Glass Canopy on Flat Roof - Steels or Wooden Joists Support?

[KenExtends]

My structural steel woes continue:

I have a 'small' area 4.3m x 2.8m including supporting wall (depth to 100m) that is single storey where I want to put a glass canopy as part of the otherwise flat roof.

The glass canopy is 2.5m x 1.5m approx. My plans show two 'huge' 152 steel beams across the 2.8m span just to support the approx 250 kg mass of the canopy plus the normal roofing material.

The builder has advised me that it is perfectly normal just to have wooden joists - doubled, if necessary, to support such a small additional load, however, the engineer who calculated the steels insists that steels are needed due to snow load and for safety.

I have tried to make sense of the calculations, however, the best I can come up with is that wooden joists can support a tonne or more (depending on size of joist and type of wood), and therefore, steels are completely unnecessary - and just a waist of money in both cost and construction.

Am I correct?

 

[noseall]

Structural engineers and BCO's alike have said to us in the past that if we feel that someone is 'over egging the pudding' as it were, feel free to offer an alternative provided you can show it will work and that we are prepared to underwrite it.

We have also had common sense strategy passed too.

 

[tony1851]

can you post a drawing of the structure? or perhaps some of the figures?

 

[KenExtends]

Untitled

• KenExtends
• 16 Mar 2012
• 1

Untitled

• KenExtends
• 16 Mar 2012
• 1

Untitled

• KenExtends
• 16 Mar 2012
• 1

The person who drew up the plans put a crude lean-to sloping roof on the original plans, hence the strange terminology in the above text. The first version of the steels did not include these structural elements!

 

[tony1851]

The figures have come out Ok but the drawing is indeciferable. Do you have a photo, or possibly elevation drawings?
Certainly the figures for bending moment look well below the capacity of the specified steel beam for that span.

 

[KenExtends]

Here is an extract from the plans:

Untitled

• KenExtends
• 17 Mar 2012
• 1

The cross section mentions the joists as being 150mmx40mm, but the remainder of the joists are 225mm x 47mm

Untitled

• KenExtends
• 17 Mar 2012
• 1

In looking at the problem more closely (in answering your question), I was told by the builder that the roof had to be flat as it was impossible to achieve what I originally wanted - a canopy with tiles flush to the slope fo the glass to give the illusion of a part tiled/part glass roof (as the beam would get in the way as it would sit on the wall - and, I had to have a completely flat roof. Therefore, has the problme arisen because they have tried to put 'cranked' beams in on the plans?

Please also note that during the process, I belatedly discovered that the plans were drawn up on Apple - line by line, and not by CAD, therefore, there was no consistency throughout my drawings and as the drawing process had taken so long [he didn't tell me about the commercial job he took on] - I have to confess that by the time the steels were added and the canopy roof changed I was not as diligent as I had been when correcting the numerous initial errors caused both by the separate nature of each of the original drawings and other mistakes that he had made (he rushed the work as well), as we just wanted the builder to start and we had to get BC sign off - this whole process took over four months...

 

[tony1851]

The latest drawings make it a little clearer. If the SE initially assumed a cranked beam would be necessary to achieve the effect you wanted, steel would probably be the best choice.
However, if the roof is to be flat, and the span is 2.8m, then timber would be an economic choice.
2 no. 47x200 C16 grade timbers bolted together would support a uniform load of 10.9kN.
2 no. 47x200 C24 grade timbers bolted together would support a uniform load of 15kN.
If you allow 0.75kN/sq.m. for your snow load and, say. 1.0 kN/sq.m. for dead load, then the C16s would support 6.2 sq. m. of roof. (the appropriate safety factors are built into these figures). Note that if the loading was not evenly distributed along the beam, these figures would need to be slightly modified.
The beams could be notched onto the bottom flange of the main steel beam.
(if you want to see the figures, leave a message).
Hope this helps.

 

[KenExtends]

Tony

This is most useful and your earlier prompting has helped clarify the situation I found myself in.

It would be useful to see the numbers behind your calcs as I like to learn about what is going on, and in the (unlikely?! - well this is the most stressful time) event of us ever having to undertake another extension of this magnitude, then I'll be better prepared next time.

Once again, thanks.

Ken

 

[tony1851]

see note in Messages

 

[Static]

Wow.. even i cant justify why hes used such a large steel for such a small load.. even cranked the size of steel seems excessive..