Academia.eduAcademia.edu

A COST BASED APPROACH TO DESIGN OF RESIDENTIAL STEEL ROOF SYSTEMS

Profile image of Barzin MobasherBarzin Mobasher

Abstract

A comprehensive system for the design of residential steel roof truss systems is presented. The research involved three distinct stages. In the first stage, components of the truss systems were tested in order to determine their member properties subjected to axial force and bending moments. Finite element simulations of these tests were carried out to further verify the calculations obtained using the AISI-LRFD code guidelines. The AISI-LRFD code based design curves were used for the actual design, the laboratory experiments and the finite element results provided additional checks and verification of the AISI values. The second stage of the research involved the development of an integrated design system that would automatically design a roof truss given minimal input and using the design curves as the performance constraints. A design optimization scheme based on the genetic algorithm was adopted to handle sizing, shape and topology variables in the design problem. A software system was developed to design the lowest cost truss given the input parameters. The third stage of the research involved full-scale testing of typical residential steel roofs designed using the developed software system. Roof trusses were loaded to failure. The full scale testing procedure established the factor of safety while validating the analysis and design procedures. Evaluation of the test results indicates that the present design system provides enough reserve strength for the structure to perform as predicted.

Figures (15)
In order to design the different truss members, it is necessary to construct the design curves that indicate the allowable internal stresses in any member. For the open section members, this task is achieved in three different ways. First, mechanical testing is carried out on individual members. Second, a finite element simulation (of the test procedure) is conducted. Finally, the AISI-LRFD code is used to compute the same values. The motivation is to ensure that the AISI code based values are applicable with adequate factor of safety to the cross-sections used.
In order to design the different truss members, it is necessary to construct the design curves that indicate the allowable internal stresses in any member. For the open section members, this task is achieved in three different ways. First, mechanical testing is carried out on individual members. Second, a finite element simulation (of the test procedure) is conducted. Finally, the AISI-LRFD code is used to compute the same values. The motivation is to ensure that the AISI code based values are applicable with adequate factor of safety to the cross-sections used.
Fig. 3 shows a typical load-displacement response in compression for various length specimens of 18 gage. The test is terminated in the post peak region of the response when the vertical displacement exceeds the prescribed value. Figure 4 shows the (mean) ultimate load obtained for the different chord specimens. Note that as the length of the section increases the axial capacity decreases. In addition the scatter in the data increases as the length decreases and local buckling dominates the failure.
Fig. 3 shows a typical load-displacement response in compression for various length specimens of 18 gage. The test is terminated in the post peak region of the response when the vertical displacement exceeds the prescribed value. Figure 4 shows the (mean) ultimate load obtained for the different chord specimens. Note that as the length of the section increases the axial capacity decreases. In addition the scatter in the data increases as the length decreases and local buckling dominates the failure.
2.2 Finite Element Simulations Flexural response of various sections were determined using a four point bending test (Fig. 5). The objective of this test was to compute the maximum bending moment that the sections can withstand under braced conditions. The plastic moment capacity was of primary importance at this stage of testing; hence the torsional buckling was neglected in the flexural mode. Specimens were simply supported on the loading fixture. A constant span of 24" was used for all the specimens. Two line loads spaced at 4.75" apart were used to apply the flexural forces. In order to prevent local crushing of the specimen due to stress concentration, steel plates 1/4" in thickness were used under the loading points. Specimen deflections were measured using a Linear Variable Differential Transformer (LVDT). Tests were terminated at a vertical displacement of 0.8" for negative moment and 0.4” for positive moments. Two replicate tests per section were conducted for each positive and negative bending moment tests. Positive moment was defined as causing compression in the flange, where as negative moment caused tension in the flange.
