-beams taking the weight of the heater across the span. Flat bar will be sitting across the top and bottom flanges of the C-section/I-beams through which will thread and bolt the M10 suspension rods for the heater. The bottom flat bar will be bolted against the bottom flange using the threaded rod in effect to secure the heater fixings from sliding length ways along the beam.

I'll post up some hand drawn sketches a bit later. Pictures worth a 1000 words and all that

 

[Bob c]

nothing to do with load bearing ,
seems a good idea to put the heater up there out the way but,.....
how big is the space heater , if it was say 100k btu`s,it`s going to get very hot up there ,hot rises up as well and all tha

just my thought`s thats all

 

[MickMoor]

That sounds fine to me.

 

[Pheasant Surprise]

nothing to do with load bearing ,
seems a good idea to put the heater up there out the way but,.....
how big is the space heater , if it was say 100k btu`s,it`s going to get very hot up there ,hot rises up as well and all tha

just my thought`s thats all

You wouldn't believe it from my bodgy mounting ideas, but the space heater is actually designed to be mounted at height, well between 3.5 and 4 metres ideally. It's a proper externally flued gas fired condensing heater, louvred with fan control, designed to throw the heat along about 30 metres rather than just up and into the roof. About 170,000 btu.