AC 2010-2330: STRUCTURAL EVALUATION OF A TRUSS PEDESTRIANBRIDGE

Jorge Tito-Izquierdo, University of Houston, Downtown

Alberto Gomez-Rivas, University of Houston, Downtown

© American Society for Engineering Education, 2010

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Structural Evaluation of a Truss Pedestrian Bridge Abstract

The superstructure of a steel pedestrian bridge located near the University of Houston Downtown campus was selected to expose the students of the Structural Analysis and Design program to the evaluation of an existing structure. The bridge is a continuous structure of 304’0” making three spans with support-to-support distances of 57’0”-190’0”-57’0”. The steel superstructure consists of two vertical trusses with 10’0” height and spaced by 10’0”; a top horizontal truss, and a concrete slab at the bottom that is supported on steel beams spanning between the vertical trusses. The bridge is supported by straps at the ends and by pin-type supports on the central bents. All the steel joints are welded. The substructure consists of concrete frames which are constructed over drilled shafts. The project tasks are to obtain the material take-off and cost estimating; obtain the natural frequency; make the structural modeling; and verify if the structure is able to withstand the loads indicated in current codes. This type of project is motivating for the students because they see a real application of their studies. The course assessment indicates that the project leads to an excellent compliance of the course outcomes.

Introduction

Senior Steel Design is one of the capstone courses of the Structural Analysis and Design program at the University of Houston Downtown. Students taking this course are in their Senior year and they were previously introduced to classes of Structural Analysis and Steel Design. The course is taught every other semester, and typically there are between 25 and 30 students enrolled. One of the outcomes defined for this capstone course is the evaluation of existing structures, for which this project is selected. It consists in the structural evaluation of an existing pedestrian bridge in order to determine if it is able to withstand the loads indicated in current construction codes. The selected existing pedestrian bridge has the characteristics needed for a capstone project level, such as easy access to the site, availability of as-built drawings, adequate complexity level for Senior students, and feasibility to complete the study during the academic semester. This project was repeated during two consecutive years permitting the students compare results with the previous year, and improve the quality of the study. It is not intended to repeat in the near future, but it will be used as a model for other similar projects.

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Pedestrian Bridge Description

As shown in Figure 1, the pedestrian bridge selected is used to connect two parts of a residential area separated by highway I-45, located south of beltway I-610 in Houston, Texas. The drawings are dated May-June 1974 and are available for the public at the Texas Department of Transportation (TxDoT), which is the institution in charge of the design, construction and operation of this pedestrian bridge. The bridge is a continuous truss with a total length of 304’0” making three spans with support-to-support distances of 57’0”, 190’0”, and 57’0”. The superstructure of the bridge consists of two vertical trusses made of steel I-shapes spaced 10'10" and with a center-to-center height of 10’0”, a top steel truss, and a bottom concrete slab supported by transversal steel beams every 19'0" which are attached to the vertical trusses. The useful width of the bridge is 8’0” that is the free space between the curbs. Both the handrail and the safety mesh are attached to the curbs. All the joints of the steel structure are welded. The vertical trusses are on roller-type supports on the ends, and pin-type supports on the central bents, as shown in Figure 1. The substructure consists of concrete bents which are constructed over drilled shafts. Student Work Plan

The student project consists in the structural evaluation of the existing superstructure of the pedestrian bridge. The class is divided into four working groups. Each group makes the evaluation of the same project, sharing information for comparison purposes, but working separately. The schedule for partial delivery of the project tasks are clearly defined by the

IH-610

IH-45

Figure 1. Location and panoramic view of the pedestrian bridge

N

pin-support strap-support

rigid nodes

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instructor beforehand. The following tasks are scheduled for completion by the end of the academic semester: 1. From the TX-DoT documents, select the drawings related to the superstructure of the

pedestrian bridge and make new CAD drawing. Include any field detail observed during the visit.

2. Compute the materials required to construct the superstructure of the pedestrian bridge (Take

off), estimate the actual cost of the materials, and estimate the dead load of the structure. 3. Using a commercial software, like SAP2000, perform the structural modeling of the

superstructure of the bridge. 4. Input the load cases according to the current code ASCE/SEI 7-05:

a. Dead load b. Live load, consider different possibilities to apply the live load. c. Wind load

5. Make the load combinations indicated in ASCE/SEI 7-05 6. Perform dynamic tests to find the natural frequency of the bridge and compare with SAP2000. 7. Perform the verification of the steel members. Use the computer program SAP2000, verify

the most stressed members with an alternative method. Verification of the drawings and material take-off. In order to perform this task, the class visits the site where they compare the main dimensions and details of the drawings with the constructed structure. As the original drawings are from 1974, the students need to investigate some sections that are currently not used. The student's job is to assess whether the drawings of the superstructure match well with the constructed bridge. After the completion of this task, the students are more confident with the drawing notations. The students make an AutoCad version of the drawings referred to the superstructure of the bridge, permitting a clearer reading of the geometry, as shown in Figure 2. The material take-off is obtained from the construction drawings. The take-off permits an estimate of the actual cost of the materials, as well as the estimating of the dead load of the superstructure. The take-off is done using a spreadsheet prepared by the students and considering the weight of the steel sections indicated in the literature or commercial catalogs. A typical take-off is shown in Figure 3. The estimated dead load of the superstructure is 502 kips. Considering a plan area center to center of the trusses and the total length of the bridge (10’10” x 305’0”) the weight per square foot is 152 lb. The estimated cost of materials is about $661,000 based on the prices of the year 2008 in Houston, TX.

