Serviceability, limit state, bar anchorage and lap lengths...

29NOVEMBER 2013 The IndIan ConCreTe Journal

Point of View

Serviceability, limit state, bar anchorage and lap lengths in ACI318:08 and BS8110:97: A comparative study

Ali S. Alnuaimi and Iqbal Y. Patel

This paper presents a comparative calculation study of the deflection, control of crack width, bar anchorage and lap lengths of reinforced concrete beams using the ACI 318 and BS 8110 codes. The predicted deflections by the ACI code were larger than those by the BS. In both the codes, the short-term deflection decreases with the increase in the dead-to-live load ratio but the long-term deflection increases. In addition, the limits on the maximum bar spacing to control crack width vary significantly in the two codes. While the BS code predicts a constant bar spacing regardless of the concrete cover, the ACI reduces it with the increase in cover thickness. In both codes, the tension anchorage length decreases with the increase in concrete strength. The tension anchorage and lap lengths vary with the values of the term

. The BS code requires a greater anchorage length in compression than the ACI code does. The compression lap length requirement in the BS is more than that in ACI code for the concrete of compressive strength less than 37 MPa and the former stipulates longer lap lengths for higher concrete strengths.

In the absence of a national design code, the structural engineers in Oman use the ACI 318 and BS 8110 structural design codes to calculate deflections, crack

width, and anchorage and lap lengths.1,2 They find these codes useful for complying with the legal stipulations there. However, designers and project owners frequently compare the stipulations in the two codes seeking points of similarities and differences. Yet, no comprehensive work of this kind is available in literature, though several researchers have used these codes for estimating deflection, crack control and lap length development in reinforced concrete constructions. The following highlights the findings of select researchers.

Nayak and Menon, conducted experimental investigation on six one-way slabs, monitored their short-term deflection and compared the existing provisions given in IS 456:00, BS 8110, ACI 318 and Euro-code 2 with the experimental results.3,4,5 They found considerable disparities among the three codes. The AC I318 and Euro Code 2 generally predicted acceptable deflection at the least and largest deflection points respectively whereas the BS and IS codes gave an acceptable intermediate value. Santhi et al compared the total deflection including the creep and shrinkage for a two-way slab using AC I318:00, BS 8110:97 and IS 456:00 and found that the total deflection based on ACI 318:00 and BS 8110:97 were almost similar for the different slab thicknesses studied while the IS 456:00 gave much larger deflection in most of the cases.6

The IndIan ConCreTe Journal NOVEMBER 201330

Point of View

Bacinskas et al statistically investigated the accuracy of the long-term deflection predictions made by the various design codes including Eurocode 2, ACI 318, ACI 435, SP 52-101 and the flexural layered deformation model proposed by Kaklauskas.7,8 They found that the Eurocode 2 overestimates the long-term deflection while ACI 318 and ACI 435 underestimate it. The SP 52-101 slightly overestimates the deflection and has the lowest standard deviation among the various code methods studied. Lee and Scanlon conducted parametric study on the control of deflection of reinforced concrete slabs, and compared the various design provisions in the ACI 318:08, BS 8110:97, Euro-code 2 and AS 3600:01.9,10 They concluded that although the minimum thickness values are easy to apply, limitations need to be placed on the applicability of current ACI 318:08 values due to the assumption that the slab thickness is independent of applied dead and live loads and no limits are specified on the applicable range of span lengths. They proposed a unified equation. Bischoff and Scanlon came to a similar conclusion.11

Bacinskas et al developed a model for calculating the long-term deflection of cracked reinforced concrete beams considering creep, shrinkage and the tension-stiffening.12 They compared the ACI 318 and Eurocode 2 provisions with 322 experimental results. Their finding was that the deflections predicted by the ACI 318 were strongly dependent on the loading duration but the results had high variations. However, the predicted deflections by the Eurocode 2 and the proposed model were quite similar and independent of the loading duration.

