REINFORCED CONCRETE STRUCTURAL DESIGNC4301/UNIT7/ 1 UNIT 7 DESIGN DETAILS OF BEAMS OBJECTIVES GENERAL OBJECTIVE To understand the principles in providing the design details of reinforced concrete beams according to BS 8110 requirements. SPECIFIC OBJECTIVES At the end of this unit you will be able to; 1. 2. determine the amount of nominal cover to reinforcement. check whether the minimum and maximum areas of reinforcement are within limits. 3. 4. 5. 6. determine the distance between bars. calculate anchorage lengths. calculate lap lengths. calculate the cut-off distance for curtailments. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 2 INPUT 1 7.1 Introduction Reinforced concrete structural elements are designed so that they meet the requirements of Ultimate Limit State (ULS) and Serviceability Limit State (SLS). In addition to these requirements, they should also satisfy other requirements such as the concrete mix, nominal cover to reinforcement and detailing of the reinforcements. These requirements if fulfilled will ensure that the elements are durable; this is in construction, economical and cost-effective. 7.2 Concrete Cover Nominal cover is the thickness of concrete which protects all reinforcements including links from the surrounding environment in which they are exposed to. Adequate cover should be provided in order to protect the reinforcement from fire and corrosion. The amount of cover to be provided is given in Table 3.4 and 3.5, BS 8110. It can be seen that the nominal cover to be provided depends on various affecting factors. They are as follows; 1) 2) 3) exposure conditions grade of concrete and fire resistance. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 3 The limits on water/cement ratio and cement content will automatically be assured by specifying the minimum grades of concrete indicated in the Table3.3, BS8110. In actual structure, the nominal cover dimension shown in the drawings can never be maintained at 100%. Therefore, some tolerances must be provided. The actual cover to all reinforcements should never be less than the nominal cover minus 5 mm. The nominal cover should also comply with the recommendations for bar size, aggregate size and concrete cast against uneven surfaces. These are elaborated as follows; a) The nominal cover to a main bar should not be less than the size of the main bar if it is a single bar, or the equivalent size if they are in pairs. b) The nominal cover should not be less than the nominal maximum size of aggregates. This is to facilitate the compaction of fresh concrete. c) Concrete cast directly against earth as in foundations, the nominal cover should not be less than 75 mm. If blinding is provided, the nominal cover should not be less than 40 mm. In choosing the appropriate cover for a particular structural element, we should take the largest value derived from the following factors; a) b) c) bar size environmental conditions fire resistance REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 4 7.3 Minimum and maximum area requirements BS 8110 recommends that the minimum crack width should not exceed 0.3mm. This is to avoid corrosion of the reinforcements. Observing the detailing rules regarding the minimum areas of reinforcement and also the maximum spacing of the bars will satisfy this requirement. Minimum areas of reinforcement are given in Table 3.27 of the code. Reference should also to be made to clause 3.12.5.3. The area of reinforcement provided must exceed the minimum given in Table 3.27 to ensure that the structural element will not be under reinforced. The maximum area of reinforcement requirement is to ensure that the structural element will not be congested with too much steel. Clause 3.12.6 of the code states that the maximum area of reinforcement in a beam should not exceed 4% of the gross concrete sectional area. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 5 ACTIVITY 7a TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE TO THE NEXT INPUT! 7.1. From the given information below, determine the nominal cover required for simply supported beam. Exposure condition: moderate Concrete grade: 35 Fire resistance: 1 .5 hour Maximum aggregate size: 20 mm For the given beam section below, calculate the minimum and maximum areas of reinforcement and then decide whether the steel provided is satisfactory. b = 250 h = 700 4T20 Figure 7.1: Typical section of beam REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 6 FEEDBACK 7a Now, let¶s check your answers. 3.1 From Table 3.4, BS8110: Nominal cover = 35 mm From Table 3.5, nominal cover = 20 mm Maximum aggregate size = 20 mm Therefore, the nominal cover is the greatest value derived from all the three conditions, i.e. 35 mm. 1. From Table 3.25, BS 8110 : Minimum % of reinforcement = 0.13 % This implies that, 100 As ! 0.13 Ac 0.13 v 250 v 700 100 The minimum area of reinforcement is 228 mm2 . ¡ s = 228 mm2 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 7 Maximum area of reinforcement ! 