SI HOY ESCUCHÁIS SU VOZ La escucha como arte

The discussed paper compared the normalized maximum loads attained by the splice specimens with deformed bars and plain bars and concluded that the splice specimens with plain bars can resist maximum loads, which are approxi- mately 60% of those identical specimens with deformed bars with the same nominal diameter. The discussers wonder why the authors did not conduct more comparisons, as the limited comparisons with only two specimens can hardly convince readers.

The discussed paper showed that the CEB-FIP Model Code11 _{provisions for average bond stress underestimate}

Pmax and proposed two empirical formulas. The discussers

made a comparison between the predicted efficiency of the CEB-FIP Model Code11 _{and the two proposed equations.}

The results are shown in Table 3 and several conclusions can be drawn.

1. The CEB-FIP Model Code11 _{provision statisti-}

cally underestimates the test results for all three kinds of bars. Among them, the bars with a diameter of 19 mm (0.75 in.) made the worst prediction; however, it seems that the predicted accuracy increases with the increases of the bar diameter.

2. Both Eq. (2) and (2a) statistically underestimate the experimental results for the bars with diameters of 19 and 25 mm (0.75 and 1 in.), while they overestimate the results for the 32 mm (1.25 in.) bars.

3. For estimating the bars with diameters of 19 mm (0.75 in.), Eq. (2) and (2a) are superior to the CEB-FIP Model Code,11 _{while for the 25 and 32 mm (1 and 1.25 in.)}

bars, the superiority is hardly reflected.

Table 3—Comparisons between predicted efficiency of CEB-FIP Code11 _{and two empirical equations}

Specimen ID

Normalized maximum load L = Pmax/√fc′, kN/√MPa Ratios between predicted and experimental results

Test: L1 CEB-FIP Model Code11 provisions: L2 Eq. (2): L3 Eq. (2a): L4 R1 = L2/L1 R2 = L3/L1 R3 = L4/L1

19-305 8.50 5.06 7.57 7.48 0.60 0.89 0.88 19-410 9.14 7.88 10.32 10.05 0.86 1.13 1.10 19-510 9.58 10.70 13.09 12.50 1.12 1.37 1.30 19-610 17.80 14.00 14.99 14.95 0.79 0.84 0.84 Average ratio for bar diameter db = 19 mm (0.75 in.) 0.69 0.87 0.86

25-410 16.20 12.40 12.47 13.22 0.77 0.77 0.82 25-510 18.40 16.00 14.73 16.45 0.87 0.80 0.89 25-610 20.60 19.40 18.20 19.67 0.94 0.88 0.95 25-810 29.70 25.10 26.63 26.12 0.85 0.90 0.88 Average ratio for bar diameter db = 25 mm (1 in.) 0.81 0.83 0.85

32-410 15.60 14.60 17.83 16.92 0.94 1.14 1.08 32-610 25.10 22.30 25.99 25.18 0.89 1.04 1.00 32-810 31.80 28.80 35.87 33.44 0.91 1.13 1.05 Average ratio for bar diameter db = 32 mm (1.25 in.) 0.92 1.14 1.07

Disc. 109-S22/From the March-April 2012 ACI Structural Journal, p. 235

Behavior of Lap-Spliced Plain Steel Bars. Paper by M. Nazmul Hassan and Lisa R. Feldman Discussion by M. John Robert Prince and Bhupinder Singh

Research Scholar, Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand, India; PhD, Assistant Professor, Department of Civil Engineering, Indian Institute of Technology Roorkee

to achieving bond failure in splice beam specimens. If the intention is to enforce a bond failure, then it is desirable to restrict the splice lengths to values significantly less than the critical splice lengths. Preferred splice lengths in splice beam tests are typically less than 20 bar diameters. The use of relat- ively large splice lengths in some of the specimens by the authors—as, for example, in Specimens 19-610 and 25-810, each having a splice length of 32db—may be the reason for

the measured moment capacities of these specimens being larger than their nominal yield moment capacities.

