ARTÍCULO ORIGINAL

DOI: 10.17533/udea.redin.n77a18

A practical method to compute shear force and bending moment envelope curves for isostatic bridges subjected to vehicle live loads

Desarrollo de un método práctico para calcular las curvas envolventes de cortante y momento en puentes isostáticos para cargas vivas

Cuerpo Académico UAGro-CA-93: Riesgos Naturales y Geotecnología, Unidad Académica de Ingeniería, Universidad Autónoma de Guerrero. Av. Lázaro Cárdenas s/n. Col. La Haciendita. C. P. 39070. Chilpancingo, México.

* Corresponding author: Sulpicio Sánchez Tizapa, e–mail: sstizapa@uagro.mx

ISSN  0120–6230

e–ISSN 2422–2844

(Received May 12, 2015; accepted September 7, 2015)

In order to obtain the envelope curve of shear force and bending moment induced by vehicle live loads along Mexican isostatic bridges with spans from 15.0 to 50.0 m, first and second-degree equations are calculated by considering the position from the initial right joint to the final left joint of a beam as the independent variable. Additional to the professional use of this elementary tool, it can be used in academic courses of bridge design in order to avoid the illegal use of commercial software. By using a simple-short algorithm developed in a free software application, the theoretical envelope curves are obtained. In order to simplify the process, these curves are used to calculate the coefficients of shear force and bending moment equations by means of a statistical analysis. The minimum value of correlation coefficient for both methodologies was 0.98. This proposed method could be also extended to other vehicle loads used worldwide.

Keywords: Bridges, envelope curve, shear force, bending moment, free software

RESUMEN

Este artículo presenta un método práctico para calcular la curva envolvente de cortante y momento en puentes isostáticos con longitudes entre 15,0 y 50,0 m, considerando las cargas vivas del reglamento mexicano. La curva de cortante es una función de primer grado mientras que el momento flector es de segundo grado, en ambos casos la distancia a partir del extremo izquierdo de la viga es la variable independiente. Esta herramienta puede utilizarse ya sea en el diseño de puentes en el campo profesional o en cursos académicos, evitando así el uso ilegal de software profesional. Mediante un algoritmo simple desarrollado en software libre se calcularon las envolventes de ambos elementos mecánicos, utilizadas posteriormente para evaluar, mediante un análisis estadístico, los coeficientes de las ecuaciones respectivas. El mínimo coeficiente de correlación entre ambos métodos fue 0,98, lo cual muestra la capacidad del método. Este proceso puede ser implementado para cualquier tipo de carga viva en puentes.

Palabras clave: Puentes, curva envolvente, cortante, momento, software libre

1. Introduction

Design of bridges is a complex process where some branches of Civil Engineering converge, for example: Structural Analysis, Hydraulic-Hydrology, Geotechnical Engineering, Earthquake Engineering, and Design of Reinforced or Precast Concrete. To simplify this process, it is necessary to develop some basic tools for computerized calculation. Thus, coefficients of Ft-degree and Second-degree equations to define the envelope curve of shear force and bending moment caused by the most common Mexican standardized vehicle live loads were calculated.

In the field of design of bridges, either in Civil Engineering education or professional work, maximum values of shear force and bending moment for isostatic superstructures are often evaluated by the influence line method. This could have two disadvantages: a) it is a complex task, b) only some points of the envelope curve are calculated. Another alternative may be to use professional software for structural analysis. However, for academic institutions of Civil Engineering or small design companies located in developing countries, the latter option can result in high costs. Therefore, illegal use of professional structural analysis software is very frequent. In fact, this situation reaches 65% of the whole software employed in Latin America [1].

For structural design, the envelope curve of bending moment is used to define either the length of longitudinal reinforcement or the trajectory of tendons in precast concrete beams. For steel superstructures, this curve can also help to define the position of reinforcing flexural plates. In the same case, the backbone curve of shear force defines the type or characteristics of shear reinforcing.