Subramanian suggested simple formulae, involving the clear cover and calculated stress in reinforcement at service load, to control crack width.13 He criticised the provision made in the Indian code IS 456:00 for crack width calculation and commended the ACI 318:02 provisions. Alam et al criticised the Euro code 2 for under estimating the crack width and crack spacing due to neglecting the structural member size influence which they found had significant effect.14

Khan et al compared the value of bar development lengths obtained using ACI 318:99, BS 8110:85 and IS 456:00. IS code gave the development length 8 percent and 11 percent more than that by BS and ACI codes respectively. The development length obtained in compression using IS code was 3.5 percent and 17 percent more than that used by BS and ACI codes respectively.15 Subramanian compared the IS 450:00

provisions for the development length with the ACI code. He suggested a formula to improve the existing IS provision.16,4 The formula includes factors such as bar diameter, concrete cover spacing of bars, transverse reinforcement, grade and confinement of concrete around the bars, type of aggregate, type of bars and coating applied on bars, if any.

Haitao et al compared the experimental test results of lap length development from eighteen reinforced concrete beams with eight international codes requirements.17 They found that all the codes were conservative in specifying lap length development for small diameter bars and that ACI 318:05 and ACI Committee 408 provided the worst agreement for large diameter bars. Chul et al studied the experimental results of 72 test specimens for compressive lap splices using concrete compressive strengths of 80 and 100 MPa.18 The effect of concrete strength, splice length and transverse reinforcement were assessed. They proposed a simple equation, which provides shorter lengths than the ACI 318:08 does. Sarki et al reviewed the BS 8110 and Euro-code 2 recommendations on steel bar lap lengths and concluded that the British code gave the best safety indices in all the cases they evaluated.19

It is clear from these references that most of the research work compare experimental results with the codes’ requirements or proposed models. But no comprehensive work was found in the literature comparing the ACI 318:08 and BS 8110:97 codes in terms of deflection, control of crack width, and anchorage and lap lengths for different conditions including live-to-dead load ratios, concrete strength, area of reinforcement, and bar type or diameter. Accordingly, a comparative study with these parameters was conducted on single-span, continuous rectangular reinforced concrete beams.

Control of deflectionACI 318:08 provisions for deflection calculationACI 318:08 has two approaches for controlling deflection. The first indirect approach consists of setting suitable upper limits on the span-depth ratio. In the second approach, the deflections are controlled directly by limiting the computed deflections to the values specified in the code (Table 9.5 (b)). In this study, the second approach was adopted as follows:

Short-term deflectionThe initial or short-term deflection Δi is calculated using Equation 1. The PCA notes explain the details in this

Continued on page 35


Point of View

regard particularly the ACI 318:05 sections 9.5.2.2 and 9.5.2.3: 20

...... (1)

where, Ma = service moment, Ie is the effective moment of inertia, and K = deflection coefficient given in Table 1.Mo = Simple span moment at mid-spanMa = Service support moment for cantilever or mid-span moment for simple and continuous beams

For each load combination (i.e. dead + live) the deflection is calculated using an effective moment of inertia i.e. (Ie)d, (Ie)d+l and (Ie)sus with the appropriate service moment Ma. The incremental deflection caused by the addition of load, such as the live load, is then computed as the difference between the deflections computed for any two-load combination. Therefore, immediate deflection due to the live load is given by Equation 2:

(Δi)l = (Δi)d+l – (Δi)d ...... (2)

This calculated deflection should be less than the allowable deflection given in Table 9.5 (b) of ACI 318:08.

Long-term deflectionAccording to section 9.5.2.5 of the ACI 318:08, an additional long-term deflection due to the combined effects of shrinkage and creep from sustained loads is given by Equation 3:

...... (3)

where, = multiplier for the long-term effect. As per section 9.5.1 of ACI 318:08; the sustained load includes dead load and that portion of the live load which is sustained. Equation 4 gives total deflection:

...... (4)

This computed total deflection should not exceed the limits given in Table 9.5 (b) of ACI 318:08.

BS 8110-2:85 provisions for deflection calculation BS 8110-2 is based on the calculation of a section’s curvature subjected to the appropriate moments, with an allowance for creep and shrinkage effects.21 Deflections are calculated from these curvatures. In BS 8110-2, a reduction in the applied moment causing deflection is made, as in reality the concrete below neutral axis can carry limited tension between cracks. Its effect, called the tension stiffening, can be looked upon as a reduction of moment causing deflection expressed as (M - ΔM), where ΔM is the moment carried by the tension in concrete.