4 v Ac 100 4 v 250 v 700 100 Thus, for 4T20 (As = 1260 mm2 ), since 0.13% < As < 4% bh, therefore, the area of reinforcement provided is all right. ` Oh, you are clever! You can do it. When you study and understand it, everything is not difficult for you. Come on. We will do the next part. ¢ ! 7000mm 2 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 8 INPUT 2 7.4 Spacing of reinforcement BS 8110 specifies the minimum and maximum distances between tension reinforcement. The minimum distance is based on the need to achieve good compaction of the concrete around the reinforcement. The limit on the maximum distance is to ensure that the maximum crack width is under control to prevent corrosion of the embedded bars. For singly reinforced simply supported beams, the clear horizontal distance between tension bars, denoted as sb must lie within the following limits; a) For fy = 250 N/mm2 ; hagg + 5 mm or bar size sb 300 mm b) For fy = 460 N/mm2 , hagg + 5 mm or bar size sb 160 mm Where hagg is the maximum size of coarse aggregate used in the concrete mix. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 9 If the beam is provided with more than one layer of reinforcement, the clear vertical distance between bars should not be less than diagrammatically in Figure 7.2 below; 2 hagg . This is shown 3 2 hagg 3 hagg + 5 mm Fig 7.2: Minimum Clear Distance REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 10 ACTIVITY 7b For the given beam section below, determine the clear distance between bars and then decide whether the spacing requirements are satisfactory. Assume that the cover is 40 mm and maximum aggregate size is 25 mm. 325 mm T16 T20 T16 FEEDBACK 7b REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 11 16 20 16 ab ab 40 40 Let the clear distance between bars is ab . Then, a b + 2 (40 + 16 + 20 + 20) = 325 mm Therefore, a b = 133 mm From Table 3.30, BS8110, For fy = 460 N/mm2 and assuming no redistribution of moment, Clear distance between bars = 160 mm hagg + 5 mm = 25 + 5 = 30 mm Since hagg + 5 mm or bar size sb is all right. 160 mm, the spacing requirement INPUT 3 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 12 7.5 Anchorage of bars Reinforcement bars subjected to direct tensile force must be adequately anchored so that they will not slip-out of the concrete. Bars subjected to bending should also be provided with adequate anchorage to ensure that the design stress (0.87 fy for mid-span) will not be reduced. Bars are anchored in the concrete in order to develop this value. Figure 7.3 gives some clarification on this matter. Bar slip when anchorage is not adequate Load Bar slip when anchorage is not adequate Support Support Fig 7.3 Anchorage Failure Anchorage is normally achieved by extending bars beyond the point at which they are theoretically no longer required. This length is equal to the greater of the: REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 13 a) b) effective depth of the member 12 times the bar size In order to be effective, anchorage is also dependent upon the bond strength between bar, concrete and surface area in contact. Anchorage length is calculated using the following equation: L! 0.87 f y * 4F f cu Where * = bar size and F = bonding coefficient from Table 3.28 of the code. Sometimes it is possible to use straight bars when available space is limited. In this case, anchorage is provided using hooks or bends. The anchorage values of hooks and bends are shown in Figure 7.4a and 7.4b that¶s given in the next page. 4 r REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 14 Figure 7.4a: Anchorage length for 90 0 bend Note that the anchorage length for 900 bend = 4r but it is not greater than 12 Figure 7.4b: Anchorage length for hook Note: Anchorage length for hook = 8r but it is not greater than 24 Note that the anchorage length for 90 bend = 4r but it is not greater than 12 Note: Anchorage length for hook = 8r but it is not greater than 24 For mild steel bars minimum radius, r = 2 For high yield bars minimum radius, r = 3 or 4 £ £ £ £ £ 4 r £ £ for sizes 25 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 15 It should be noted that the radius of the bend is greater for high yield bars than for more ductile mild steel bars. Bending is to be done in accordance to BS4466. The reductions, which are made for bends and hooks, are formed in accordance with the standards for bar bending. Refer to Figure 7.4a and 7.4b. ACTIVITY 7c REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 16 Calculate the anchorage length that is required for the bar shown below; 7.2 ? Anchorage length = _______________________ 2T20 7.3 ? d = 550mm 3T25 Anchorage length = _______________________ 7.4 Given, fy = 460 N/mm2 , fcu = 30 N/mm2, bar size is 16 mm of Deformed Bar Type 2 High Yield Steel as tension reinforcement. Calculate the anchorage length for this bar. FEEDBACK 7c REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 17 Compare your answers with the calculations that are given below. Please refer to your lecturer if you get any confusion. Enjoy it! 7.2 Anchorage length = 12 * = 12 x 20 mm = 240 mm This is to be extended beyond the centre line of support. 