Anchorage of Headed Reinforcing Bars in Concrete

Headed reinforcing bars serve as a viable alternative to hooked bars for anchorage in concrete because they provide a more efficient anchorage mechanism and limit congestion of the reinforcement. This study is part of a comprehensive study of the anchorage behavior of the headed bars. The work described in this report includes tests of 32 No. 8 headed bars anchored in simulated column-foundation joints represented by bars anchored in slabs, all but two with reinforcement in the plane of the slab, and six lapped-slplice specimens without confining reinforcement containing No. 6 headed bars and an analysis of these tests along with test results from 23 studies by other researchers of 84 exterior, seven roof-level interior, and seven knee beam-column joints subjected to reversed cyclic loading. The headed bars in the column-foundation joint specimens had net bearing areas ranging from 4 to 15 times the area of the bar Ab; some of the headed bars contained large obstructions adjacent to the bearing face of the head that exceeded the dimensional limits for HA heads in ASTM A970-16; embedment lengths ranged from 6 to 8.5 in.; reinforcement in a plane perpendicular to the headed bars included combinations of bars placed symmetrically about the headed bar, parallel and close to the long edges of the specimen, bars placed symmetrically about and close to the headed bar in the short direction of the specimen, and bars oriented in both the long and short directions of the specimen; concrete compressive strengths ranged from 4,200 to 8,620 psi; and stresses in the bars at failure ranged from 49,500 to 117,000 psi. The No. 6 headed bars had a net bearing area of 4Ab and a lap length of 12 in. The center-to-center spacing between the spliced bars was 1.67, 2.33, or 3.53 bar diameters db; clear concrete cover to the bars was 2 in.; concrete compressive strengths averaged 6,360 and 10,950 psi; and stresses in the bars at failure ranged from 75,010 to 83,560 psi. For the beam-column joints subjected to reversed cyclic loading, headed bar sizes ranged between D12 (No. 4) and D36 (No. 11), net bearing areas ranged from 1.7 to 11.4Ab, and embedment lengths ranged from 8 to 22.6db; concrete compressive strengths ranged from 3,480 to 21,520 psi and steel yield strengths ranged from 53,650 to 149,930 psi; all but four specimens contained hoops, spaced at 2.2 to 6.8db (1.8 to 5.9 in.), as confining reinforcement parallel to the headed bar within the joint region; clear cover and minimum center-to-center spacing between the bars ranged from 1.4 to 9.9db and from 2 to 11.2db, respectively. Experimental anchorage strengths are compared with values based on descriptive equations for anchorage strength and design provisions for development length of headed bars for members with concrete compressive strengths up to 16,000 psi and steel yield strengths up to 120,000 psi that recognize the contribution of confining reinforcement without specifying minimum limits on bar spacing or clear cover. The descriptive equations and design provisions were developed based on tests of simulated beam-column joints under monotonic loading as part of the comprehensive study. The comparisons are used to expand the applicability of the descriptive equations to members subjected to reversed cyclic loading and develop simplified design guidelines allowing for the use of headed reinforcing bars in wide range of reinforced concrete members. Changes in the provisions of ACI 318-14 for the development length of headed bars and in ASTM A970 for head dimension requirements are also proposed. The results of this study show that reinforcement perpendicular to headed bars in columnfoundation joints does not improve the anchorage strength. Headed bars with obstructions exceeding the dimensional limits for HA heads in ASTM A970-16 provide adequate anchorage strength. Headed bars did not provide sufficient anchorage in knee beam-column joints subjected to reversed cyclic loading. The descriptive equations and proposed design provisions developed based headed bars in beam-column joint specimens tested under monotonic loading, in which the anchorage strength of the headed bar is a function of embedment length, concrete compressive strength, bar spacing, bar diameter, and confining reinforcement within the joint region, are applicable to a wide range of reinforced concrete members, including beam-column joints subjected reversed cyclic loading, lap splices, and column-foundation joints, and allow the minimum clear spacing of 3db between headed bars permitted in joints in special moment frames in accordance with Section 18.8.5.2 of ACI 318-14 to be reduced to 1db, allowing for the use of more closely spaced headed bars. The anchorage strength of the headed bars calculated using anchorage provisions of Chapter 17 of ACI 318-14 with a strength reduction factor of 1.0 provides a very conservative and highly variable estimate of anchorage strength for headed bars compared to the proposed design provisions