2.2 Finite Element Simulations Flexural response of various sections were determined using a four point bending test (Fig. 5). The objective of this test was to compute the maximum bending moment that the sections can withstand under braced conditions. The plastic moment capacity was of primary importance at this stage of testing; hence the torsional buckling was neglected in the flexural mode. Specimens were simply supported on the loading fixture. A constant span of 24" was used for all the specimens. Two line loads spaced at 4.75" apart were used to apply the flexural forces. In order to prevent local crushing of the specimen due to stress concentration, steel plates 1/4" in thickness were used under the loading points. Specimen deflections were measured using a Linear Variable Differential Transformer (LVDT). Tests were terminated at a vertical displacement of 0.8" for negative moment and 0.4” for positive moments. Two replicate tests per section were conducted for each positive and negative bending moment tests. Positive moment was defined as causing compression in the flange, where as negative moment caused tension in the flange.
Table 1 Comparison of Results for the Axial Compression Test Finally, the AISI-LRFD design code was used to compute the corresponding values of the different cross-sections. The details of the code provisions and the relevant calculations are not shown here. They are however available in a research report [Mobasher and Situ, 1996]. The values for the Axial Compression obtained from the laboratory, AISI-LRFD design code and finite element analysis are compared in Table 1.
Table 1 Comparison of Results for the Axial Compression Test Finally, the AISI-LRFD design code was used to compute the corresponding values of the different cross-sections. The details of the code provisions and the relevant calculations are not shown here. They are however available in a research report [Mobasher and Situ, 1996]. The values for the Axial Compression obtained from the laboratory, AISI-LRFD design code and finite element analysis are compared in Table 1.
Fig. 6b The local buckling mode of buckling obtained from a linear finite element simulation for flexural loading.
Fig. 6b The local buckling mode of buckling obtained from a linear finite element simulation for flexural loading.
In the study presented the most critical sections are in the top left and right chord, and marked with an ‘x’ (Fig. 6(b)). In other words, the combined axial and bending effects in the top chord govern the performance of the truss.
In the study presented the most critical sections are in the top left and right chord, and marked with an ‘x’ (Fig. 6(b)). In other words, the combined axial and bending effects in the top chord govern the performance of the truss.
Fig. 8(a) Schematics of the instrumentation of the truss, load cells, and dial gauges
Fig. 8(a) Schematics of the instrumentation of the truss, load cells, and dial gauges
Fig. 7(c) Connections details at the top of the King Post
Fig. 7(c) Connections details at the top of the King Post
The truss loading was continued until a satisfactory level of factor of safety with respect to the design loads was achieved without any visual signs of failure in a member or at a joint. The deflection of the truss was recorded at three nodal points on the bottom chord using dial gages with a resolution of 0.001” and a range of 1” throughout the loading history. Fig. 9 represents the total load applied vs. the deflection response of the truss. As shown in the figure, the deflection of the bottom chord member is quite uniform throughout the length of the member for a major portion of the load applied. An initial linear response is seen until about 1600 lbs. The slopes of the curves change at this point and the second nearly linear response is observed until about 2500 lbs. Beyond that point, the response is nonlinear.
The truss loading was continued until a satisfactory level of factor of safety with respect to the design loads was achieved without any visual signs of failure in a member or at a joint. The deflection of the truss was recorded at three nodal points on the bottom chord using dial gages with a resolution of 0.001” and a range of 1” throughout the loading history. Fig. 9 represents the total load applied vs. the deflection response of the truss. As shown in the figure, the deflection of the bottom chord member is quite uniform throughout the length of the member for a major portion of the load applied. An initial linear response is seen until about 1600 lbs. The slopes of the curves change at this point and the second nearly linear response is observed until about 2500 lbs. Beyond that point, the response is nonlinear.

Key takeaways

  • Based on this observation, AISI-LRFD Code guidelines were used in the subsequent sections for the truss design.
  • (2) The truss structure is defined by the specification of the panel points, the maximum unbraced length of a bottom chord member or top chord members.
  • In other words, the combined axial and bending effects in the top chord govern the performance of the truss.
  • To prevent the truss from side sway buckling, a three-truss system was assembled with trusses placed @ 2' apart.
  • The deflection of the truss was recorded at three nodal points on the bottom chord using dial gages with a resolution of 0.001" and a range of 1" throughout the loading history.