Short-term curvaturesThe curvature of a section is the larger value obtained by considering the section either un-cracked or cracked as appropriate.

The curvature of the short-term deflection for un-cracked section is given by Equation 5:

...... (5)

The curvature for the cracked section is given by Equation 6:

...... (6)

where, M = applied moment, Ec = modulus of elasticity of concrete, Ig = gross moment of inertia, Mc is the moment of resistance of concrete in tension, and M-Mc is the moment causing deflection.

Long- term curvature In calculating the long-term curvatures, the effects of creep and shrinkage are considered.

Equation 7 gives the long-term curvature due to permanent load:

...... (7)

Equation 8 gives the curvature due to shrinkage:

Support type K

Cantilever 2.4

Simple beams 1.0

Continuous beams

Table 1. Deflection coefficient K (PCA Table 10.3)


Point of View

...... (8)

where; = modular ratio as per section 3.6 of BS 8110-2,

= free shrinkage strain as per clause 7.4 of BS 8110-2, Ss = the first moment of area of reinforcement about Neutral Axis and I= the moment of inertia either for un-cracked or cracked section depending on whether the curvature due to loading is derived from the un-cracked or cracked section.

Total long-term curvature = long-term curvature due to the permanent load plus short-term curvature due to the non-permanent load plus the shrinkage curvature given in section 3.6 of BS 8110-2:85.

Calculation of deflection from curvatureThe deflection is given by Equation 9:

...... (9)

Deflection limitsTable 2 shows the total deflection limits in the ACI and BS codes.

Crack width control ACI 318:08 provisions to control crack width The section 10.6.4 of ACI 318 does not purport to predict crack width and gives a simple equation which directly limits the maximum reinforcement spacing as shown in Equation 10.

...... (10)

where; s is the maximum centre-to-centre spacing of flexural tension reinforcement nearest to extreme tension face, fs is the calculated stress in reinforcement at service load and cc is the clear cover from the nearest

surface in tension to the surface of flexural tension reinforcement.

BS 8110-2:85 provisions to control crack width Two approaches are given for the control of crack width in BS 8110-2:85. The first approach is deemed-to-satisfy approach, in which the maximum spacing of bar is limited to control the cracking. The other approach requires actually calculating the crack width and keeping it within the limit.

Section 3.12.11.2.4 of BS 8110:97-1 specifies the clear spacing between bars in tension as shown by Equation 11:

...... (11)

where; fs is service stress in the reinforcement and db is the bar size.

Design surface crack width w is given by section 3.8.3 of BS 8110-2 as shown in Equation 12:

...... (12)

where; acr = distance from the compression face to the point at which the crack width is being calculated, = average strain, Cmin = minimum cover to the tension steel, h = overall depth of the member and x = depth of the neutral axis.

Anchorage lengthACI 318:08 provisions for anchorage length Anchorage length in tension For normal weight concrete, the anchorage length in tension is specified by section 12.2.3 of the ACI 318:08 as shown in Equation 13:

...... (13)

where; ψt = reinforcement location factor (Taken as =1 for the bottom reinforcement), ψe = coating factor (taken = 1 for uncoated reinforcement), ψs = reinforcement size factor (taken = 0.8 for dp ≤ 20 mm dia and = 1.0 for dp > 20 mm dia), cb = bar spacing or cover dimension = the

SituationDeflection limits

ACI BS

Members supporting non-structural elements that are not likely to be damaged by large deflection

Span/240 Span/250

Members supporting non-structural elements that are likely to be damaged by large deflection

Span/480 Span/500 ≤ 20mm

Table 2. Limits of total deflection using ACI and BS codes


Point of View

smaller of 1) distance from centre of bar being developed to nearest surface and 2) one half the centre-to-centre spacing of bars being developed, and Ktr = transverse

reinforcement index = where; Atr = total area of all transverse reinforcement which is within the spacing, S = maximum spacing of transverse reinforcement within ld (centre-to-centre), and n = number of bars being developed. The code permits to use Ktr = zero as a design simplification even if transverse reinforcement is

present. The term cannot be taken greater than 2.5 to safeguard against pull-out type failure. To simplify computation of ld, preselected values for the

term are chosen to as shown in section 12.2.2 of ACI 318;08.

