Roof Truss Research Articles

Behavior of Complex Hat Shapes Used as Truss Chord Members

Cold-formed steel roof truss systems that use complex hat shape members for both top and bottom chord elements are a growing trend in the North America steel framing industry. When designing cold-formed steel sections, a structural engineer typically tries to improve the local buckling behavior of the cold-formed steel elements. The complex hat shape has proven to limit the negative influence of local buckling. However, a distortional buckling mode can be the control mode of failure in the design for the chord member with an intermediate unbraced length. The chord member may be subjected to both bending and compressive load because of the continuity of the top and bottom chord members. These members are not typically braced between each panel point in a truss. Numerical analyses using finite strip and finite element procedures were developed to compare with experimental results. A parametric study on geometric imperfection was also conducted to investigate the factors that affect the ultimate strength behavior of a particular complex hat shape. Better understanding of the flexural behavior of these complex hat shapes is necessary to obtain efficient, safe designs of a truss system. The results of these analyses will be presented in the paper.

初代国会仮議事堂の小屋組について : わが国の国会仮議事堂に関する研究 その2

The purpose of this paper is to explain the wooden roof truss of the first provisional Diet Building built in 1890 in Japan and to reconstruct it, referring to pictures (colored woodblock prints, copperplate prints and the like) of the provisional Diet Building and to other examples with similar construction in Japan and Germany. The roof truss used in the provisional Diet Building was called the "German roof frame" at that time in Japan and was a composite construction reinforced with iron tie bars. We can find several parallels in festival halls of those days in Germany. The reason seems to be that both of them were temporary buildings which had to be built in a short period.

長野県下諏訪町・下諏訪倉庫繭倉群の建設経緯と構法について(建築歴史・意匠)

This report aims to explain the structural futures of the cocoon warehouses of Shimosuwa Soko Company and the developing process of the warehouses in Suwa area. The four cocoon warehouses, mud-walled and multi storied, were built in 1901 and 1912. To realize more stories and spaces in newer ones, the company adopted the new structure plan containing the wider spans between beams, European roof truss, and the two storied pillars. As the new plan is similar to others in this area, it can be said the unique and local building plans including western technique were developed in a short term.

Effect of flexibility of substructures upon thermal behaviour of spherical double layer space truss domes. Part II: Gradient & partial loading

This paper presents the second part of a study on the influence of the type and properties of support conditions on the thermal behaviour of spherical double layer space truss roofs. Whereas the first part focused on the consequences of uniform thermal loadings, this paper studies the effects of support flexibilities on the behaviour of such structures subjected to gradient and partial loadings. It is observed that the thermal forces are very high and depend on the magnitude of temperature and the dome size. It is also pointed that the important point in partial loading is the unsymmetrical deformation and behaviour of the dome, while in gradient loading it is the rotational deformations. Finally, it is concluded that the use of rigid supports should be carefully examined.

Strength of Bolted Aluminum Alloy Tension Members

Aluminum alloy tension members are incorporated in structural and mechanical systems, examples of which include bracing in railcars, members in roof trusses, and transmission towers. In practice, it is difficult to connect all elements of a complex cross section. Procedures used to estimate the net section rupture strength of a tension member through a bolted end connection must account for the accompanying reduction in efficiency. Current provisions within the Aluminum Design Manual fail to adequately address situations other than single or double angle tension members attached by a single leg. During the course of this investigation, 72 aluminum alloy tension members consisting of angles, channels, and structural T's were fabricated and tested to failure. Variables considered during the course of the study included bolt pattern, connection length, cross-section profile and area, connection eccentricity, and alloy. Experimental results were accompanied by finite-element analysis of several of the test specimens. A design rule is proposed and compared to the experimental data as well as several other methods for calculation of the net section rupture strength.

Static Behavior of a Historic Roof Structure

Historic roof structures of large monumental buildings are often complex with static behavior that is difficult to grasp. Yet, understanding their static behavior is necessary to adequately preserve and maintain them. The aim of this paper is twofold. The more general aim is to develop use of the finite element method in combination with visualization and to demonstrate its potential both for providing an understanding of historic structures and their static behavior and for supporting a dialogue regarding these matters. The more specific aim is to examine the static behavior of one particular roof structure and the damages to it that occurred. The roof structure selected was that of the medieval Swedish castle of Glimmingehus. A detailed computational model of the roof truss was developed, in terms of which a set of possible causes of the damages observed was simulated. Reliable causes were found and three-dimensional visualizations were carried out to demonstrate their respective influence on the overall structural behavior. As expected, use of visualization in the analysis was found to be very helpful for interpreting the relationship between the computational model and the computed results.