7.3 Anchorage length measured from the face of support; ! d 12* 2 550 12( 25) 2 ! ! 575mm 7.4 From Table 3.28 of BS8110, F ! 0.50 Anchorage length, ! 0.87 f y v * 4F f cu ¤ ! 0.87 v 460 v 16 4 v 0.5 v 30 ! 584.5mm INPUT 4 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 18 7.6 Curtailment of reinforcement Steel reinforcement can be cut-off or their numbers can be reduced at suitable places in the beams for various reasons. They are as follows; a) Bending moment decreases on either side at mid-span. The corresponding area of bending reinforcement will be reduced because smaller bending moments require a smaller number and hence smaller areas of reinforcement. b) The reduction in number of bars helps in reducing cost and they are easier to handle too. c) Congestion of bars can be avoided thus enhancing the compaction of fresh concrete. Simplified rules for curtailment of bars are given in Clause 3.12.10.2 of BS8110. These are shown diagrammatically in Figure 3.24 of the code for simply supported, continuous and cantilever beams. The simplified rules are used when the following conditions are met; a) b) The beams carry predominantly uniformly distributed loads. In the case of continuous beams, the spans are approximately equal. ACTIVITY 7d REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 19 Calculate the curtailment distance indicated in the given figures below; 7.5 Simply supported beam ? 10.0m Distance=______________________________ 7.6 Cantilever 4 T20 ? Distance = ____________________________________ FEEDBACK 7d REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 20 Let¶s check the answers together. 7.5 0.08 l = 0.08 x 10,000 mm = 800 mm 7.6 l 3000 ! 2 2 ! 1500 mm 45 * = 45 x 20 = 900 mm Since l " 45* , the curtailment distance is taken as 1500 mm. 2 ´Are your answers correct? ¶Yes!· Congratulations if you have got the right answers. ¶No?· Never mind if your answers are wrong. Please correct them and make sure you do not repeat it. INPUT 5 7.7 Laps in reinforcement REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 21 Lapping of bars is necessary in order to transfer stresses from a bar to another bar. Bars that have been joined in this way acts as a single length bar. The lap should be sufficiently long. The minimum lap length should not be less than 15 times the bar diameter or 300 mm, whichever is the greater, as stated in Clause 3.12.8.11.of BS8110. For tension laps, it is normally equal to the tension anchorage length, but will often need to be increased as outlined in clause 3.12.8.13 of the code. The anchorage length, L, is calculated using the following equation; L = LA * Where * is the diameter of the smaller bar and LA is to be obtained from Table 3.29 of the code. For compression laps, the lap length should be at least 1.25 times the compression anchorage length. Note that a longer lap length is required at the top and corner of the beam section. This is because at the top of the section, fresh concrete is less compact and contain more water than at anywhere else in the section, while at the corner of the section, bars are less restrained. ACTIVITY 7e Fill in the blanks. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 22 7.7 Lapping of bars is required to __________ stresses from one bar to the other. 7.8 7.9 7.10 Lap length to be provided must not be less than ______________. Longer lap lengths are required at __________. For T20 Deformed Type 1 bars, tension lap length is equal to __________ mm if concrete of grade 25 is used. 7.11 For T16 deformed Type 2 bars, the compression lap length is equal to ______ mm when concrete of grade 30 is used. 7.12 The minimum lap length for bars in question 5 is equal to __________mm. 7.13 The compression anchorage length, L for bar in question 4 is equal to ______mm. 7.14 i) Calculate the lap length for bars given in question i to iv. Cover = 40mm ii). Cover = 30 m REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 23 iii) 65 mm iv) 60 mm Cover = 40 mm FEEDBACK 7e REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 24 7.7 Transfer 7.8 15 * or 300 mm 7.9 top and corner of section. 7.10 51* ! 