Anchorage length in compressionThe anchorage length in compression is given by section 12.3.2 of the ACI 318:08; as shown in Equation 14:

...... (14)

For fc’> 32 MPa, (0.043fy)db governs the length.

British Code (BS 8110:97) provisions for anchorage length Section 3.12.8.3 of the BS 8110:97 specifies anchorage length in tension and compression as given by Equation 15:

...... (15)

where; = is the bar size, fs = is the ultimate tensile or compressive stress in reinforcement (0.95fy) and fbu =

the ultimate anchorage bond stress = with = bond coefficient = 0.5 for bar Type 2 in tension and = 0.63 in compression.

Lap lengthACI 318:08 provisions for lap length Lap length in tension Section 12.15.1 of the ACI 318:08 specifies the tension lap lengths for class A and class B splice as 1.0ld and 1.3ld respectively but not less than 300 mm. Section 12.15.2 of CAI318:08 specifies tension lap splice condition. Since in practice class B splice condition is more common it was considered in this research.

Lap length in compressionSection 12.16.1 of the ACI 318:08 specifies the compression lap length as 0.071fydb for fy ≤ 420MPa and 0.13fy – 24 for fy > 420MPa but not less than 300 mm in both cases. These values are applicable for 21 < fc’≤ 70 MPa (normal strength concrete). For fc’≤ 21 MPa, lap length shall be increased by one-third.

BS 8110:97 provisions of lap lengths Lap length in tensionAs per section 3.12.8.13 of BS 8110:97; in general the lap length in tension = 1.0 times tension anchorage length.

Lap length in compressionSection 3.12.8.15 of BS 8110:97 specifies the lap length in compression = 1.25 times compression anchorage length.

Results and discussionDeflectionFrom the equations presented above, it can be seen that the BS method does not use Ieff as does the ACI method. Instead, EcI for short-term and long-term loadings are calculated separately using appropriate Ec(short-term) and Ec(long-term). Further, in the ACI code, long-term deflection is calculated with the combined effect of creep and

BeamNo.

ServiceDL,

kN/m

ServiceLL,

kN/m

DL:LLratio

Ult. mom.Mu ,

kN/m

ServiceMd ,

kN/m

ServiceMl ,

kN/m

ServiceMsus ,kN/m

Tensionsteel, As ,

mm2

BR1.0 25 25 1.0 600 200 200 250 2754

BR1.3 25 20 1.3 536 200 160 240 2376

BR1.7 25 15 1.7 472 200 120 230 2029

BR2.5 25 10 2.5 408 200 80 220 1706

Table 3. Applied loads and provided reinforcement for simply supported beam using ACI and BS (Span, L = 8m)


Point of View

shrinkage, whereas in the BS code, deflections due to creep and shrinkage are calculated separately. The ACI and BS limits on deflection, for the same situations, are close to each other.

Table 3 shows the applied loads and provided reinforcement for simply supported beams using ACI and BS codes having a span length (L) of 8m and subjected to uniformly distributed loads with different DL:LL ratios. Figures 1 and 2 show the total calculated and allowable defections for the beams in Table 3. It is assumed that the beam is supporting non-structural elements that are not likely to be damaged by a large deflection. The beam was 350x750 mm with an effective depth of 625 mm. The characteristic compressive strength of concrete fcu was 30 MPa and the cylinder compressive concrete strength fc’ was 24 MPa. The yield strength of steel was taken as 460 MPa. It was assumed that 25% of the live load as permanent. The time dependent factor for sustained load, ξ, as required in the ACI code, was taken as 2.0 (i.e. factor for a period of 5 years or more). The 30 year creep coefficient, Ф, as required by the BS code, was taken as 2.0 for ambient relative humidity of 60% and age of loading as 14 days. The 30 years free shrinkage strain, εcs, as required in BS code, was taken as 0.000027 for ambient relative humidity of 60%. From Figure 1, it is clear that the predicted short-term deflection from both codes, decreases with the increase of the dead load to the live load ratio. Contrarily, the long-term deflection increases with increasing dead load to live load ratio. Figure 2 shows that the predicted total deflection is almost constant for each code with a small drop when the dead load to live ratio was 2.5. The maximum allowable deflections are constant for each code with values of ACI being 4.2 per cent larger than that of the BS. The predicted deflections using ACI code are more than those using BS code for short-term, long-term and

