Anchorage, laps and cover
Cover check to links and main bars (Eurocode 2 cnom = cmin + Δcdev)
- where
- minimum cover for bond to the main bars (mm)
- main bar diameter (mm)
- 5 mm if the aggregate is larger than 32 mm, otherwise 0
- nominal cover needed to the main bars (mm)
- minimum cover for durability (mm)
- allowance for deviation (mm)
- nominal cover needed to the links (mm)
- link diameter (mm)
- spacer size needed, cover to the outermost bar (mm)
- margin, provided minus needed (mm)
- cover provided by the spacers (mm)
- cover provided to the main bars (mm)
Explain this equation
Checks that the spacers you are using give the Eurocode 2 nominal cover, allowing for the link sitting outside the main bars. Use it before an inspection or when choosing spacers.
cb,m is the cover needed for bond to the main bar: its own diameter φ, plus k = 5 mm when the aggregate is over 32 mm. cdur is the durability cover for the exposure and design life from BS 8500 or the drawing. Δcdev is the allowance for fixing tolerance, 10 mm in the UK. φl is the link size and c the spacer size you are using.
The nominal cover is the largest of the bond, durability and 10 mm minimums plus the deviation allowance. The links need their own cover, and the main bars need theirs measured through the link, so the spacer size cs is the larger of the link requirement and the main-bar requirement less the link diameter.
Sense check: 25 mm bars with 10 mm links and 30 mm durability cover need 40 mm to the main bars and 40 mm spacers.
Watch out: fire resistance or casting against ground (40 or 75 mm) can need more.
Checks the spacer size against Eurocode 2 nominal cover, allowing for the link sitting outside the main bars, so you can confirm the right spacers before the steel is inspected.
Enter your values to see the result.
Good to know
Minimum cover is the largest of the bar diameter (for bond, plus 5 mm if the aggregate is over 32 mm), the durability cover for the exposure class from BS 8500 or the drawing, and 10 mm. A deviation allowance (10 mm in the UK National Annex) is added to give the nominal cover cnom that spacers are sized to. The link needs its own cover, and the main bars need theirs measured past the link, so the spacer size is the larger of the two. Casting against blinding needs at least 40 mm and directly against soil 75 mm.
Common mistake
Choosing spacers for the cover to the main bars and forgetting the link sits outside them, so the links end up 8–12 mm short of cover.
Limits
Bond and durability cover only. Fire resistance (axis distance), abrasion, casting against ground and special finishes can need more; bundled bars use an equivalent diameter. The cover stated on the drawings governs.
Sources
- BS EN 1992-1-1:2004 Eurocode 2 (Table 3.1 fctm, fctk,0.05; 3.1.6 fctd; 4.4.1 cover; 8.4 anchorage, Eq. 8.2–8.6; 8.7 laps, Eq. 8.10–8.11)
- UK National Annex to BS EN 1992-1-1:2004+A1:2014 (γc = 1.5, αct = 1.0, Δcdev recommended 10 mm)
- The Concrete Centre, Eurocode 2 FAQs (deviation allowance Δcdev usually 10 mm)
- LABC Warranty, How to get it right: using reinforcement in your foundations (spacers at no more than 1 m centres; mesh lap rule of thumb two wire spacings + 50 mm; minimum covers)
- BS 7973-2:2001 Spacers and chairs for steel reinforcement and their specification. Fixing and application of spacers and chairs and tying of reinforcement
Results are calculations from the published methods and figures shown, and depend on the data you enter. Always work to the engineer’s drawings and bending schedules, BS 8666, BS EN 1992, BS EN 1090-2, BS 5531/the BCSA National Structural Steelwork Specification and manufacturers’ data. Lifting must be planned under LOLER by a competent person.