Joint Details in Post and Beam Framing

Although most wood design is now undertaken by firms specializing in this work, nonspecialists have occasion to design in wood from time to time. As with other structural materials, even a simple understanding of the characteristics of the material will enable designers to avoid many subsequent problems. The intent here is to present several relatively simple examples. Although nails may be the most commonly used connectors, bolts and lag screws tend to be used when member thickness increases from 38 to 89 mm ~1 1 2 to 3 1 2 in.!. With wood, the ideal connector when members overlap would be a device that develops shear over the entire contact face. Although this is a characteristic of adhesives, their use on-site is problematic. The development of the lightweight galvanized truss plate has revolutionized fabrication of house roof trusses by eliminating the lap joint, yet providing distributed shear connections. Similarly, the glulam rivet or nail with appropriate nail plates 4.8 to 6.4 mm ~3/16 to 1 4 in.! in thickness, has provided distributed shear connections for larger members. Concurrently with the development of the truss plate and glulam rivet, the Swiss are believed to have developed the equivalent of a two-sided truss plate that could be inserted between the contact faces of lap joints. As with truss plates, presses were needed

Strength evaluation of oil palm stem trussed rafters

Research on the suitability of using oil palm stems as trussed rafters is reported in this paper. Previous studies indicated that certain portion of the stems had good potential to be used as structural components. Fink type trusses made from parts of oil palm stems were designed, fabricated and tested. Full-size load test was carried out since it was considered to be one of the most reliable methods of establishing the structural adequacy of roof trusses as well as their behaviors and modes of failure. The results obtained were compared to the results of hardwood timber trusses of similar configurations. The results indicate that oil palm stem has the potential to be exploited for use as roof trusses with some design modifications and proper grading of the oil palm stems.

Behavior of thin gusset plates in compression

The use of cold-formed steel members for roof and floor truss applications is gaining widespread acceptance in the United States. However, there is little technical information regarding the behavior and design of thin gusset plates in compression. Thus, a study was initiated at the University of Missouri-Rolla aimed at investigating the behavior of thin steel gusset plates in compression. Key parameters that were considered in the experimental study were the thickness of the gusset plate, the width and length of the gusset plates, the fastener location, and the fastener pattern. Both a plate model and a column model were investigated for computing the strength of a thin plate in compression. Based on the findings of this study, design recommendations are proposed.

Helix Pedestrian Bridge

Amgen Incorporated, one of the world's largest biotechnology firms, recently developed a new research center on the Elliot Bay waterfront of Seattle, Washington. As part of the research center, a pedestrian bridge was built to connect the campus with a major transportation corridor. The owner's goal for the new pedestrian bridge was to create a signature gateway to Amgen's campus. The pedestrian bridge is a three-arch structure that depicts a three-dimensional helix. The overall shape of the bridge both reflects the helical DNA that connects all life and references the vital research under way at Amgen. The arches create the outline of the basic DNA helix, and the deck support and roof trusses complete the remaining components of the helix. The abstract double helix provided many design and construction challenges. The slim profile, large enclosed areas, and suspended deck meant wind and pedestrian vibration would be critical design considerations. The unique shape made it difficult to apply the standard building or bridge design codes. The contractor was faced with erecting this twisting structure over active railroad tracks while meeting tight geometric specifications. The bridge stands today as a testimony to the owner's vision, teamwork, innovation in bridge design, and creative construction techniques. The bridge combines both form and function to show how an ordinary pedestrian bridge can become an extraordinary statement while serving its intended function. The project has won broad community support in the few months that it has been open.

