Fire resistance of external wall

[GB]

A single storey building, 14m high, compartment area of 8000m2, Storage building, with a 3 storey office area in a corner occupying 300m2 - boundary distance of 18m - does the external wall AND elements of structure need to be fire resisting to 90 minutes?

I am reading BS9999 and am confused??

[Tom Sutton]

Have you checked out ABD (B3, B4)?

[GB]

Hi Tom,

Yes I have had a look at these which has pointed me to BRE 197 due to the elevation dimensions being 100m x 14m high. Reading tables within BRE 187 states that with the 18m boundary distance, I would only be allowed 40% of the external wall to be unprotected. Am I correct in assuming therefore that the wall should have a 60 minute FR period with the exception of windows and doors as long as they are less than 40% of that total area?

[kurnal]

The methodology in br187 describes how to encompass the unprotected areas into a rectangle and to assess the area of the rectangle against the total area of the elevation.

[wee brian]

I've not checked the tables, but yes if it says 40% unprotected then 60% of the wall needs to be FR.

[colin todd]

Bet you were not able to add up 40 and 60 and get it to come to 100 when you worked in building control, Bri.

[Phoenix]

Hi GB,

If any fire resistance is required on the external envelope then it will be for the period that is appropriate for the building from Table A2 in ADB or Table 25 or 26 (as applicable) from BS 9999.

As for the percentage of unprotected walling that you can have, I think you got 40% from Table 1 BR 187 for an enclosing rectangle 15 m high and 100 m wide.  40% of the enclosing rectangle is 40% x 15 x 100 = 600 sq m so, at 18 m, you can have 600 sq m of unprotected walling.  The other 800 sq m should be FR.  I'm assuming here that all 1400 sq m of facing wall encloses the storage area, as opposed to partly enclosing a staircase or part of the office accommodation.   

If your building is sprinklered you won't need any FR at all as you can halve the boundary distances in Table 1.  Your building only occupies 93.3% of the enclosing rectangle so remember that 100% of your building is still only 93.3% of the enclosing rectangle.

The above only holds true if your boundary is parallel with the face of the building at a distance of 18 m over its entire length.  If it isn't and if the boundary is further away than 18 m along some of this side of the building then you can fiddle with the figures and obtain more beneficial figures for the required amount of FR.

If this is an existing building then I wonder how it got built in the first place but I would say that there is a good chance that it will not have to comply with the functional requirement of B4 retrospectively and you would only need to consider how the lack of FR might impinge upon the requirements of the RR(FS)O, bearing in mind how the neighbours might be affected.

If this is a new building then you should read BR 187 carefully and apply the recommendations of that document.  Do not try to use the tables or apply the guidance in ADB as it won't apply and, even if it did, it would be more onerous.

 

[GB]

Many thanks,

I have purchased BR 187 and gone through it which has been a very useful exercise.

In relation to the FR periods, what is the forum view in relation to CONSTRADO construction methods with the steelwork and any resultant effect / reduction on the FR periods of structural elements?

[col10]

Fire resistance periods are not altered but  CONSTRADO allows the portal frame rafters to be unprotected.  The rafters only need protection because of the theoretical argument that  the rafters and columns in a portal frame form one structural element.  Rafters aren't required to be protected in a non portal design.

  CONSTRADO recommends; low melting point roof lights and a beefed up foundation design, to vent the fire and give more rigidity to the columns, which seem sensible and reasonable cost.  Expansion of the rafters pushing over a masonry wall onto fire fighting personnel,  is a concern.  There can be a false sense of security behind a thermally insulated, impermeable wall, and our buildings are recently more insulated and more impermeable.

[wee brian]

Isn't CONSTRADO old news?

http://shop.steel-sci.com/products/120-p313-single-storey-steel-framed-buildings-in-fire-boundary-conditions.aspx

[col10]

Yes , the guidance document is no longer called CONSTRADO, and the guidance is within the 2002 SCI document.

[Tom Sutton]

This is out of my depth but does this document have anything to do about your discussion http://fire-research.group.shef.ac.uk/Downloads/YYS_Prague.pdf.