2. Materials and methods

2.1. Mexican vehicle live loads

Mexican Standards define two kinds of virtual vehicle live loads for bridge design. Figure 1a shows the vehicle live load used in bridges with span greater or equal to 15.0 m in roads Type D, it is called IMT20.5. For roads Type ET, A, B and C, the vehicle live load is called IMT66.5; when the length of the bridge is less than 30.0 m, this vehicle live load has six loads, Figure 1b. If the bridges span is greater than 30.0 m, the system has three point loads and a distributed uniform load, Figure 2a. The weight of this virtual vehicle live load is 654 KN [2].

In some cases, bridge design is still based on other vehicle live loads defined by earlier Mexican standards, for example: Trucks T3S3 and T3S2R4 with weight of 478 KN and 758 KN, respectively, Figure 2b - Figure 3 [3].

2.2. Methodology proposed

In order to evaluate the backbone curve of shear force and bending moment, a practical algorithm in a free software application [4] was developed. The process consists of the following steps:

If 0 ≤ X ≤ L1

In Eqs. (1-4), the distance X defines the analysis section. In this study, the load P1 was placed in 101 positions (variable L1); for each load position, the mechanical internal elements (shear force and bending moment) were calculated in 101 points along the longitudinal axis of the beam (distance X), including the initial and final sections (X=0.0, X=L). This information was stored in an array called [M1] (101x101). Each row of this array has 101 values of shear force or bending moment that correspond to position of load P1 defined by L1. If the vehicle live load has three loads (P1, P2, and P3), additional arrays [M2] and [M3] are calculated.

Step 2. Figure 6 shows a defined position of the load distribution. In this situation and respect to the node A, the load P3 is over it, the load P2 is positioned at distance b, and the load P1 is at distance b+a. For this load system position, mechanical internal elements (101 values) of each individual load array correspond to: a) n2 -row of [M1] array for load P1, b) n'1- row of [M2] array for load P2, and c) first row of [M3] array for load P3. The n'1 and n2 values are obtained according to Eqs. (5) and (6); Figure 7 shows the position of these mechanical elements in arrays [M1], [M2], and [M3].

According to Eq. (6), the number of rows of load system array [MG] for a vehicle live load composed by m-loads can be evaluated by Eq. (7), the Ʃ symbol shows the total length of the load train system and lj is the distance between two consecutive loads.

Step 3. For the superposition process, the proposed algorithm transforms each individual matrix [Mi] (101, 101) in a general matrix [MGi] (T, 101), then a transformation matrix [TR] (T, 101), which premultiplies the matrix [Mi], is required, Eq. (8). Eq. (9) defines the limits of index of these arrays: l, k, and p.

The relation among the index of general array [MGi] and individual array [Mi] must be established. In upward order from P1 to P3, where i has values from 1 to 3, Eqs. (10-12) were obtained, Figs. 8a, 8b, and 8c.

The row n1 of Eq. (11) can be evaluated by Eq. (13). In the same way, Eq. (14) values the [MGi] array associated to the load Pi when i >1, Eq. (15) defines the value of n(i-1), where li is the distance between two consecutive loads.

For any load system, the distance among loads defines the first cell where each load begins to cause either shear force or bending moments in the beam. The first black block of Fig 8d shows the first value effect of load P1 in the first section of beam analysis. The second black block indicates the cell that records the first effect of load P2, this cell is on the n1- row. The third black block, in the n2- row, contains the first effect of load P3.

3. Results and Discussions

3.1. Envelope curve of shear force and bending moment

Four load systems of the Mexican Standard were analyzed. According to the common practice of bridges design in Mexico, spans from 15.0 m to 35.0 m were considered for the analysis of the IMT20.5 vehicle. Three additional envelope curves of shear force and bending moment for IMT66.6 vehicle, T3S3 vehicle, and T3S2R4 vehicle, were calculated for 25.0 m to 50.0 m spanned bridges. As it be can seen in Figure 9, both, the envelope curve of bending moment and shear force, can be defined by a Second-degree and a First-degree equation, respectively.