Fragility Assessment of Light-Frame Wood Construction Subjected to Wind and Earthquake Hazards

A fragility analysis methodology is developed for assessing the response of light-frame wood construction exposed to stipulated extreme windstorms and earthquakes. Performance goals and limit states (structural and nonstructural) are identified from a review of the performance of residential construction during recent hurricanes and earthquakes in the United States. Advanced numerical modeling tools provide a computational platform for risk analysis of light-frame wood building structural systems. The analysis is demonstrated for selected common building configurations and construction (defined, e.g., by roof sheathing, truss spacing, and roof or shear wall nailing patterns). Limit state probabilities of structural systems for the performance levels identified above are developed as a function of 3-s gust wind speed (hurricanes) and spectral acceleration (earthquakes), leading to a relation between limit state probabilities and the hazard stipulated in ASCE Standard 7, “Minimum design loads for buildings ...

Towards integrated performance-driven generative design tools

Performance-driven generative design methods are capable of producing concepts and stimulating solutions based on robust and rigorous models of design conditions and performance criteria. Using generative methods, the computer becomes a design generator in addition to its more conventional role as draftsperson, visualizor, data checker and performance analyst. To enable designers to readily develop meaningful input models, this paper describes a preliminary integration of a generative structural design system, eifForm, and an associative modeling system, Generative Components, through the use of XML models. An example is given involving generation of 20 lightweight, cantilever roof trusses for a saddle shaped stadium roof modeled in Generative Components. Synergies between the two systems and future extensions are discussed.

Design optimization of N-shaped roof trusses using reactive taboo search

Abstract 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 27 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 closed-form objective function (OF) and constraints, multimodal objective function and costly CPU time per objective function evaluation. Three implementations of general-purpose genetic algorithms (GAs) are tested for this problem, along with a version of taboo search called reactive taboo search (RTS). In this study, the raw version of RTS exhibited better performance than the tested versions of GA but lacks some of the GA capabilities to span the search space. A modification of RTS that uses a population-based exploitation of the search history is proposed. The optimization results show that the introduced modification can further improve the performance of RTS.

Capacity of timber roof trusses considering statistical system effects

For structural timber systems such as trusses, system effects related to strength variation within timber members has a significant effect on reliability. The system effect originates from the reduced probability that weak sections of timber coincide with the most stressed sections in the truss. In this paper, the strength variation within and between timber members is described with a statistical model earlier calibrated against data for Norway Spruce of Scandinavian origin. A method is presented to account for buckling effects when expressing the interaction between axial loading and bending in all sections of the truss members. The interaction is expressed in terms of a combined stress index, CSI, defined so that the failure criterion is CSI=1. For a given truss with given load, the cumulative distribution of CSI can be determined by Monte Carlo simulations based on the statistical model. By comparing the results with corresponding results obtained from deterministic engineering design, the statistical system effect is quantified. It is found that within member variability gives an extra capacity of 12% for Norway Spruce and 24% for Radiata Pine. Design rules proposed for Eurocode accounting for system effects in trusses give results slightly on the safe side compared to the results from the present study.

The ‘Belfast’ Roof Truss—Worth Conserving?

Timber trusses have been used in many forms since ancient times. The ‘Belfast’ timber truss was developed around the mid-nineteenth century to meet the demand for efficient, lightweight, and longer span roofs, brought about by the Industrial Revolution. It is a very efficient structural form with a curved top member that behaves essentially as an arch in combination with a horizontal tie member. Several thousand still exist, many in buildings of historic interest. This paper outlines the development of this truss type, explains why it is an efficient structure, and demonstrates that trusses can be replicated in historic buildings almost exactly as the original. This is supported by comparisons of the actual and theoretical load-carrying behaviour, based on the results obtained from new trusses, tested in laboratory conditions, and on trusses tested in existing buildings. In addition, the findings in a report of load tests on prototype trusses in 1906 have been included in the comparative study. All the work on the analysis, design, fabrication, and testing of ‘Belfast’ trusses has resulted in a better understanding of their behaviour, which is not only of historic interest, but also fundamental to the repair and restoration of existing trusses. As well as their significance in historic building conservation, the paper proposes that ‘Belfast’ trusses are an attractive alternative to other structural roof types.

Stability and ductility of thin high strength G550 steel members and connections

High strength cold-reduced steel is typically of stress grade G550 (550 MPa nominal yield and tensile strength) and less than 1 mm thick. The steel has been used for many years for sheeting and decking but is now being used for structural members such as roof trusses and stud walls of steel framed houses. The paper summarises a major research program on the stability and ductility of this steel which has been proceeding for several years at the University of Sydney. The paper relates the Sydney research to the work of others being undertaken around the world.