51 v 20 = 1020 mm 7.11 7 = 37 x 16 = 592 mm 7.12 1.25 x 29 = 1.25 x 29 x 16 = 580 mm 7.13 41 = 41 x 20 = 820 mm 7.14 i) Lap length = 1.4 x tension anchorage length = 1.4 x 37 = 1.4 x.37 x 20 = 1036 mm ii) iii) Lap length = 1.4 x 37 Lap length = 1.4 x 37 = 1.4 x 20 = 1036 mm = 1.4 x 20 = 1036 mm iv) Lap length = 2.0 x tension anchorage length = 2.0 x 37 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 25 = 2.0 x 37 x 20 = 1480 mm. SUMMARY This unit should give you some understanding of the design details of reinforced concrete beams. The design details requirements that we have discussed are as follows; REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 26 1. Minimum concrete cover This is given by clause 3.3.1 of BS 8110 and is dependent on: a) b) c) d) e) f) diameter of bar type of structural member exposure conditions of concrete type of cement used type of aggregate used fire resistance required 2. Laps These may be required« a) b) c) to reduce bar handling length at construction joints in confined spaces The minimum length of bars is given by« a) b) c) concrete mix type of bar type of stress The position of laps must be: a) b) at points of minimum stress staggered in adjacent bars 3. Hooks and Bends The minimum internal radius must be: REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 27 a) b) c) twice the bar diameter for mild steel thrice the bar diameter for high yield steel the radius of the anchoring bar which is to be bend round Hooks and bends must: a) b) have a minimum straight length of four times the bar diameter be marked hook up or hook down when at right angles to the plan of the detail drawing c) d) not be positioned in tensile zones not foul with other bars 4. Minimum and maximum areas of reinforcement For rectangular beams with overall dimensions b and h, the area of tension reinforcement, As should lie within the following limits; 0.24% As 4 % bh when fy = 250 N/mm2 0.13 % bh As 4% bh when fy = 460 N/mm2 5. Arrangement of bars a) b) a minimum cover must satisfy clause 3.3.1 of BS 8110 horizontal and vertical spacing must satisfy clause 3.12.11 of BS 8110 c) if different diameters, then the larger diameter bars must be ; i. in a single row placed outside REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 28 ii. iii. for two or more rows placed in the lower row placed at the upper row on top over supports 6. Curtailment of bars a) theoretical cut-off position determined from bending moment diagram b) c) d) e) actual position allowed for bond length cut-off bars in pairs symmetrical about beam centre line 50% of bottom bars to be carried through to the support simplified curtailment rules must satisfy clause 3.121.10.2 of BS8110 SELF-ASSESSMENT REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 29 Answer all the questions given in this section. Award 1 mark for every correct answer. There are TEN questions (100 % marks). Select your answer by encircling the alphabet of your choice (A to D). Questions: 1. Which of the following beam sections correctly explain the meaning of nominal A. cover, B. c? Cover Cover C. D. Cover Cover 2. The nominal cover should not be less than the following values EXCEPT « A. B. the amount shown in Table 3.4 of BS8110. the amount shown in Table 3.2 of BS8110. REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 30 C. D. (for main bars) the bar size. the nominal maximum size of aggregate. 3. For the given beam section, what is the minimum area of reinforcement, assuming high yield steel is used? 600 mm 300 mm A. B. C. D. 468 mm2 324 mm2 234 mm2 432 mm2 4. What is the maximum area of reinforcement for the beam section in Question 3? A. B. C. 7200 mm2 10800 mm2 14400 mm2 REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 31 D. 18000 mm2 5. If T16 bars are used, what is the minimum lap length required? A. B. C. D. 220 mm 250mm 300 mm 320 mm 6. For the beam and reinforcement shown below, what is the minimum horizontal clear distance between bars if the maximum aggregate size , hagg = 20 mm ? T32 T25 T32 A. B. C. D. 20 mm 25 mm 30 mm 32 mm REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 32 7. What is the minimum vertical clear distance for bars in beam section shown below? (Use hagg = 25 mm). ? A. B. C. D. 10 mm 17 mm 24 mm 31 mm 8. The beam section given below is taken at mid-span of a simply supported rectangular beam. If Type 1 deformed bars are used, the bond coefficient, is equal to « REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 33 2 T20 A. B. C. D. 0.28 0.35 0.40 0.50 9. For the given continuous beam, what is the length of curtailment for the reinforcement shown? ? A. B. C. D. 975 mm 1625 mm 650 mm 520 mm REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 34 10. For the given continuous beam of equal span of 6.5 m, what is the curtailment distance for top steel at support indicated in the figure where 40% of reinforcement will be cut-off ? (Assume d = 600 mm). ? 6.5 m A. B. C. D. 975 mm 1625 mm 300 mm 529 mm FEEDBACK ON SELF-ASSESSMENT Now check your answers with the answers given below. 1. 2. D B 6. 7. 8. 9. 10. D B C C A REINFORCED CONCRETE STRUCTURAL DESIGN C4301/UNIT7/ 35 3. 4. 5. C A D NOW TOTAL UP YOUR MARKS. Calculate your score as shown below: Score = total marks obtained 10 x 100% You should have scored 80% or more to pass this unit. If you have scored 80% or more, you may proceed to unit 8 If you have scored less than 80%, you should go through unit 7 again until you pass this unit. See you in UNIT 8. END OF UNIT 7