Sizing a structural beam - calculation 7

[kaspor]

Hello

I am trying to calculate the size of some structural beams. Each beam will be supported on one side by a timber stud and on the other hung vertically with timber from timber joists.

I have a box with UDL of 85kg/m (or 0.84kn/m). Box dimensions are 1.4m (L) x 1m (W) x 0.5m (H).

My span for these beams are 2.2m. I would like to use 90x45 MGP10 timber.

Sketch as follows:

My calculations are as follows:

For beam bending:
Moment capacity (Mc) = [F (allowable stress) x Breadth x Depth] / 6

For 90x45 MGP10 timber F = 17 MPa (as per AS1720.1 Table H3.1), Breadth = 45mm, Depth = 90mm.

Mc = (17 x 45 x 90^2) / 6 = 1032750 N/mm or 1.032 kN/m. This is > my requirement of 0.85kN/m so the beam is adequate.

For deflection:

Deflection (D) = [5 x W (load) x L^4 (span)] / [384 x E (Youngs Modulus) x I (section modulus)]

For 90x45 MGP10 timber, E (Youngs Modulus) = 10000 MPa for MGP10 (as per AS1720.1 Table H3.1)
W (load) = 0.84 kN/m = 0.84 N/mm
L (span) = 2200mm
I (section modulus) = (Breadth x Depth^3) / 12 = (45 x 90^3) / 12 = 2.73 x 10^6 N/mm4

Hence, Deflection = [5 x 0.84 x 2200^4] / [384 x 10000 x 2.73x10^6] = 9.4mm of deflection. This is < then the maximum limit of deflection in Australia so the beam is adequate.

Each beam will be supported by a timber stud and top plate on one side. The timber stud will be fixed laterally every 1300mm within an existing wall frame.

The other side of the beam will be supported from the floor joist. A vertical 90x45 timber will be secured to the floor joists and joist hangers will be used to support each beam from the vertical timber.

Appreciate any feedback anyone has

 

[Enable]

You are hanging things overhead. My feedback is don't mess around with that and get a local structural engineer to design it.

 

[rb1957]

how much load is going from the RH support into the floor joist ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[kaspor]

Thanks for the replies. The joists themselves have been engineered to carry this load. I'm just looking at the support for the unit itself.

 

[BAretired]

I agree with Enable. Get a structural engineer to design it.

BA

 

[rb1957]

my question was less about the floor joist and more about the structure you're adding, and how you're connecting this to the joist, and how the joist will feel about this load (locally).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[DayRooster]

Hire a structural engineer...

 

[Brad805]

Is that correct the unit is only 82kg?

 

[nivoo_boss]

Your Mc calculation is off with units. A 90x45 section with bending strength of 17 MPa takes a moment of Mc=1,03 kNm. If we assume that the unit is supported only by one 2,2 m beam, then the characteristic load would be approximately 0,82/2,2=0,37 kN/m. Let's multiply it with a safety factor of 1,35, then it's 1,11 kN/m. Bending moment from this is 1,11*2,2²/8=0,67 kNm. Which means you can support this thing on one beam.

Deflection calculation is off as well because you're using too large load, more correct would be to use about 0,37 kN/m. Which means you would get a deflection that is more than twice less you calculated. For timber there are other considerations in calculating deflections, such as creep. But in your case, it does not matter much since it just supports some AC unit.

I wouldn't worry too much if the unit really is as light as you say.

 

[LittleInch]

My concern would be the attachment and also the point load on what looks like one of those timber metal truss floor joists. They really don't like point loads hanging underneath them I think.

Ditto the joint planned between the horizontal and vertical member.

These are your critical points, not the beam for what is no more weight than a person

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