Seismic behavior and retrofitting of joints in traditional timber roof structures

The seismic quality of traditional timber structures, as those supporting roofs in old buildings, mostly depends on the condition of their connections. A research program has investigated the behavior of old timber joints and examined retrofitting criteria. The main parameters affecting the mechanical behavior of the connection have been singled out and their effects quantified by means of experimentation and numerical analysis. A synthetic model of the cyclic behavior has been formulated, implemented in a finite element format, and used in the analysis of timber structures in seismic conditions. The model has been verified with full-scale experimentation on a roof truss. Different reinforcing methods have been compared on the basis of experimental observation and calculated structural response. The study has shown that a satisfactory post-elastic response, comparable to that of new structures, may be reached for suitably retrofitted structures.

Assesment of probabilistic system effects on the reliability of timber trusses

In structural engineering the system effect is known as the possibility of transversal collaboration among parallel load-bearing elements due to mechanical load sharing. The statistical system effect, related to the variation of strength within structural timber elements, is investigated in this paper. The system investigated is a roof truss of W-type. The strength variation within and between timber members is described with a statistical model based on stochastic variables representing position of weak sections and length of weak regions, strength of weak sections and strength between weak sections. With the statistical model, the cumulative distribution of a Combined Stress Index (CSI), which is a measure of the stress levels in the truss, is determined with Monte Carlo simulations. The results from the simulations are used to determine the reliability level of the truss considering the effect of within member variation of the timber strength. It is shown that the load bearing capacity of a truss is increased significantly for a given reliability level, since the probability is low that sections with locally high stresses will coincide with weak sections. The increase in the estimated load bearing capacity for a W-truss is of the order 15-20% for spruce of Scandinavian origin and 30-35% for Radiata Pine. (Less)

Forensic Engineering Structural Failure Review By Finite Element Analysis

A Florida Engineer Was Charged By The Florida Board Of Professional Engineers With Incompetence And Negligence Because Of His Design, And His Fabrication Of The Metal Roof Trusses For A Hanger/Home In Florida. The Engineer Personally Designed, Supervised Fabrication, And Specified The Field Modifications Required Due To Conflicts Of The Bearing Height, And Truss Damage During Shipment Of The Trusses To The Jobsite. A Complaint Was Filed By The Homeowner And Investigated By The Boards Consulting Engineer. The Consultant Investigated And Alleged That The Modified Heel Connection On The House Truss Was Grossly Inadequate To Transfer The Code Mandated Loads For This Home. No Failure Has Occurred Since The Home Has Not Been Subjected To Full Code Mandated Wind Loads. The Following Analysis Follows The Investigation, And Formal Hearing Before An Administrative Law Judge In Florida. An After The Fact, Finite Element Analysis Was Performed By W. Bracken To Verify The Consultants Allegations. This Analysis By Bracken Was Performed After The Conclusion Of The Case Against The Engineer Of Record.

The behaviour of a new type of connection system for light-weight steel structures applied to roof trusses

The Rosette-joining system is a completely new press-joining method for cold-formed steel structures. One Rosette-joint has a shear capacity equal to that of approximately four screws or rivets. The Rosette thin-walled steel truss system presents a new fully integrated prefabricated alternative to light-weight roof truss structures. The trusses are built up on special industrial production lines from modified top hat sections used as top and bottom chords and channel sections used as webs which are joined together with the Rosette press-joining technique to form a completed structure easy to transport and install. A single web section is used when sufficient but can be strengthened by double-nesting two separate sections or by using two lateral profiles where greater compressive axial forces are met. An individual joint in the truss can be strengthened by introducing a hollow bolt into the joint hole. The bolt gives the connection capacity a boost of approximately 20%. A series of laboratory tests have been carried out in order to verify the Rosette truss system in practice. In addition to compression tests on individual sections of different lengths, tests have also been done on small structural assemblies and on actual full-scale trusses of a span of 10 metres. Design calculations have been performed on selected roof truss geometries based on the test results, FE-analysis and on the Eurocode 3 and U.S.(AISI) design codes.