total deflections. The differences between the ACI and BS results in short-term, long-term and total deflections increase with the increase in dead load to live load ratio the maximum values being 8.58, 20.68 and 27.51 per cent respectively for the given conditions. These differences are attributed to the different approaches adopted in ACI and BS for calculating EI, as discussed earlier. The differences in the long-term case increase at a larger rate than those in the short-term case. This large difference could be attributed to the fact that in the ACI code, a combined effect of creep and shrinkage is considered, whereas in the BS code these effects are calculated separately. This has in-turn affected the total deflection with difference increasing as the dead load to live load ratio increases from 24.36 to 27.51 per cent. While comparing the total predicted deflection with the deflection limits (Figure 2), it was found that, in the ACI code, the estimated deflection is larger than allowable limits, which means that the ACI limits can be violated by the ACI equations used in estimating the total deflection. The BS code estimated deflections remain within the allowable limits. This indicates that the ACI limits should always be observed for possible violation.

Control of crack widthAs pointed out earlier, ACI code does not give explicit crack width calculation. The control of cracking is deemed satisfactory as long as the limit on the bar spacing is satisfied. The BS code furnishes two approaches, a deemed-to-satisfy approach and the calculation of crack width. In deemed-to-satisfy approach, the maximum bar spacing is controlled in a similar way as in the ACI code. It was shown in Equation 10 that the ACI procedure is a function of service stress and concrete cover, whereas the BS provision given in Equation 11 is


Point of View

a function of service stress and bar size. Figure 3 shows the effect of concrete cover on the bar spacing using

these equations. The values used were , fy = 460 MPa, and dp = 20 mm. It is clear that the limits on the maximum spacing between bars vary significantly between ACI and BS codes. The BS code has a constant spacing regardless of the concrete cover, whereas the ACI bar spacing reduces with the increase in the concrete cover thickness. The difference between the two codes’ decreases as the concrete strength increases (Figure 3). With the given data, the highest difference is 57.8% when concrete strength is 30 MPa and the lowest is 0.6 %when the concrete strength is 70 MPa.

Anchorage length in tensionFrom Equations 13 and 15 for anchorage length in tension, it can be seen that both the ACI and BS codes’ equations are the functions of concrete and reinforcement yield strengths and bar diameter. However, the ACI equation is more detailed and takes into account the location of reinforcement, coating factor, bar spacing, effects of small cover, and confinement provided by transverse reinforcement. Table 4 shows the deduced equations of tension anchorage length using both the codes for different values of fc’ which is taken as 0.8 of fcu (using ψt = ψe = 1.0, and ψs = 0.8 for dp≤ 20mm Dia and = 1.0 for dp>20mm Dia). In the case of ACI, pre-selected

values of the term were adopted which were 1.0, 1.5 and 2.5. The resulting anchorage length in tension, (Table 4), are plotted in Figure 4. It can be seen that in both the codes, the tension anchorage length decreases with the increase in concrete strength.

When the term has a

value of 1 and 1.5, ACI requires more tension anchorage length than the BS does; varying from 14.1 to 114 percent respectively for concrete strength change from 30 to 40

MPa. Whereas, when the term has a value of 2.5, BS requires more anchorage length; varying from 16.8 to 46 per cent for concrete strength change from 30 to 40 MPa. In this regard it clear that the ACI provision

Code fcu , MPa30 35 40

db≤20 db>20 db≤20 db>20 db≤20 db>20

ACI(ClassB)

(cb+Ktr/db)=1.0 (0.146fy)db (0.185fy)db (0.135fy)db (0.169fy)db (0.126fy)db (0.158fy)db


cb+Ktr/db)=2.5 (0.058fy)db (0.073fy)db (0.054fy)db (0.068fy)db (0.040fy)db (0.050fy)db

BS (0.087fy)db (0.087fy)db (0.09fy)db (0.09fy)db (0.076fy)db (0.076fy)db

Table 4. Equations for anchorage length in tension, using both ACI and BS codes


Point of View

is more conservative since in most cases it requires a longer anchorage length than the BS.