Related papers

Cost-based design of residential steel roof systems: A case study
Barzin Mobasher

1999

Abstract The cost effectiveness of steel roof systems for residential buildings is becoming increasingly apparent with the decrease in manufacturing cost of steel components, reliability and efficiency in construction practices, and the economic and environmental concerns. While steel has been one of the primary materials for structural systems, it is only recently that its use for residential buildings is being explored. A comprehensive system for the design of residential steel roof truss systems is presented.

View PDFchevron_right
OPTIMIZATION OF STEEL ROOF TRUSSES
Ijetrm Journal

Journal ijetrm , 2024

The main purpose of this project was to study and apply the topology optimization on nine different truss geometry as plane truss by only using angle section. The need of this study arises when time is a constraint in the project, and it is difficult or taking much time to choose effective and economical truss geometry during the design period. Its main purpose is to determine the minimum angle section which can be used to design a truss geometry which should be safe and should be able to take loads which are common in Hamirpur region. Design loads were distributed to the joints so that no moment is generated over the members. Total five span (6m, 7m, 8m, 9m,10m) were analysed and designed with the guide of STAAD Pro in various geometries mentioned till the minimum steel take off was achieved. Optimal geometries from each span of each 9 trusses ( (Pitched Pratt Roof Truss, Pitched Howe Roof Truss, Fan Roof Truss, Pratt Roof Truss, Howe Roof Truss, Warren Roof Truss, Fink Roof Truss, Diamond Roof Truss, K roof truss) with pin and roller support ,were compared to determine whether it is the same effective geometry for different combinations of spans and heights. This work and analysis shows that no fixed most effective geometry can be determined for different as well as same span, height nor height over span ratio. For each case different geometry was obtained. However, close results were obtained where it does help to provide a good guideline in choosing a truss that does not waste much material. From the results it has been concluded that the warren truss geometry can be considered as the most effective geometry in terms of bearing loads. In this study it has been attributed that the arrangement of the web members and chord members has been done in symmetric manner which helps in better distribution of loads in trusses. It was also observed that if we increase the angle between the chords (tension and compression) then the truss geometry distributes the loads in more effective way. From the results obtained, an optimality curve has been derived for a better understanding of correlation between span, optimum depth and minimum self-weight for various configurations.

View PDFchevron_right
Automated Design of Steel Trusses
Barzin Mobasher

Abstract: Designing an automated procedure for the optimal design of any structural system poses special challenges. Converting this methodology into a practical tool is even more challenging. In this research, a point-and-click software system is developed for the optimal design of roof truss systems. The starting point is a roof template containing minimal user input-outline of the truss, truss spacing, load information, and cost figures. A ground structure is constructed as the starting point of the design iterations.

View PDFchevron_right
Design optimization of N-shaped roof trusses
Karim Hamza, Kazuhiro Saitou

Design optimization of a class of plane trusses called the N-Shaped Truss (NST) is addressed. The parametric model of NST presented is intended for real-world application, avoiding simplifications of the design details that compromise the applicability. The model, which includes twenty-seven discrete variables concerning topology, configuration and sizing of the truss, presents a challenging optimization problem. Aspects of such challenge include large search space dimensionality, absence of a closedform objective function and constraints, multimodal objective function and costly CPU time per objective function evaluation. Three implementations of general-purpose genetic algorithms (GA) are tested for this problem, along with a version of taboo search called reactive taboo search (RTS). The RTS exhibited better performance than the tested versions of GA. Performance study of the algorithms provides some good insight to some weaknesses in GA and RTS as well as future prospective combination of them to gain better performance.

