steel selection

[russellsmiler]

Hoping this forum can help me with this.
I am building a 2 storey extension and want to knock through the existing external 2 leaf wall to eventually form an opening so as to enlarge the kichen area.

The opening size will be 3200mm, the steel sections will have Min 150mm End bearing onto C35 concrete padstones, the walls are in exellent very tough condition so see absolutely know problem in that respect.

The steel beams (one for each wall ) will be drilled and using spacer sleeves and all thread bar will be connected at 600mm C/C between the flanges.

The loadings as i see it are
2150mm of rendered brickwork above
3200mm high of 35 degree pitched roof (slate) rafters at 450mm c/c onto wallplate
No floor joists or ceiling joists are built into the loaded walls.

A dilemma i have is that i am to turn a window above the beams into a doorway to access new bedroom and want to keep flush floorboards and flush ceilings downstairs so are looking to use steel beam at depth no greater than 175mm (obviously this as to be achievable given the span loading etc)

Would anybody be happy enough to help me through the selection and calculation to determine a satisfactory steel beam.

Am i right in that as long as the maths, specification and installation are correct then building control will sign this off

Sketch below is of side of house, you can see that the masonry loading is less on the right hand side.


View media item 65289
I used to be able to do the calculations but are know to fuzzy in the head to understand my old textbooks and written examples, but would love someone to help me with this.

Many thanks
Russell

 

[Alastairreid]

I stand to be corrected here, but as far as i'm aware your steel has to be designed/ specified and signed off by a qualified structural engineer who would also carry the appropriate indemnity insurance.

It wont be the remit of the la engineers to check and sign off your drawings.

 

[russellsmiler]

hello Alastair

Yes thats what my logical way of thinking was telling me however!

Math is math..so would i be right in thinking

the proffesional indemnity of a structural enginner would only be called upon if a problem appeared and it was found to be specified wrong, meaning that if it specified correctly then it would not fail.

(obviously installation is the builders resposibility and the structural engineers PI as nothing to do with this)

 

[tony1851]

I stand to be corrected here, but as far as i'm aware your steel has to be designed/ specified and signed off by a qualified structural engineer

alastair;
That's a common misconception; anyone can do structual calcs. The Council's job is to check the calcs as submitted - they can't insist on a person having a formal qualification.

 

[tony1851]

the proffesional indemnity of a structural enginner would only be called upon if a problem appeared

Not necessarily.

Most engineers have small print in their calcs which excludes any liability for failure of any part of the structure other than the beam and padstones.
For example, the wall supporting the end of the beam may be in such poor brickwork below the plaster that it isn't capable of supporting the reaction from the beam. How would the engineer know the condition before work starts?

By the way, two 178x102 UBs would do.

 

[Alastairreid]

I stand to be corrected here, but as far as i'm aware your steel has to be designed/ specified and signed off by a qualified structural engineer

alastair;
That's a common misconception; anyone can do structual calcs. The Council's job is to check the calcs as submitted - they can't insist on a person having a formal qualification.

Mores the pity.

http://www.structuralengineersgrantham.co.uk/appointing_an_engineer.html

The sooner it is a criminal offence to offer structural advice without appropriate qualifications and professional indemnity insurance the better!! Would you go to a an uninsured doctor or dentist?

 

[russellsmiler]

Hello Tony
Thanks for your interest in my topic!

Your footnote of 2 no 178 x 102 UB will do, is that like calculated??

Just for my own revisit to what i was taught, is there three criteia that the beam as to meet to be acceptable??

I seem to remember deflection first then shear, but not sure what other criteria the beam might have to satisfy..is it resistance to lateral buckling??

Would be nice to relearn a little here if possible.

Thanks
Russell

 

[tony1851]

Your footnote of 2 no 178 x 102 UB will do, is that like calculated??

From experience on similar spans, yes, particularly if no floor is supported.
The actual range of sections used in domestic work is usually quite small;
127x76, 152x89, 178x102, 203x102 and 203 x 133 cover 95% of situations.
Its about choosing what appears to be the smallest suitable section, and then doing the calcs to show that it works. If it works, you try the next smallest section to see if that could work. If it doesn't, you go next one up and try again.

In your situation, the two main points to consider are lateral buckling of the beam(s), and deflection.

I allow for lateral buckling by assuming the beam is unrestrained, and then reducing the allowable stress in the beam to bring it within the buckling resistance moment for the span (these are easily had from published tables).

For deflection, you just use the standard equation for uniform loading
5*W*L^3/384*E*I