Anchorage length in compressionFrom the Equations 14 and 15 for compression anchorage length, it can be seen that both codes’ equations are functions of concrete and steel strengths and bar size. Table 5 shows equations and calculated values for the anchorage length in compression, as a multiple of bar size, using both ACI and BS codes, for different values of fc’. It can be seen that the BS code requires approximately 40 per cent more anchorage length in compression than the ACI code.

Lap length in tensionAs discussed earlier, ACI and BS codes take into account bar coating factor, effect of small cover and location of bars. However, ACI further considers the effect of close bar spacing and confinement provided by transverse reinforcement. Table 6 was prepared based on the equations of tension lap length in both codes for different values of fc’. The reinforcement location factor was taken as 1.0 and reinforcement assumed as uncoated. In the case of ACI, pre-

Concrete strengthfcu , MPa

Equation of anchorage length in compression, ldc

Anchorage length in compression, ldc

% Diff. ofldcACI BS ACI BS

30 (0.049fy)db (0.069fy)db 23db 32db 40.5

35 (0.045fy)db (0.071fy)db 21db 29db 40.5

40 (0.043fy)db (0.060fy)db 20db 27db 38.6

Table 5. Equations for anchorage length in compression, using both ACI and BS codes, as multiple of bar size

fy = 460MPa, fc’ =0.8fcu

fcu , MPa30 35 40

db≤20 db>20 db≤20 db>20 db≤20 db>20

ACI(ClassB)



cb+Ktr/db)=2.5 (0.075fy)db (0.094fy)db (0.070fy)db (0.088fy)db (0.052fy)db (0.065fy)db

BS (0.087fy)db (0.087fy)db (0.090sfy)db (0.09fy)db (0.076fy)db (0.076fy)db

Table 6. Equations for lap length in tension

selected values of term are adopted which are 1.0, 1.5 and 2.5. Figure 5 shows resulting lap length in tension, as a multiple of bar size, using both ACI and BS

codes. It can be seen that when the term is 1 or 1.5, ACI needs more lap length; varying from 48.4

to 178.2 percent. When the term is 2.5, BS asks for 12.3 percent more lap length than ACI when the diameter of bar ≤ 20 mm but ACI required 11.3 percent more lap length when the diameter of bar >20 mm.


Point of View

Lap length in compressionThe provision of compression lap length discussed earlier suggests that in the ACI code, the compression lap length is a function of bar size and yield strength of steel and is independent of the concrete strength. Whereas, in BS code, the compression lap length is equal to 1.25 times the compression anchorage length. Table 7 gives the equations for lap length in compression, as a multiple of bar size, using both ACI and BS codes and Figure 6 shows the resulting values, for the different grades of concrete. It can be seen that for concrete having fcu = 30 MPa, BS required 10.5 per cent more lap length than the ACI whereas for the higher concrete grades, the differences of lap length is negligible as it varies from -4.5% to 2.3%. It is clear that BS code requires more compression lap length than does the ACI code till concrete compressive strength is 37 MPa, beyond that, ACI requires more compression lap length.

Concluding remarks The predicted short-term deflections from both ACI and BS codes, decrease with the increase in the dead load to the live load ratio; ACI values being larger than the BS by a maximum 8.58 per cent for the given conditions. Contrarily, the long-term deflections increase with increase in the dead load to live load; ACI values being

•

larger for a maximum of 20.68 per cent for the given conditions.

In both codes, the total deflection decreases with the increase in the dead-to-live load ratio with the ACI values being larger than the BS values with a maximum 27.51 per cent for the given conditions.

The values of the deflection limits in both ACI and BS codes are close to each other regardless of the dead-to-live load ratios with the values of ACI being 4.2 per cent larger than those of the BS.