Headed Bars in Beam-Column Joints Subjected to Reversed Cyclic Loading

Descriptive equations developed for the anchorage strength of headed bars in beam-column joints under monotonic load are evaluated for beam-column joints subjected to reversed cyclic loading. Test results from 23 studies that include 84 exterior and seven roof-level interior beam-column joints are used in the evaluation. Concrete compressive strengths and reinforcement yield strengths ranged from 3480 to 21,500 psi (24 to 148 MPa) and 53,700 to 150,000 psi (370 to 1030 MPa), respectively. Headed bar sizes ranged from slightly smaller than a No. 4 (No. 13) to No. 11 (No. 36) with net-bearing areas ranging from 1.7 to 8.6 times the bar area. The embedment lengths and center-to-center spacing between the headed bars ranged from eight to 18 bar diameters and from two to eight bar diameters, respectively. Analysis of the test data shows that descriptive equations based on headed bars under monotonic loading are also applicable to headed bars in beam-column joints subjected to reversed cyclic loading. These comparisons were used to justify the single approach used within the ACI Building Code for calculating the development length of headed bars

Conventional and High-Strength Headed Bars—Part 2: Data Analysis

Equations to characterize the anchorage strength of headed bars were developed, incorporating key factors affecting anchorage strength: concrete compressive strength; embedment length; bar diameter; spacing between the bars; and confining reinforcement parallel to the headed bars. Results from tests of 138 exterior beam-column joints, 64 without and 74 with confining reinforcement within the joint region, were used to develop the equations. Concrete compressive strengths ranged from 4050 to 16,030 psi (27.9 to 110.6 MPa) and bar stresses at failure ranged from 33,100 to 153,160 psi (228 to 1056 MPa). The bearing area of the headed bars ranged from 3.8 to 9.5 times the area of the bar. Some headed bars contained obstructions adjacent to the head that exceeded the dimensions permitted for HA heads in ACI 318-14 and ASTM A970-13a but are now permitted by ASTM A970-18. The test results show that headed bar anchorage strength is proportional to the concrete compressive strength raised to the power 0.24. The contribution of confining reinforcement is proportional to the area of confining reinforcement parallel to the headed bar within eight to 10 bar diameters of the headed bar. Headed bars with obstructions larger than those permitted in ACI 318-14 that meet the provisions in ASTM A970-18 exhibit anchorage strengths that are similar to those that meet the provisions in ACI 318-14

Conventional and High-Strength Headed Bars—Part 1: Anchorage Tests

Results of an experimental program on the anchorage strength of headed reinforcing bars are presented. Two hundred and two exterior beam-column joint specimens with concrete compressive strengths ranging from 3960 to 16,030 psi (27.3 to 110.6 MPa) were tested under monotonic loading. Key parameters included concrete compressive strength, embedment length, bar size, head size, spacing between headed bars, and confining reinforcement within the joint region. Bar stresses at failure ranged from 26,100 to 153,200 psi (180 to 1057 MPa). Specimens exhibited concrete breakout, side-face blowout, or a combination of the two failure modes, with concrete breakout being the dominant failure mode. A comparison of bar stress at anchorage failure with the stress calculated based on ACI 318-14 shows that ACI 318-14 provides a very conservative estimate of anchorage strength for No. 5 (No. 16) bars and low concrete compressive strengths. The estimate becomes progressively less conservative with increasing bar size and concrete compressive strength

Does program linking with insurance makes agriculture insurance sustainable?

Agriculture insurance is most common forms of risk transfer in agriculture. It is often compulsory for borrowers of agricultural loans in low and middle income countries. This study tries to find out the status of compulsory agriculture insurance in Nepal and its sustainability through answering question “are compulsory agriculture insurance programs making agricultural insurance sustainable? Or we have to think differently for its sustainability. Household survey were conducted using pre-tested semi structured questionnaire in eight districts. Altogether 377 insurer farmers (132 crop and 245 livestock farmers) were selected from the list of target population using simple random sampling technique. Similarly, five cases were selected from the study districts. Result shows that Government of Nepal (GoN) has developed both cost of production and value based insurance products based on farmers demand. Basically, premium rate is fixed as five percent to cost of production based and seven percent to value based insurance for most of crops and livestock. Different governments programs such as youth self-employment program, youth focused program, spring rice promotion program and other grant/subsidy programs under different mega projects of GoN have started to link agriculture insurance with their programs. Insurance has been made mandatory to get such any subsidy support from the government for promoting agriculture insurance simultaneously. However, this study found that this strategy did not adequately work. But if they feel the enterprises is risky and realize the importance of agriculture insurance and can get higher returns from the enterprises, they were willing to participate in agriculture insurance. Most of farmers who participated in government grant program have limited understanding of crop insurance so that they have discontinued insurance after the end of grant/subsidy program. Therefore, it is necessary to revisit the existing provision of grant linked insurance and need to focus more on creating awareness on importance of agriculture insurance for its sustainability