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Figure 3. Typical material take off and cost calculation

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Structural evaluation of the superstructure.

Previous to this project, the students have two classes of structural analysis, where they are introduced to matrix analysis and finite elements that are the basis of SAP2000 and other commercial software. Additionally, in the classes of Steel Design and Reinforced Concrete Design the students use this program to solve small structures. Previously to start the structural modeling of the bridge, there are two sessions dedicated to review concepts and to explain the structural modeling of this large structure. As shown in Figure 4, the structural analysis is performed using the computer program SAP2000, which permits the students to gain experience and confidence with a commercial software, as well as to understand its strengths and limitations. The structural modeling is done following the construction drawings and it is verified continuously to correct errors due to incorrect data input or wrong assumptions. The total dead load calculated by the software compares well with the result of the material take-off, obtaining a margin of error of about 1%. The students correct input mistakes to obtain a good comparison between the sum of dead load reactions and the estimated weight of the structure. It is interesting to note that the typical errors are as gross as mistakes with the system of units, density of materials, type of joints, and load position. This comparison permits the students realize the importance of verifying the software input using a known parameter. The original drawings specify that the straps of the end supports was installed after the bridge deflects due to all the dead loads, and also provide the deflections values. The SAP2000 software is used to verify these deflections for which the structure is modeled with supports at the central bents only and considering that the concrete has no elasticity, as shown in Figure 5. There is a 12% difference in the deflection values, which may be explained because the computer model considers rigid nodes, while the original calculations might have considered the nodes as free pinned.

Figure 4. A SAP2000 model of the pedestrian bridge superstructure

Concrete Slab

Rigid joints

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The AASHTO code requires the use of an 85 psf live load used for the analysis of pedestrian bridges, which is applied on the slab surface. The reactions due to the live load are computed by the software and compared with the hand calculation. Figure 6 shows the locations where the live load is considered in order to verify the structure with the worst situation. The case of LL-c is neglected because it induces low stresses due to the end spans are short comparing with the central span. The wind load is important in the city of Houston, that is a hurricane prone area. This load is estimated using the current ASCE/SEI 7 code, considering a basic wind speed of 110 mph. Figure 7 shows the spreadsheet developed to help with the calculations of wind loads acting on the bridge. The students are required to verify the total input wind load comparing them with the reactions computed by the computer program. Two load cases are considered for wind load, the horizontal and the vertical wind loads.

Deflections (ft):

Node From Drawings From SAP2000 Diff.

L0 0.166 0.146 12%

L8 -0.295 -0.261 12%

Figure 5. Deflections due to dead load considering supports at central bents only

L0

L8

Figure 6. Alternate of Live load location

a. Live load along all the bridge (LL-a)

b. Live load at the central span (LL-b)

c. Live load at the end spans (LL-c), neglected

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The fourth task is the verification of the steel sections. The students used the capabilities of the commercial software, verifying some structural members with hand calculations and with a spreadsheet prepared by the instructor. This double check permits an understanding of the software assumptions avoiding errors due misinterpretation of some concepts. Dynamic tests

Field dynamic tests are performed to find the natural frequency of vibration of the bridge. The tests are done using an accelerometer set to read vertical accelerations and located at the center of the bridge (node L8 of Figure 8). The reading are stored in a data logger model Xplorer GLX, and processed using the software DataStudio, which is purchased from PASCO®. The structure starts vibrating when a student jumps at a similar pace than the computed natural frequency at the center of the bridge. After the jumping stops, the bridge continues vibrating with enough amplitude, permitting the data logger to record the vertical accelerations. The natural frequency is obtained using the Fast Fourier Transformation method included in the software DataStudio. The program SAP2000 is used to obtain the theoretical natural frequencies and corresponding vibration modes. The first natural frequency is 2.2 Hz which corresponds to a torsional mode. Figure 8 shows the second mode with a theoretical natural frequency of 2.9 Hz and

Figure 7. Wind load calculations following ASCE/SEI 7

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corresponding to a vertical wave mode. This second mode is in complete agreement with the natural frequency measured after the student jumped at center of the bridge that is also 2.9 Hz. The theoretical and experimental frequencies are identical because during the modeling of the bridge structure very few assumptions are done, mainly due the joints are totally welded and the concrete slab has no joints along all the bridge. Also, the central supports are real steel pins and the end supports are straps which model very good as rollers. The good agreements between the field and theoretical natural frequencies permit confidence in the structural model, including the assumptions and dead loads. Structural evaluation of the bridge

The superstructure of the bridge is evaluated using the current ANSI/AISC 360-05 steel code. The software SAP2000 is used to evaluate all the steel members of the bridge superstructure, for which two models are considered: 1) Bridge with supports on the central bents and with the straps at end bents. 2) Bridge with only two supports located on the central bents, and all the loads acting.