3. In structural testing, it is common to load beam speci- mens, for example, in the load-controlled mode up until the peak loads and in the displacement-controlled mode after- ward to capture the post-peak response. Load-controlled testing in the ascending branch facilitates detection and recording of cracking behavior, which may be difficult when testing is done in the displacement-controlled mode. Did the authors have any specific purpose for using only displace- ment-controlled loading in their experiments?

4. On the basis of the measured capacities of Specimens 25-410 and 25-610, the authors suggest that “splice speci- mens reinforced with plain bars are capable of resisting peak loads that are approximately 60% of those recorded for identical specimens reinforced with deformed bars of the same diameter.” There is insufficient direct empirical evidence to back up this assertion, which implies that bond strength of plain bars is a significant fraction (60%) of the bond strength of deformed bars. It is well-known that adhe- sion and friction is the dominant bond mechanism in plain bars and these get quickly lost due to slip at early stages of loading. It is reckoned that the structural capacities of Speci- mens 25-410 and 25-610 attributed by the authors to the bond resistance of plain bars has more to do, rather, with the development of an alternate load-resisting mechanism in the form of arch action in these beams, consequent to a more or less complete loss of bond at early stages of loading. Inter- estingly, the authors reported the development of arch action in their beam specimens reinforced with the plain bars. The authors should be complimented for carrying out a

meticulously planned and carefully executed experimental investigation into the bond behavior of plain steel bars. The following observations are made for their consideration and response:

1. In bond studies, for test results to be directly comparable with each other, it is very important that the confinement conditions of the longitudinal reinforcement bars across different test specimens be identical. Concrete confine- ment to the longitudinal reinforcement is usually expressed in terms of the dimensionless parameter c/db, where c may

either be the side clear cover cs or the bottom clear cover cb

and db is the diameter of the longitudinal reinforcement bar.

For most practical beams, c/db lies in the range of 1 to 1.5.

The c/db for the authors’ beams reinforced with the 19, 25,

and 32 mm (0.75, 1, and 1.25 in.) diameter bars are 2.65, 2.02, and 1.56, respectively, when c is taken as the bottom clear cover cb. These values indicate that the confinement of

longitudinal reinforcement in the authors’ beam specimens was neither uniform nor realistic. The practical range of c/db

values mentioned previously usually result in a splitting type of bond failure, whereas the authors reported a pullout type of bond failure in their beam specimens, which is expected, given their relatively high c/db. Because cover requirements

were generally less stringent at the time when most of the historical structures were constructed, the use of relatively large c/db by the authors needs clarification.

All the beams tested by the authors had a constant thick- ness of 305 mm (12 in.) and this implies that the cs/db across

their specimens varied, depending on the diameter of the longitudinal reinforcement. Ideally, for identical confine- ment conditions, both cs/db and cb/db should be nominally

equal to each other and these values should, in turn, be constant across all the test specimens. For this to happen, the effective depth and the width of the beam specimens should vary, depending on the diameter of the longitudinal reinforcing bars. The authors may want to clarify this issue.

2. The upper-bound splice length of 32.4 times the longit- udinal bar diameter adopted by the authors is not conducive

Disc. 109-S22/From the March-April 2012 ACI Structural Journal, p. 235

Behavior of Lap-Spliced Plain Steel Bars. Paper by M. Nazmul Hassan and Lisa R. Feldman Discussion by Andor Windisch

ACI member, PhD, Karlsfeld, Germany

The authors report on tests that should help readers better understand the behavior of lap splices; however, the success is quite limited.