The ACI estimated total deflection is always larger than the ACI limits for the given conditions. This implies that the limits should always be observed for possible violation.

The differences in short-term deflections estimated by ACI and BS could be attributed to the different approaches adopted in ACI and BS code for calculating EI. In ACI, the effective moment of inertia, Ieff, is used, whereas the BS procedure calculates EcI for short term and long term loading using separate Ec(short term) and Ec(long term).

The differences in the long-term deflections estimated by ACI and BS could be attributed to the fact that in ACI consider combined effect of creep and shrinkage, whereas in BS the effect of creep and shrinkage is calculated separately.

Limits on maximum spacing of bars to control crack width in rectangular beams vary highly between ACI and BS codes. The BS code has a constant value regardless of the concrete cover while the ACI bar spacing reduces with the increase in the concrete cover to reinforcement. ACI code allows more spacing than BS code does for low grades of concrete and difference in values

•

•

•

•

•

•

Concrete strengthfcu , MPa

Equation of lap length in compression

Lap length in compression

% Diff. ofcompression

lap lengthACI BS ACI BS

30 (0.078fy)db (0.086fy)db 36db 40db 10.5

35 (0.078fy)db (0.089fy)db 36db 37db 2.3

40 (0.078fy)db (0.075fy)db 36db 34db -4.5

Table 7. Lap length in compression as a multiple of bar size in ACI and BS codes

fy = 460MPa, fc’ =0.8fcu


Point of View

between two codes decreases as the concrete cover thickness increases.

In both the codes, the required tension anchorage and lap lengths decrease with the increase in concrete strength.

The ACI code lap length in compression is constant regardless of the concrete strength while the BS code length decreases with the concrete strength.

For the term 1≤ < 2.5, ACI code equations result in more tension anchorage and lap lengths than the BS. Whereas, when the term

has value of 2.5, BS code equation result in more tension anchorage and lap lengths than the ACI. In most cases, the ACI is more conservative by requiring a longer anchorage length.

BS code asks for more compression anchorage length than ACI code does.

BS code demands more compression lap length than ACI code until concrete compressive cube strength is 37 MPa, beyond which ACI requires more compression lap length than BS code.

Based on the above it can be stated that the provisions in the ACI code are more conservative than those in the BS code for both short- and long-terms deflections, which give the ACI provision a superior reliability. However, the ACI limits for deflection are violated by the results of the ACI equations, which allow the limits to always dictate the length. On the other side, the BS Code is more conservative in terms of bar spacing to limit crack width, anchorage and lap lengths. Therefore, it is not easy to give preference to one code over the other. However it is a fact that SI units are becoming more and more enforced internationally, building material and references available in Oman are mostly in SI units. Therefore in order to unify the knowledge of the code requirements among municipality and site engineers, it is recommended to use the BS code as a first choice until a national code is established.