Technical efficiency of hybrid maize production in eastern terai of Nepal: A stochastic frontier approach

Maize is the second most important crop after rice in terms of area and production in Nepal. This article analyzes the technical efficiency and its determinants of hybrid maize production in eastern Nepal. Using a randomly selected data from 98 farmers (41 from Morang and 57 from Sunsari) in eastern Nepal, the study employed a stochastic frontier production model to find the production elasticity coefficients of inputs, determinants of efficiency and technical efficiency of hybrid maize farmers. The results showed that maize production responds positively to increase in amount of urea, DAP and the area planted, where as it is negative to seed quantity. The study indicate that farmers are not technically efficient with a mean technical efficiency 79 %. Socioeconomic variable age had a negative and significant while the household size had a positive and significant related to maize output. The younger farmers were observed more technically efficient than older farmers. Larger the members in the household higher the maize production. It is recommended that farmers should increase their fertilizer dose and farm size while they should decrease their seed rate for efficient production

Agro-morphological Diversity of High Altitude Bean Landraces in the Kailash Sacred Landscape of Nepal

Many varieties of bean are widely grown across diverse agro-ecological zones in Nepal. And opportunities exist for improving the crops and enhancing their resilience to various biotic and abiotic stressors. In this context, an experiment was conducted from June to October 2016 in Khar VDC of Darchula district to study the phenotypic traits of nine landraces of bean (Phaseolus vulgaris L.). The bean landraces were planted using randomized complete block design in three sites (Dhamidera, Dallekh and Sundamunda villages), with three replications in each site for their comparative analysis. The study considered the following phenotypic traits: days to emergence, days to 50% flowering, days to 90% pod maturity, number of nodes, pod length, pod width, number of pods, number of seeds per pod and weight and grain yield for 100 seeds. Kruskal-Wallis test showed significant differences in the landraces both within and among locations. KA-17-08-FB and KA-17-04-FB were late  flowering (63 and 65 days respectively) compared to other landraces whereas KA-17-07-FB flowered earliest (within 42 days). In all three sites, three landraces namely KA-17-07-FB, KA-17-04-FB and KA-17-06-FB were found to be relatively more resistant to pest and diseases than other landraces. Eight out of nine landraces in Dhamidera and Dallekh villages and seven out of nine in Sundamunda village produced seeds greater than 1.0 t/ha. Among the nine varieties KA-17-02-FB was the highest yielding variety, with an average yield of 3.8 t/ha. This study is useful for identifying suitable landraces for future promotion based on their maturity, grain yield, diseases resistance and other qualitative and quantitative characteristics