View PDFchevron_right
Sizing, Shape, and Topology Optimizations of Roof Trusses Using Hybrid Genetic Algorithms
Yoyong Arfiadi

Procedia Engineering, 2014

Structural optimizations have received great attention from structural engineers. Several optimization methods have been proposed including evolutionary strategies and genetic algorithms. This paper considers hybrid genetic algorithms for roof tr uss optimizations. Practically, roof truss optimizations are unique. In this case, the pitch angles are usually governed by roof covering types. In the optimization process, the pitch angle is set to constant, while the coordinates of the joints are determined by genetic algorithms. The optimization process utilizes hybrid genetic algorithms, i.e., a combination of binary and real coded genetic algorithms. Genetic algorithms are optimization methods that have been used successfully for various problems. For the sizing, shape and topology optimizations considered in this paper, the area of cross section and the number of members connected to every node are optimized using binary coded genetic algorithms, while the coordinates of the nodes are determined using real coded genetic algorithms. The optimization process for binary and real coded algorithms is done subsequently. The use of real coding for joint coordinates of structures gives the program the flexibility to obtain the final position of the joints. The arithmetic crossover is used to tackle this matter. In every generation, a portion of new individuals is inserted randomly replacing the old individuals. This can be considered to increase the variability of the population. In addition, the fittest individual is always transferred into the next generation. The penalty to the individuals that are violating the constraint is set to a minimum fitness in this paper. It can be shown that the proposed procedure is able to obtain the optimum design of roof truss structures.

View PDFchevron_right

Related topics

  • Seismic analysis and design
  • Integrated design
  • Factor of Safety

    • Related papers

    Effects of Structural Optimization on Practical Roof Truss Construction
    Nenad Kostic

    Applied Engineering Letters : Journal of Engineering and Applied Sciences

    In truss structural optimization the most frequently optimized factor of a structure is its weight. The minimization of weight contributes not only to savings in material, but also in other aspects of the structure such as number of elements used, number of welds needed, outer surface area, etc. This research aims to show the difference in optimal solutions for four different topological cases of a typical trapezoidal roof truss looking at their effects on overall outer surface area. The truss layouts are optimized for sizing, and a combination of sizing and shape with a minimal weight objective function. In order to ensure the most practically applicable solutions the example optimized in this paper uses dynamic constraints for buckling, stress constraints, and nodal displacement constraints. The overall outer surface area for all cases is compared, as surface protection accounts for a substantial part of the total cost of roof truss construction. Optimal solutions show a lack of correlation between weight and surface area, which is discussed in the conclusion.

    View PDFchevron_right
    Optimization of Steel Roof Framing Taking into Account the Random Nature of Design Parameters
    Urszula Radoń

    Materials

    The main subject of this paper is an optimization of steel roof framing used as a load-bearing structure in commercial pavilions. The authors wanted to draw attention to the necessity to take into account the uncertainty in the description of design parameters during optimization. In the first step, using geometrically nonlinear relationships, a static-strength analysis is performed. The decisive form of loss of stability in this steel roof framing is the jump of the node (snap-through), and not the buckling of the most stressed structure bars. Therefore, when creating the limit function, it was decided to make a condition limiting the permissible displacement. Values of the implicit limit function were calculated with Abaqus software based on the finite element method. Reliability analysis, and robust and deterministic optimization were performed using Numpress Explore software. Numpress Explore software communicates with the Abaqus software to perform analysis. The task ended with...

    View PDFchevron_right
    Shape Optimization of Roof Truss
    Rajwinder Singh bansal

    International journal of engineering research and technology, 2016

    The aim of this thesis is to concentrate on the impacts of various truss shapes in the design of plane truss by utilizing angle section. The need of this study emerges where in some cases it is difficult or requiring much investment to pick a successful and optimum truss shape during the design period. In researching the adequacy of different truss shapes, an aggregate of 20 truss shapes (Pitched Pratt Roof Truss, Pitched Howe Roof Truss, Fan Roof Truss, Pratt Roof Truss, Howe Roof Truss, Warren Roof Truss, Fink Roof Truss, Diamond Roof Truss, Low Profile Roof Truss, vault Roof Truss, Mono Roof Truss, Studio vault Roof Truss, Polynesian Roof Truss, Flat Roof Truss, Parallel Chord Scissor Roof Truss, Sloping Flat Roof Truss, Barrel Vault Roof Truss, Room-In-Attic Roof Truss, Half Scissor Roof Truss) with pin and roller support are chosen. The design loads are circulated to the joints so that there is no moment to be opposed by the members. The different spans and depth of 20 shape tr...