•

•

•

•

•

•

References_______American Concrete Institute (ACI318:08), Building Code Requirements for Structural Concrete and Commentary, 2008, Farmington Hills, MI, USA._______British Standard Institution (BS8110:97 Part-1), Structural Use of Concrete, Code of Practice for design and construction,1997, London, UK.Nayak S.K. and Menon D., Improved procedure for estimating short term deflections in RC slabs, The Indian Concrete Journal, July 2004, v78, n7, p19-25._______Indian standard code of practice for plain and reinforced concrete for general building construction, IS 456:2000, Bureau of Indian Standards, New Delhi, India. Eurocode 2, Design of concrete structures, general rules and rules for buildings, BS EN 1992.Santhi A.S., Prasad J. and Ahuja A.K., Effects of creep and shrinkage on the deflection of RC two way Flat plates, Asian journal of Civil Engineering (Building and Housing), 2007, v8, n3, p267-282.Bacinskas D., Gribniak V. and Kaklauskas G., Statistical analysis of long-term deflections of RC beams, Creep, Shrinkage and Durability, Mechanics of Concrete and Concrete Structures, Taylor & Francis Group, London, UK, 2009, ISBN 978-0-415-48508-1.Kaklauskas G., Flexural layered deformation model of reinforced concrete members, Magazine of Concrete Research, 2004, v56, n10, p575-584.Lee Y. H. and Scanlon A., Comparison of one-and two-way slab minimum thickness provisions in building codes and standards, ACI structural Journal, March-April 2010, v107, n2, p157-163.Scanlon A. and Lee Y. H., Unified span-to-depth ratio equation for non-pre-stressed concrete beams and slabs, ACI Structural Journal, Jan-Feb 2006, v103, n1, p142-148.Bischoff P. H. and Scanlon A., Span-depth ratios for one-way members based on ACI318 deflection limits, ACI Structural Journal, Sep- Oct 2009, v106, n5, p617-625.Bacinskas D. Kaklausks G., Gribniak V., Sung W. P. and Shih M. H., Layer model for long-term deflection analysis of cracked reinforced concrete bending members, Mech. Time-Depend Mater, 2012, v16, p117–127, DOI 10.1007/s11043-011-9138-9.Subramanian N., Controlling the crack width of flexural RC members, The Indian Concrete Journal, November 2005, v79, n11, p31-36.Alam Y. S., Lenormand T., Loukili A., and Region J. P., Measuring crack width and spacing in reinforced concrete members, Fracture Mechanics of Concrete and Concrete Structures -Recent Advances in Fracture Mechanics of Concrete - B. H. Oh, et al.(eds), 2010, Korea Concrete Institute, Seoul, ISBN 978-89-5708-180-8.Khan M. S., Reddy A. R., Shariq M. and Prasad J., Studies in Bond strength in RC Flexural Members, Asian Journal of Civil Engineering (Building and Housing), 2007, v8, n1, p89-96.Subramanian N., Development length of reinforcing bars – need to revise Indian code provisions, The Indian Concrete Journal, August 2005, v79, n8, p39-46Haitao L., Xiaozu S., Andrew d. J., Evaluation of adequacy of development length requirements for 500MPa reinforcing bars, Advances in Structural Engineering, June 2011, v14, n3, p367-378.Chul C. S., Ho L. S. and Bohwan O., Compression splices in high-strength concrete of 100MPa and less, ACI Structural Journal, November- December 2011, v108, n6, p715-724.Sarki Y. A., Murana A. A., and, Abejide S. O., Safety of Lap Lengths Prediction in Reinforced Concrete Structures, World Journal of Engineering and Pure and Applied Science, 2012, v2, n3, pp 98 -106.Notes on ACI318:05, Requirements for structural Concrete with Design applications, ACI Building code, 2005, Edited by: Mahmoud E. Kamara and Basile G. Rabbat, Portland Cement Association, 5420 Old Orchard Road, Skokie, Illinois 60077-1083._______British Standard Institution (BS8110:85 Part-2), Structural Use of Concrete, Code of Practice for Special Circumstances, 1985, London, UK.

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.


Point of View

Ali S. M. Alnuaimi holds an MSc in structural engineering from the University of Southern California, USA and PhD in structural engineering from the University of Glasgow, UK (Title of thesis: Direct design of reinforced and partially pre-stressed concrete beams for combined torsion, bending and shear). He is an

Associate Professor at the Department of Civil and Architectural Engineering, College of Engineering, Sultan Qaboos University, Oman. He has published 29 journal papers and 28 conference papers. Before being an academic he worked as structural engineer and Director of projects at Sultan Qaboos University for five years. Currently, he is also the chair of the projects committee at Sultan Qaboos University. Dr. Alnuaimi’s main research interests are structural design and analysis, estimating construction cost, and administration of contracts.

Iqbal Y. Patel holds an MSc in Civil Engineering from Sultan Qaboos University, Oman. He is a structural engineer at Muscat Municipality, Oman. He has more than 25 years of experience in structural design of concrete and steel structures along with Project Management experience. He is proficient with American

concrete and steel design codes ACI318, AISC360 as well as structural design software STAAD, ETABS, SAP, SAFE and familiar with ASCE7, UBC, IBC. Prior to this he worked as a civil and structural engineer in India, Saudi Arabia, and in private companies in Oman.

Serviceability, limit state, bar anchorage and lap lengths...

Documents

Transcript of Serviceability, limit state, bar anchorage and lap lengths...