Anchorage of Conventional and High-Strength Headed Reinforcing Bars

Headed bars are often used to anchor reinforcing steel as a means of reducing congestion where member geometry precludes adequate anchorage with a straight bar. Currently, limited data on the behavior of headed bars are available, with no data on high-strength steel or high-strength concrete. Due to a lack of information, current design provisions for development length of headed reinforcing bars in ACI 318-14 limit the yield strength of headed reinforcing steel to 60,000 psi and the concrete compressive strength for calculating development length to 6,000 psi. Current design provisions for developing headed bars in ACI 349-13, which are based on ACI 318-08, apply the same limits on the material strengths (60,000 psi and 6,000 psi, respectively, for headed bars and concrete). These limits restrict the use of headed bars and prevent the full benefits of higher-strength reinforcing steel and concrete from being realized. The purpose of this study was to establish the primary factors that affect the development length of headed bars and to develop new design guidelines for development length that allow higher strength steel and concrete to be utilized. A total of 233 specimens were tested, with four specimen types used to evaluate heads across a variety of applications. Two hundred two beam-column joint specimens, 10 beam specimens with headed bars anchored near the support in regions that are known as compression-compression-tension (CCT nodes, 15 shallow embedment specimens (each containing one to three headed bars for a total of 32 tests), and 6 splice specimens were evaluated. No. 5, No. 6, No. 8, and No. 11 bars were evaluated to cover the range of headed bar sizes commonly used in practice. Concrete compressive strengths ranged from 3,960 to 16,030 psi. A range of headed bar sizes, with net bearing areas between 3.8 and 14.9 times the area of the bar, were also investigated. Some of these heads had obstructions larger than allowed under current Code requirements. In addition, the amount of confining reinforcement, number of heads in a specimen, spacing between heads, and embedment length were evaluated in this study. The results of this study show that provisions in ACI 318-14 and ACI 349-13 do not accurately account for the effect of bar size, compressive strength, or the spacing of headed bars in a joint. The effect of concrete compressive strength on the development length of headed bars is accurately represented by concrete strength raised to the 0.25 power, not the 0.5 power currently used in the ACI provisions. Confining reinforcement increases the anchorage strength of headed bars in proportion to the amount of confining reinforcement per headed bar being developed. Headed bars with obstructions not meeting the Class HA head requirements of ASTM A970 (heads permitted by ACI 318-14 and ACI 349-13) perform similarly to HA heads, provided the unobstructed bearing area of the head is at least 4.5 times the area of the bar. Headed bars exhibit a reduction in capacity for values of center-to-center spacing less than eight bar diameters. These results are used to develop descriptive equations for anchorage strength that cover a broad range of material strengths and member properties. The equations are used to formulate design provisions for development length that safely allow for the use of headed reinforcing bars for steels with yield strengths up to 120,000 psi and concretes with compressive strengths up to 16,000 psi. Adoption of the proposed provisions will significantly improve the constructability and economy of nuclear power plants and other building structures.Electric Power Research Institute, Concrete Steel Reinforcing Institute Education and Research Foundation, BarSplice Products, Incorporated, Headed Reinforcement Corporation, and LENTON® products from Pentair

Collaborative exploration and collection of native plant genetic resources as assisted by agrobiodiversity fair

This article describes the agrobiodiversity fair aided exploration and collection expedition of native plant genetic resources in southern Lalitpur, jointly organized by the National Agriculture Genetic Resources Centre (NAGRC) and Group of Helping Hands (SAHAS) Nepal. In-district one-day agrobiodiversity fairs were organized in February and December month of 2019, altogether two times, and these agrobiodiversity fairs were used as a tool to explore plant genetic resources found in Bagmati and Mahankal Rural Municipalities of Lalitpur district. To collect these explored genetic resources during agrobiodiversity fairs, the joint field expedition, key informant survey, diversity rich farmers discussion was used as a collection tool. The present study explored, inventoried, collected and conserved 148 accessions of 44 crop species, the highest number (18 accessions) was of chayote followed by 10 accessions each of soybean, cowpea and maize and 9 accessions of common bean. Collections are generally new and unique. Many landraces, mostly from rice (13 landraces) were identified as extinct from the surveyed areas and few are under extinction mainly due to attraction of farmers to new high yielding varieties. The collected species with orthodox seeds were tested for germination ability and those that passed a minimum of 85% germination, were preserved in seedbank of NAGRC. NAGRC plans to characterize these accessions in the coming seasons depending upon the season of crop growing. The current expedition collected eight species for which mode of propagation is vegetative or those for which seed storage behavior falls under intermediate mode. NAGRC has been started expanding field genebank coverage using these accessions

Technical Efficiency of Hybrid Maize Production in Eastern Terai of Nepal: a Stochastic Frontier Approach

Maize is the second most important crop after rice in terms of area and production in Nepal. This article analyzes the technical efficiency and its determinants of hybrid maize production in eastern Nepal. Using a randomly selected data from 98 farmers (41 from Morang and 57 from Sunsari) in eastern Nepal, the study employed a stochastic frontier production model to find the production elasticity coefficients of inputs, determinants of efficiency and technical efficiency of hybrid maize farmers. The results showed that maize production responds positively to increase in amount of urea, DAP and the area planted, where as it is negative to seed quantity. The study indicate that farmers are not technically efficient with a mean technical efficiency 79 %. Socioeconomic variable age had a negative and significant while the household size had a positive and significant related to maize output. The younger farmers were observed more technically efficient than older farmers. Larger the members in the household higher the maize production. It is recommended that farmers should increase their fertilizer dose and farm size while they should decrease their seed rate for efficient production