Curtailment

Wherever possible simplified methods of curtailing reinforcement would be followed. The following is intended to show how a rigorous assessment of curtailment of reinforcement might be undertaken. End support A: bottom steel at support Check anchorage

As simply supported, 25% of As should be anchored in support. 25% x 595 = 148 mm2

Check anchorage length

Envelope of tensile force:

To resist envelope of tensile force, provide reinforcement to al or lbd beyond centreline of support. For members without shear reinforcement, al = d = 232 By inspection, ued = 0, lbd = lbd,min = max(10$ 100 mm)

Indirect support:

As anchorage may be measured from face of indirect support, check, force to be resisted at solid/rib interface:

<9.3.1.1(4), 9.3.1.2(1) & Note, 9.2.1.4(1) & NA>

<9.3.1.1(4), 9.2.1.3(1), 9.2.1.3(2), 9.2.1.3(3) Fig. 9.2> <9.2.1.3>

<9.3.1.1(4), 9.2.1.4(2), 9.2.1.4(3), Fig. 9.3b>

0.666

= 29.3 x 1.25 = 36.6 kN / rib Fs = 18.6 x 106 / (220 x 103) + 36.6 = 121.1 kN

Anchorage length a/b.rqd > L

bd where:

a = conservatively 1.0

a"ed = design stress in the bar at the ULS

= 121.1 x 1000 / 314 = 385 MPa fbd = ultimate bond stress = 2.25 771 % ft.d where n = 1.0 for good bond conditions n2 = 1.0 for bar diameter < 32 mm fct.d = act fct.k / X;

= 1.0 x 2.2 / 1.5 = 1.47 MPa fbd = 2.25 x 1.47 = 3.31 MPa .•. /b,rqd = (20 / 4) (385 / 3.31) = 581 mm /bmin = max[10p 100 mm] = 200 mm

.'. /bd= 581 mm measured from solid/rib intersection.

i.e. 31 mm beyond centreline of support****. End support A: top steel

Assuming partial fixity exists at end supports, 15% of As is required to extend at least 0.2 x the length of the adjacent spanssss, Asreq = 15% x 525 = 79 mm2 / rib

: 0.26 x 0.30 x 350666 x 159 x 257 / 500 = 68 mm2 / rib

Use 2 no. H12 T1 / rib in rib and 2 H10 T1 / rib between ribs

Support B (and C): top steel

At the centreline of support (2 H16 T + 3 H12 T) / rib are required. The intention is to curtail in two stages, firstly to 2 H16 T / rib then to 2 H12 T / rib. Curtailment of 2 H16 T / rib at support (capacity of 2 H12 T / rib + shift rule)

Whilst this would comply with the requirements of Eurocode 2, it is common practice to take bottom bars 0.5 x a tension lap beyond the centreline of support (= 250 mm beyond the centreline of support; see MS1 in SMDSC[21]).

§§§§ It is usual to curtail 50% of the required reinforcement at 0.2/ and to curtail the remaining 50% at 0.3/ or line of zero moment (see MS2 in SMDSC[21]).

Assume use of 2 H12 T throughout in midspan: Assuming z = 211 mm as before, MR2H12T = 2 x 113 x 434.8 x 211

= 20.7 kNm / rib (23.0 kNm / m) (Note: section remains under-reinforced) From analysis MEd = 23.0 kNm / m occurs at 2250 mm (towards A) and 2575 mm (towards B).

Shift rule: a/ = z cot Q/ 2 Assuming z = 211 as before a, = 1.25 x 211 = 264 mm

■ 2 no.H12 T are adequate from 2250 + 264 = 2513 mm from B towards A and 2575 + 263 = 2838 mm from B towards C.

■ Curtail 2 no.H16 T @ say 2600 from BA and 2850 from BC Curtailment of 3 no.H12 T / rib at support (capacity of 2 H16 T / rib + shift rule)

= 36.9 kNm / rib (41.0 kNm / m) (Note: section remains under-reinforced)

From analysis MEd = 41.0 kNm / m occurs at 1310 mm (towards A) and 1180 mm (towards C).

Shift rule: at = 263 mm as before

.'. 2 no.H16 T are adequate from 1310 + 263 = 1573 mm from B towards A and 1180 + 263 = 1443 mm from B towards C.

■ •■ Curtail 3 no. H12 at say 1600 from B (or C).

Curtailment Reinforcement

Figure 3.16

Curtailment of top reinforcement at B per rib

Figure 3.16

Curtailment of top reinforcement at B per rib

Support B (and C) bottom steel at support

At the support 25% of span steel required

Try 1 no.H16 B / rib (201) This reinforcement may be anchored into indirect support or carried through.

Support B (and C): bottom steel curtailment BA and BC

To suit prefabrication 2 no.H20 / rib will be curtailed at solid/rib interface, 1000 mm from BA (B towards A) and BC. From analysis, at solid/rib interface sagging moment = 0. From analysis, at solid/rib interface + a,, i.e. at 1000 + 1.25 x 244 = 1303 mm at 1305 mm from BA sagging moment = say 5 kNm / rib at 1305 mm from BC sagging moment = 0

Greener Homes for You

Greener Homes for You

Get All The Support And Guidance You Need To Be A Success At Living Green. This Book Is One Of The Most Valuable Resources In The World When It Comes To Great Tips on Buying, Designing and Building an Eco-friendly Home.

Get My Free Ebook


Responses

  • Melody Smith
    What is curtailment of reinforcement?
    6 years ago
  • katrin
    What is curtailment of steel?
    5 years ago

Post a comment