Slim Concrete Columns EC Vs BS - NHBC [PDF]

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Slim concrete columns – fire resistant design Patrick Piggott Major Projects Engineer, NHBC



Slim concrete columns – fire resistant design NHBC no longer accept the use of blade columns



• Not true! • Fire resistance sometimes overlooked • Appropriately designed • Exposure to fire – realistic?



Slim concrete columns – fire resistant design • Slim column - definition • Concrete in fire.



• Exposure - realistic? • Design approaches. • Supplementary protection. • Buildability.



Definition Slim or Blade Column



Columns 1



1



1



=< 4



Walls



Considered a Wall 1



>4



Slim concrete columns – fire resistant design Column 250mm or less



Heat transfer



Column core remains relatively cool.



Heat degradation



Concrete



Steel Reinforcement



1200oC concrete melts! 800oC steel strength lost



600oC strength / stiffness lost 350oC strength / stiffness loss starts



400oC strength loss starts



Heated column 500oC temperature contour (isotherm)



Load bearing based on reduced concrete section



400 500 800



Reinforcement 400 - 800oC - reduced strength



Degree of exposure – Compartment Wall Compartment Wall 60 mins FR 0 - 60 mins (Fire - one side)



Column 120 mins FR



61 - 120 mins (Fire – more than one side)



Fire breaks through wall



Degree of exposure – External Wall



0 - 30 mins (Fire - one side)



31 - 120 mins (Fire – more than one side)



External Wall 30 mins FR or less Column 120 mins FR



Fire breaks through wall



Degree of exposure • Realistic assessment of exposure to fire. • Will be challenged if not robust. So: • Engage early with NHBC Engineer for advice.



Design Codes Eurocode 2 - BS EN 1992



BS 8110



• Refined Tabular



• Basic Tabular



• Simplified Calculation



• Fire Testing



• Advanced Calculation



• Fire Testing



Design Approach Eurocode 2 - BS EN 1992 – Tabular Method - Columns • Assumes square columns - slim columns can be assessed.



• Sides exposed. • Load during fire • Min. column width • Min. depth to main rebar



Design Approach Eurocode 2 - BS EN 1992 – Tabular Method Walls • Use if width to depth exceeds 1:4. • Sides exposed. • Load during fire • Min. column width • Min. depth to main rebar



Design Approach Eurocode 2 - BS EN 1992 – Calculation Models



• Heat transfer. • Capacity reduction - heat and degradation.



Reinforcement 400 - 800oC has reduced strength



Reduced section



• Load capacity during fire situation. Depth



Width



Design Approachcolumns – Design Approaches BS 8110 – Tabular Method Columns • Assumes square columns (no provision for slim columns).



• Sides exposed. • Load during fire. • Min. column width • Min. cover to main rebar



Design Approach BS 8110 – Tabular Method Walls • Use if width to depth exceeds 1:4. • Sides exposed. • Load during fire. • Min. column width • Min. cover to main rebar



Design Approaches Eurocode 2



BS 8110



• Refined prescriptive (tabular) design; conservative.



• Basic prescriptive (tabular) design; very conservative.



• Performance based (engineered) design; realistic / less conservative.



• Performance based design – Not covered.



• Modelling / design of actual slim columns not possible.



Design Alternatives Equivalent time of fire exposure – BS EN 1991-1-2 (Eurocode 1) Determines the severity of a real fire. The duration of structural fire resistance required can then be reduced to suit the real fire.



Design Alternatives Supplementary Fire Protection Fire-boards or surface coatings. Independent third party fire testing required. Installation must follow the tested arrangement exactly!



Design Approach - Summary • Eurocode 2 - flexible approaches realistic design possible. • BS 8110 - prescriptive - very conservative design. • Fire testing - unlikely to be cost-effective. • Supplementary protection - consider with care.



Retrospective Justification • Discuss with the NHBC Engineer.



• If exposure unrealistic use of Eurocode 2 can be considered – subject to appropriate caution. • Exhaust all desk based options.



Buildability • Slim columns with enhanced cover. • Concrete placement and compaction.



• Workmanship critical!



180



• Reduce aggregate size.



Clear gap c. 50mm



Slim concrete columns – fire resistant design Summary – Key Points • Realistic exposure to fire. • Eurocode 2 - more accurate / economical design. • BS 8110 - limited scope and conservative – two side exposure of walls requires alternative justification. • With appropriate design - no need for supplementary protection. • Contact NHBC Engineer early for advice.