    View PDFchevron_right
    ROOF TRUSS DESIGN FOR INDUSTRIAL BUILDINGS
    Editor Ijasre

    International Journal of Advances in Scientific Research and Engineering (ijasre), 2019

    The roof trusses are mostly depended on the internal forces in the structure, dismember the structure and analyze separate free-body diagrams of individual members or combination of members. In this paper, the design of roof trusses frame, rivet diameters, and area of all members have to be calculated. General design theory of structural analysis, the equilibrium of rigid body, method of joints, tension and compression of members, mechanical properties of materials, design of riveted joints and area of trusses members in the project of Thilawa Zone (1) constructed by PREFAB STEEL Company Limited. This paper deals with the design and analysis of roof truss.

    View PDFchevron_right
    Optimization of Steel Truss Using Genetic Algorithm
    ٍِSalim Yousif

    Passer journal of basic and applied sciences, 2024

    In this paper, an optimization study is presented, focusing on steel trusses. The main goal of this study is to reduce the weight of truss structures using a Genetic Algorithm (GA), which is a widely acknowledged evolutionary-based method known for its efficiency in solving intricate optimization problems. The design problem formulation takes into account various constraints, such as displacement, tensile stress, and minimum size requirements. These constraints are implemented in MATLAB, utilizing the ANSI/AISC 360-16 Specification as a guideline for designing tension and compression members. To determine the optimal design, the approach involves considering discrete design variables. This is achieved by selecting sections from a database containing all available steel sections specified in the AISC Steel Construction Manual, ensuring practical and feasible design solutions. The efficiency of the algorithm is validated through its application to several plane truss types. Through a comparison of the outcomes obtained from the proposed algorithm with the results generated by CSI-ETABS software, it is demonstrated that this approach consistently yields better weight optimization. Overall, the study showcases the effectiveness of the GA-based algorithm in optimizing the weight of steel trusses. The results and implications of the findings are thoroughly discussed in the paper; this study has the potential to make a substantial contribution to the field of structural optimization and design.

    View PDFchevron_right
    Structural Analysis of a Genetic Algorithm Optimized Steel Truss Structure According to BS 5950
    Engr. solomon oyebisi

    2018

    A modern technique in structural optimization known as genetic algorithm was implemented in this paper to optimize a plane steel truss structure under point loadings and is subject to stress and displacement and buckling constraints. The genetic algorithm was developed in the MATLAB software. The genetic algorithm was run thrice on the plane truss structure and the run with the best result was picked as the final optimized truss structure. For each run a minimum of 500 initial population was set. The optimized truss structure gotten from the algorithm were analyzed and designed under dead and imposed loadings to compare and determine the percentage weight reduction and check the feasibility of the optimized truss structure. The software used to analyze and design according to British standard for steel design, BS 5950 was the SAP 2000 software. The results of the analysis and design in the SAP 2000 software showed the feasibility of the optimized truss as it passed all stress and di...

    View PDFchevron_right
    Weight optimization of large span steel truss structures with genetic algorithm
    Cristian Mojolic

    2015

    A genetic algorithm for first principles global structure optimization of supported nano structures Abstract. The paper presents the weight optimization process of the main steel truss that supports the Slatina Sport Hall roof. The structure was loaded with self-weight, dead loads, live loads, snow, wind and temperature, grouped in eleven load cases. The optimization of the structure was made using genetic algorithms implemented in a Matlab code. A total number of four different cases were taken into consideration when trying to determine the lowest weight of the structure, depending on the types of connections with the concrete structure ( types of supports, bearing modes), and the possibility of the lower truss chord nodes to change their vertical position. A number of restrictions for tension, maximum displacement and buckling were enforced on the elements, and the cross sections are chosen by the program from a user data base. The results in each of the four cases were analyzed in terms of weight, element tension, element section and displacement. The paper presents the optimization process and the conclusions drawn.