3.2. Coefficients of shear force and bending moment equations

In order to simplify the evaluation of coefficients of bending moment equations, two half of the curves were plotted. In this way, it was necessary to define which segment could be used for the evaluation. Figure 10 presents the two half parabolas of the four analyzed curves. The dotted line is the left half parabola used in this process. Eqs. (17) and (18) define the shear force and bending moment, where X is the distance from the left joint to the analysis section, the domain is 0 ≤ X ≤ L/2.

Eqs. (19) and (20) [5] give the coefficients of Eqs. (17) and (18), V and M are the values of shear force and bending moments of envelope curve. Tables 1, 2, 3 and 4 show the coefficients of four load systems (IMT 20.5, IMT66.5, T3S3, and T3S2R4).

Figure 11 shows the evaluated curves with the coefficients of Table 4 and the envelope curve evaluated for the T3S2R4 vehicle. As it can be seen, there is an adequate correlation between two procedures. For the four load systems and spans of analyzed bridges, the minimum value of correlation factor is 0.98

3.3. Examples of application to evaluate the envelope curves

The domain of variable X is 0 ≤ X ≤ L/2 (22.0 m), Figure 12 shows the graphs of Eqs. (21) and (22). In order to obtain the envelope curve along the whole bridge span, a half right curve is plotted by considering the mid-span as the axis of symmetry.

Example 2. Evaluate the envelope curve of shear force and bending moment for a T3S3R4 vehicle considering a 44.0 m bridge span. According to Table 4 for a bridge span equal to 44.0 m, with the coefficients of shear force and moment, Eqs. (23) and (24) can be obtained.

The domain of variable X is 0 ≤ X ≤ L/2 (22.0 m), Figure 13 shows the graphs of Eqs. (23) and (24). In order to obtain the envelope curve along the whole bridge span, a half right curve is plotted by considering the midspan as the axis of symmetry. Additionally, Figures 12 and 13 show the calculated curves with a professional structural analysis program [6].

The analysis developed herein was based upon Mechanics, Matrix Algebra and Statistics. These were used to develop a simple computational application either for bridge design or teaching.

In order to develop the technical capacity of schools of Civil Engineering -especially those located in developing countries-, the use of free software must be encouraged as it improves the technical ability of their students. It this way, the current common practice of using non-licensed commercial software will be soon discouraged in Latin American countries.

The obtained equations are tools to perform the analysis of isostatic superstructures for Mexican bridges. The equation coefficients of shear force and bending moment have an adequate correlation, because the minimum correlation factor is 0.98. For this reason, this methodology can be applied to any vehicle live loads similar to those herein shown. Comparison with results of commercial software indicates a maximum variation of 4% for an IMT66.5 vehicle. Thus, the proposed method can be used for the worldwide vehicle live loads.

5. References

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2.  Secretaría de Comunicaciones y Transportes (SCT), ''Cargas y acciones'', Proyecto de Puentes y Estructuras, Norma N·PRY·CAR·6·01·003/01. Ciudad de México, México, 2001.         [ Links ]

3.  Secretaría de Comunicaciones y Transportes (SCT) / Dirección General de Autotransporte Federal (DGAF), Capítulo XI del reglamento del capítulo de explotación de caminos de la Ley de vías generales de comunicación que trata del peso y otras características de los vehículos. Ciudad de México, México: SCT/DGAF, 1980.         [ Links ]

4.  Scilab, Scilab, 2014. [Online]. Available: http://www.scilab.org/. Accessed on: Dec. 5, 2014.         [ Links ]

5.  G. Canavos, Probabilidad y estadística: aplicaciones y métodos, 2^nd ed. Ciudad de México, México: McGraw-Hill/Interamericana, 1998.         [ Links ]

6.  Computer & Structures, Inc. (CSI), Sap2000® v. 17. Integrated solution for structural analysis and design. Walnut Creek, USA: Computers & Structures, Inc., 2015        [ Links ] ]]> 1 2011 56 1 1 1-12 2 Secretaría de Comunicaciones y Transportes (SCT) 2001 3 Secretaría de Comunicaciones y Transportes (SCT) / Dirección General de Autotransporte Federal (DGAF) 1980 4 Scilab 2014 5 1998 6 Computer & Structures, Inc. (CSI) 2015