In the introduction, the authors note that the reason for their research is that “the evaluation of historical reinforced concrete structures may reveal the existence of construc- tion details that do not meet current standards … deform- ation patterns that do not conform to current specifications (plain reinforcement).” It should be noted that—at least in

Europe—at the end of anchorages and lap splices of plain reinforcing bars, hooks (except in shells and thin plates) were mandatory; therefore, lap splices with straight ends could not have occurred in historical structures. Moreover, at least in Europe, the highest yield strength for plain reinforcing bars with a diameter of >8 mm (>0.3 in.) was 220 N/mm2

(31.9 ksi) with an ultimate strength of 340 N/mm2 _{(49.3 ksi);}

hence, the properties of reinforcing steels chosen for the test specimens were not representative of those in Europe.

The relevant guidelines of the CEB-FIP Model Code11 _are

misunderstood and misinterpreted. Equation (1) serves for the calculation of the length of the lap splice only. Along with bond characteristics, this length also comprises the errors at cutting and installation. The formula cannot be used for any calculation for splice lengths smaller than the required one. Moreover, the CEB-FIP Model Code11 _{explicitly notes that}

splices (with their full length) should possibly be placed at regions with less tensile stresses in the spliced reinforcement.

All results are presented and discussed on the basis of “normalized” values, where the applied and achieved loads are divided by the square root of the concrete compressive strength. Previous investigations7 _{should have shown that}

this relationship was valid. A comparison of Fig. 9 with Fig. 3 shows that this is not the case. The physical reason for the independence of the results from the concrete compressive strength (or its square root, which is related to the tensile strength) is that after the adhesion is destroyed,

the main source of the local bond resistance of plain bars is the friction between the bar surface and concrete.

Specimen 32-910 was identified by the authors as an “outlier”—in Fig. 9(c), it fits very well into the trend; nevertheless, Specimen 32-810 turned out to be an outlier. Specimens 19-610 and 25-610 were judged as outliers by the discusser.

The statement that the “CEB-FIP Model Code11 _provi-

sions for average bond stress underestimate Pmax by 15.7%

on average” is not accurate. The same is valid for Eq. (2) and (2a) also. These would only fit for beams with identical geometries and loading patterns as the test specimens.

Figure 9(a) reveals that for reinforcing bars with a diameter of 19 mm (No. 6 [0.75 in.]), a minimum splice length of approximately 23.5db yields the necessary strength. For

reinforcing bars with a diameter of 25 mm (No. 8 [1 in.]) (refer to Fig. 9(b)), the minimum required splice length is shown to be 28db. For reinforcing bars with a diameter

of 32 mm (No. 10 [1.25 in.]), Fig. 9(c) yields the relative length of 28.4db. This trend is known: for plain bars, the

required splice (and anchorage) length increases with increasing reinforcing bar diameter as the relative bonded surface decreases.

Presenting the crack patterns, the authors conclude that “vertical cracks within the shear spans are an indication of a lack of shear stresses and suggest that the load-car- rying mechanism of the specimens tended toward that of a tied arch.” Checking the maximum shear stresses at the maximum applied loads, it becomes clear that the shear stresses were far below the lower fractile value of the concrete tensile strength; thus, there was no reason for the cracks to develop as shear cracks. The shear load-bearing behavior of uncracked concrete beams has nothing to do with the tied arch model.

It would be interesting to learn the sizes of the end slips shown in Fig. 5. The end slips must somehow be equal to the sum of the crack widths measured along the splice length. Does this fit? Please clarify.

It would also be interesting if the authors reported the posi- tion of the cross sections where the test specimens failed. Were these sections within the splice lengths or just outside?

The midspan deflections shown in Fig. 6 only reveal that Response 2000 is dangerously unable to predict the brittle behavior of a simple beam with spliced tensile reinforcement.

The conclusions related to the bond stresses must be considered with caution. It is known that the measured strain/stress depends on the position of the gauge related to the next crack. The stochastic character of the relative posi- tions of the strain gauges versus the flexural cracks along the splice length makes the size and distribution of the calcu- lated bond stresses and the conclusions questionable.

AUTHORS’ CLOSURE