    View PDFchevron_right
    STUDY ON STRUCTURAL DESIGN OPTIMIZATION OF STEEL BUILDING
    Roshni Ramakrishnan

    IJEAST, 2022

    Pre-Engineered Building which has been gaining popularity over the past few years employs builtup sections for members as against hot rolled steel sections. The members' section profiles match closely with the bending moment diagram across the frames as the depth of the sections is greatly dependant on the moment across the sections and hence we see tapered section profiles in such building systems. A detailed study is carried out on the final optimized section profiles and the analysis results for the present paper. As a part of research paper "Comparitive Study of Pre-Engineered Building and Truss Arrangement Building for varying spans", models of industrial pitched roof steel building which were designed as both Pre-Engineered building system and conventional steel truss building system were analyzed and designed for dead loads, live loads and lateral load. The study focuses on the variation of member strength with respect to its dimensions,optimized section profile characteristics for a certain critical load combination and ways to reduce member forces by floating columns, lateral bracings etc.

    View PDFchevron_right
    Practical Approach to Designing Wood Roof Truss Assemblies
    Rakesh Gupta

    Practice Periodical on Structural Design and Construction, 2008

    The objective of this research was to use a three-dimensional (3D) analysis method to evaluate "system effects" in light-frame roof truss assemblies. The goal of this study was to develop an improved and practical design method for 3D roof truss assemblies used in residential construction. A truss plate manufacturer (TPM) design software was used to layout assemblies and to design individual trusses. The TPM software used a conventional design procedure (CDP) by analyzing one truss at a time in two dimensions. A commer cially available structural analysis program was utilized to model and analyze 3D truss assemblies as a system. This system design procedure (SDP) is being proposed as a tool to analyze and design 3D roof truss assemblies. Three truss assemblies, L shape, T shape, and a complex assembly, were analyzed. The structural responses including combined stress index (CSn, truss deflections, and reactions from both CDP and SDP were compared and the system effects were evaluated. From this investigation, it is concluded that there are three system effects observed by the SDP, but not accounted for by CDP. These are: reduced applied load effect, truss-to-truss support effect, and stiff truss effect. Based on this investigation, the maximum CSI for most trusses in all three assemblies reduced by 6-60% because of system effects. SDP can help to improve the design of truss assemblies by directly including system effects that are not accounted for by the CDP. 001: 1 0.1061/(ASCE) 1 084-0680

    View PDFchevron_right
    Size optimization of steel trusses using a genetic algorithm in MATLAB
    Journal of Software Engineering & Intelligent Systems

    Journal of Software Engineering & Intelligent Systems, 2017

    An important aspect of any engineering design problem is to achieve efficiency and efficacy. This can be in terms of energy consumption, performance, time, total weight and costs. In many cases, there are multiple solutions to a problem and you should select the one which satisfies better the criteria. This engineering design process is known as optimization. Optimization plays an important role in various engineering applications. Engineers are in continuity, challenged to design structures that use the least amount of resources and satisfy the structural requirements. The optimal design of structures can be decomposed into three major categories: topology, shape and size optimization. These methods have evolved with time and they may be divided in two maxi-groups: deterministic and non-deterministic algorithms. Size optimization of non-deterministic methods with genetic algorithms (GA) are investigated in this article and applied to some steel trusses in MATLAB soft R2017a. This is done by building an algorithm consisting in scripts and sub-functions, which are applied to the trusses for different constraints on stresses, displacements and buckling, depending on the case analyzed. Different values for the GA parameters are analyzed in such way to achieve the best design. The results are put in comparison with previous studies.

    View PDFchevron_right