Quick Structural Beam Calc Q

Eng33 · 18 Mar 2008

Had structural calcs professionally prepared by SE for my extension, but I am concerned that they are extremely OTT. Being an engineer by profession I am in the process of working through them, but am struggling to understand a constraint for beam deflection.

I am aware of max deflection for a beam in a building of L/360, which for my 6.3m beam comes out at 17.5mm, but he has then used: 5/TAN 30deg = 8.7mm. Where has that come from? Is it a factor of safety?

It’s a big issue as 17.5mm gives me an I value of 10’323cm^4, whereas 8.7mm is 20’765cm^4, which makes a big difference to the depth of the beam!

Thanks!

Shytalkz · 18 Mar 2008

Hi

L/360 is the allowable deflection for live load. However, to prevent unsightly cracking through excessive deflection, this should limited to an absolute value of 12mm for spans >4320mm. If the beam is being installed in an existing structure, then, as the dead load is already there, it would be sensible to consider deflection under dead plus live loading, to the same limits.

So, he's right to restrict the deflection, but I have to say that I can't understand the 5/tan30 bit, that's lost me. If you pro rata the L/360 value thus: 10323 x6.3x10^3/(360x12), that will give you 15054cm^4.

Full calc for deflection is 5wL^4/384EI, or am I in danger of teaching mother's mother to suck on yolk and albumen?

^woody^ · 18 Mar 2008

BS5950 does state span/360 and you would use all the serviceability loads and not just the live load

And you do know that that gives the maximum permitted, and you need to do the other calc to work out the actual deflection?

Shytalkz · 18 Mar 2008

Service load is actually an unfactored load, whether dead, live or both

Beam stresses are calculated from ultimate, ie factored loads: factors for Gk, Qk (and Wk if applicable) depend on load combination.

^woody^ · 18 Mar 2008

Yes there was an "all" and a "just" missing - I'll edit it

Eng33 · 18 Mar 2008

Thanks guys!

OK, some more info plus I’ve been thinking a bit more!

It’s a bungalow, the extension incorporates an attached garage space and lounge, with an upstairs loft style bedroom.

Example: the beam spans the garage space (6.3m), and supports the edge of the 1st floor (2 perpendicular steel beams), point loads from a ridge beam post and a purlin post (both equi spaced at 2.1m), plus the UDL of the beam and floor. The roof is at a pitch of 30deg, so it looks to me that the “5” in the equation is a maximum of 5mm deflection at the roof (?) in a particular plane, which when resolved into a vertical deflection is 8.7mm, does that sound correct?

^woody^ · 18 Mar 2008

LOL I think this is 3rd week stuff!

I'll skip the beam questions from now on.

Need any help with air vents?

Shytalkz · 18 Mar 2008

Woody, yes the edit made a bit more sense! But it's live (serviceable) load deflection, unless there are reasons - like that I suggested - to use service dead plus live in the calculation.

Eng: I think I've got the layout in my head. I haven't got my tables near to hand, but for that span and likely loading, I presume that it's a 254UC that he's proposed? Or is it a deeper UB section?

Tanx = o/a => a = o/tanx; thus, if your roof pitch is 30, that makes it something resolved into a horizontal direction. Nap, still don't know what it is

. But, seeing what connects to and is supported by this beam I would probably use dead plus live loading limited to 12mm deflection.

Eng33 · 18 Mar 2008

Thanks Shytalkz! Just noticed you're local, I'm in the Norwich area....

UDL is 1.7kN/m
W1 is 18.1kN, W2 is 46.3kN (equi spaced at 2.1m)

The beam in question has been sized at 457x152x52 UB!!!

Yeah, as you say for it be a tangent it looks to be a horizontal component... but how's that come into a beam deflection calc?!

Shytalkz · 18 Mar 2008

Let me guess, a certain company in an old school building did your calcs, engineer's initials RP? I'm just east of t'city...

Shytalkz · 18 Mar 2008

Can you obscure name and post the calc as a pdf?

Eng33 · 19 Mar 2008

Shytalkz – have e-mailed you the details.

Spoke to SE today, and he explained the 'odd' equation... He has used a figure of 5mm as a maximum permissible horizontal deflection at the top of the walls as a result of a vertical loading to the roof structure. Therefore when resolved into this vertical load, it equates to 8.7mm.

Shytalkz · 19 Mar 2008

Despite having looked at the calcs, that explanation still foxes me...!

Update: sussed, see email reply!

^woody^ · 19 Mar 2008

Shy, matey ...... check your email I've just sent you 50 beam calcs which I wonder if you could check and get back to me by .... shall we say next Tuesday? (you can have Monday off)

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Quick Structural Beam Calc Q

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