L0 L8

Figure 8. Fundamental mode of vibration from theory and a test

a. Fundamental mode of vibration as computed by SAP2000 (f = 1 / 0.34 = 2.9 Hz)

b. Reading using an accelerometer located in the center of the bridge (L8) (f =2.9 Hz)

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The live load considered for the structural evaluations are the cases indicated as LL-a and LL-b in the Figure 6. The load combinations are in agree with ASCE/SEI 7 code, which basically are: 1) 1.4 DL 2) 1.2 DL + 1.6 LL 3) 1.2 DL + 0.5 LL + 1.6 WL 4) 0.9 DL + 1.6 WL The load combination requiring live and wind load is conservative because these types of bridges are not used by pedestrians in the event of a hurricane. Also, it is conservative to consider the bridge without the end supports. The same load cases and load combinations are used for both models. The drawings indicate that the steel used for the truss is unpainted weathering steel A588, which is a high-strength, low-alloy material with a modulus of elasticity of 29,000 ksi, a yielding strength, Fy, of 50 ksi, and an ultimate strength, Fu, of 65 ksi. The steel A588 provides atmospheric corrosion resistance and longer life in applications where the bridge is exposed to the elements. As shown in Figure 9, the stress ratios for both models are lower than 1.0, meaning that the structure can withstand the loads required. In order to verify the program results, the students are asked to compute the stress ratio using another method. Figure 10 shows the results from a spreadsheet prepared for the Steel Design course which is used to double check the results. The loads are obtained from SAP2000 and the geometry is the input according to the drawings. This verification permits the students to understand some assumptions done by SAP2000 and to make any corrections, if it is needed.

Figure 9. Stress ratio from SAP2000

a. Stress ratio for a model with supports on central and end bents. Stress ratio = 0.645

b. Stress ratio for a model with supports on central bents only. Stress ratio = 0.972

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Project assessment

Course Assessment

The students were exposed to different aspects of a professional engineering evaluation of an existing structure, such as the cost estimating, project management, structural modeling, steel design, and field dynamic tests. Finally, each student group makes an oral presentation of their calculations and findings. Each project step is in agreement with the outcomes defined for the Senior Steel Design course. The Structural Analysis and Design program makes an indirect assessment surveying the perception of the students about the accomplishment of the objectives of the course. Figure 11 summarizes the course survey indicating that all the students agree that the course objectives were covered by the project.

Figure 10. Stress ratio computed using a spreadsheet

Figure 11. Assessment of the class using a survey of acceptance between the students

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During the oral presentation of the project and also in the written report, the students show a good understanding of the problem and they report great individual participation during the different stages of the project. The author considers that the practical nature of the project, the availability of design drawings, and easy access to the site are important factors for the good acceptance of this study. Conclusions and Recommendations

The students of Senior Steel Design, a capstone type course in the Structural Analysis and Design program at UHD, are required to evaluate the structural behavior of the superstructure of a pedestrian bridge following a schedule with partial tasks defined by the instructor. The students were able to compare the existing construction drawings with the real structure; perform the take-off and a cost estimating of the materials; make a structural modeling using a commercial software; perform dynamic tests to obtain the natural frequency; and evaluate the structure under loads from the current codes. During the project, the partial tasks are verified using alternative calculations and the structural model is verified with the dynamic tests, permitting the students gain confidence in their work. Finally, the students present the project with a written report and an oral presentation, showing the working procedure, results for every task required by the instructor, and their conclusions and recommendations. The project is well accepted by the students, observing great participation and interest to outperform the objectives of the project with excellent marks. Bibliography

1. American Association of State Highway Transportation Officials, AASHTO LRFD Bridge Design Specifications 4th edition. 2. American Institute of Steel Construction. ANSI/AISC 360-05, Specifications for Structural Steel Buildings, March 2005. 3. American Society of Civil Engineers - Structural Engineering Institute. Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-05. 4. State of Texas, State Department of Highways and Public Transportation (TxDoT). Houston Urban Project, Interstate 45 and Interstate 610 Interchange, Pedestrian Overpass at Southern St., Structure No 208. May-June 1974. 5. Computers & Structures, Inc., “SAP2000 – Integrated Software for Structural Analysis & Design, Technical Reference Manual”

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