Beam / structure Calculations

[RustyH]

Good Afternoon,

Over the years since qualifying as a mechanical engineer, Ive wanted to learn more about structural mechanics. We learnt a fair bit about about beam analysis when I done my degree 15 years ago, and the fact I went down a thermal engineering route means Ive had to dig out all the old structural mechanics books.

I've decided to set myself a challenge of designing a mezzanine floor, nothing more than a learning exercise and not to actually be assembled and used!

Ignoring all the requirements for seismic assessment, floor loading assessment etc. I just wanted to check if I was going about things the right way. Do you start from the tops, working down. So knowing your dead weights (structure weight) and live loads, then first carry out SFD and BMD on the joists, then on the main beams, then the pillars?

Thanks in advance for any help

 

[RustyH]

just changed one bit on the report and re attached

 

[CELinOttawa]

Matlab? What did you do this in? If this is Word or anything of the like, I'd love to have the original file!

 

[RustyH]

Hi CEL,

What part are you referring too the report was constructed in Word

 

[CELinOttawa]

Okay, so you stuck output results / graphics into a word file... Fair enough. Matlab and a few others will produce documents that look like this AND do the calcs. Thought you might have done something similar...

 

[RustyH]

Oh ok, no done it all in word and pasted the output results. Can ping you across a copy of the word doc if you want.

How do you think it all looks?

 

[RustyH]

When I was doing the calculations for the timber joists, the Modules of Elasticity was down graded by use of a formula (E = Ematerial / 1 + Kdef) where the Kdef value compensated for things like load duration and moisture content.

Im not looking at steel I beams, is the same downgrade of E occur during calculations

 

[CELinOttawa]

No downgrading of the modulus of elasticity for steel... The reduction is to compensate for the loss of stiffness due to moisture, etc... That doesn't happen with steel.

 

[CANPRO]

Hello Rusty, very nice presentation with the calcs.

My only comment is that it appears you only ran 1 scenario for the live load on the cantilever joist. With live load on the cantilever portion, you're reducing the deflection and bending moment on the interior span...with live load on the interior span you're countering the deflection at the tip of the cantilever. So you should check 3 cases in total a)live load over entire joist b) live load on interior portion only c) live load on cantilever portion only.

 

[RustyH]

Got you, had a feeling I was going wrong downgrading it

So you jusrt leave it as 200000 MPa

Can you also be a little more lenient on your factor of safety with steel as its a more controlled manufacturing process, whereas timber is left to nature

 

[RustyH]

Just been reading up a little on Tata Steel technical reference pages, and they mention that typical safety factors of 1.4 for dead loads and 1.6 for imposed loads are typical. That seems quite a low factor of safety if designing large steel structure. Is that really the norm?

 

[jayrod12]

That is the loading factors only. That is using LRFD which would also apply factors to the material resistance. For steel it is typically 0.9 in bending.