Disadvantages Of Probabilistic Design

After the analysis on the first design brought to light the zero for members, it was decided that these members should be removed. In order to combat the flex of such a simple design the decision was made to double the thickness. This design would have the same outer frame of two triangles with a height of 20 centimeters and a width of 40 centimeters. Instead of being spaced five centimeters apart the wood be bonded directly together using Gorilla Glue. Balsa wood, a very lightweight wood with properties similar to a dense foam, was used for the main supporting member. Basswood, a lightweight yet firm wood, was used for the horizontal member. The final design added a triple layer sheet of balsa wood to rear side of the truss. This balsa sheet was added for the purpose of mounting and supporting the main structure. Additionally, a five centimeter square frame of bass wood was added at the end of the boomilever to place the loading block for testing. Once this design was implemented two key issues were discovered. First, due to the properties of the bass wood and balsa wood it was obvious that the placement needed to be reversed. The bass wood should be used for the hypotenuse and the balsa wood should be used for the

Bridges have been used for thousands of years, beginning with natural formations, such as huge rock arches (Appendix A). The first bridges made by humans were most likely simple spans of wooden tree trunks laid across streams, or planks, such as rafts tied together (Gascoigne, 2001). These simple designs evolved over time, as new materials became available, to form the hundreds of bridges we use today. Some basic bridge designs include truss, arch, beam and suspension bridges. The most basic of these is the beam bridge (Appendix B1), which consists of a deck, spanning a relatively short distance, that is held up by a pair of abutments (vertical supports at either end). When weight is placed on

Calculations were performed to determine the effectiveness of the design of the platform. Allowing for a safety factor of 1.5 times the design weight of 10kg and considering the bridge must not be overdesigned; plans were made for the bridge to fail at 25kg, 2.5 times that of the design weight. According to the calculations, the bridge would hold a load of over 15kg and experience failure at 20kg in the members. These calculations were later disproven in the testing, breaking 8kg earlier than expected, due to unforseen errors. An analysis of the bridge design and calculations has been included at the end of this report.

Clark Eldridge, an engineer from Washington State, had designed a trusted, conventional suspension bridge that would cost eleven million dollars. He requested that amount from the Federal Public Works Administration. However, Leon Moisseiff developed a design modification that would make the total cost of building the bridge much less- only eight million dollars. He swapped out the twenty-five feet long sturdy trusses as the form of support for the much cheaper alternative of 8 foot long girders, which were proven to be an insufficient form of support from the collapse of this bridge. His design accounted for an overall thinner bridge; and since his was the cheaper option, it was chosen for construction. However, the width of the bridge in comparison to the length, as well as the absence of trusses which would provide stability, are both part of Moisseiff’s “cost-effective”, yet impractically unsafe design. Since his design was flawed, he is mainly responsible for the bridge collapse. However, the board of engineers who approved his design are also partially responsible for the bridge collapse, as this unsafe design was allowed to be constructed. Meanwhile, it is imperative to remember that theories on aerodynamics were not fully developed as of the time that this bridge was constructed. Therefore, the full effect of oscillating forces was not properly accounted for due to the

In order to create a column-free space, designers and engineers opted to use free spanning composite glue-laminated (glulam) wood-steel arches supported by inclined concrete buttresses (Chodikoff, 2009). For the arches, the glulam slabs were made from engineered timbers consisting of wood laminations bonded together with strong, waterproof adhesives, resulting in a structural component as strong as steel. To increase the lateral stiffness, and assist in carrying the bending stresses from unbalanced snow loading, each arch has steel components anchoring the glulam slabs – resulting in a composite glulam wood-steel arch (Gregory, 2010). The use of glulam for the arches allowed for the roof’s large 100-metre span and curved element to be produced (Dietsch, 2011).

Experiment Two: Stiffness Report from laboratory work performed on 12 May 2011 as a part of the unit of study CIVL2201 Structural Mechanics

During the construction, two half-spans being assembled 50 meters above ground level had a misalignment of 4.5 inches or 114mm in camber. It was suggested by John Holland & Constructions to use a kentledge to weigh down the higher section of bridge. It so happened that they had ten, eight tonne concrete blocks on site. These were placed halfway along the higher span to

Temporary loads are those loads, which are placed on a bridge for only a short period of

Prior of this time, most bridge design were based on trusses, arches, and cantilevers to support heavy freight trains. Automobiles were obviously much lighter. Suspension bridge design had been envolving into producing bridges of maximum grace, lightness and flexibility which actually was more suited for carrying lighter cars and not heavier train. Unfortunately, engineers did not fully understand the response of the suspension bridge design to these poorly understand forces. The second reson was due to the excitation of torsional mode. The Tacoma Narrows Bridge was built with shallow plate girders instead of the deep stiffening trusses of railway bridges, in which trusses allowed wind to pass through but plate girders, on the other hand, present an obstacle to the wind. As a result of its design, it was proved that the bridge was too light and unstable. And causing alloping Gertieaction which were driven by the wind were being experienced by the bridge.

The comparison of measurement and prediction of prestress losses in prestressed members is highly documented in literature. Hale et al. (2006) studied the prestress losses behavior of girders subjected to increased fiber stresses. They concluded that the previous AASHTO LRFD Specifications equations (2004) overestimated the prestress losses by roughly 50%. It was found that the NCHRP Report 496 (Tadros et al., 2003) equations predicted the losses to within an average of 6%. Currently, there have been quite a few investigations on empirical models for prestress losses for HSC. In a study conducted by Kowalsky et al. (2001) on several HPC bridge girders in North Carolina to determine the prestress losses of HPC girders. The researchers found shrinkage losses were a small component of the overall prestress losses and that the elastic shortening and creep losses were the main contributors. These larger than expected losses from elastic shortening and creep were because of a predicted modulus of elasticity that was higher than actual. The total prestress losses ranged from 12.9% to 19.1% of the initial jacking stress. In an investigation conducted by Tadros et al. (2003), seven full-scale bridge girders were instrumented in Washington, Texas, Nebraska and New Hampshire to determine the prestress losses of HPC girders. The total prestress losses measured were found to be lower than the AASHTO LRFD (1998) model. Modified expressions were proposed to AASHTO and later adopted in

The disadvantages of a metal gusset plate’s behavior while being exposed to extreme heat conditions from a fire include th fact that the galvanized metal gusset plate’s teeth do not penetrate the wood all that deep. The wood the plates are connecting, have a charring rate of 1/40 inch per minute. The truss plate’s teeth have a minimum depth of 3/8 inch. So during a typical fire, the wood will lose 3/8 inch of its body in approximately 15 minutes leaving the teeth of the metal truss plate completely exposed. This will then cause the metal truss plate to fall off allowing that joint to lose its integrity. Once a joint on a truss is compromised, the weight it previously carried will be transferred to the next closest joints. These joints in return can not carry the additional load while being exposed to the fire causing them to lose their integrity resulting in a complete failure of the truss assembly. Whereas, the adhesive used for the glued finger-jointed truss will maintain its structural integrity longer than the truss members.

Because of advancements in today’s technology in construction field, many types of bridges are being constructed depending on the requirement and their suitability for the situations.

Curved I-girder has been used in bridges in the US since the 1950s, and has a complicated behavior compared to a straight beam. Some of the problems that curved girder faced are torsion, flange lateral bending, lack of stability and buildability concern. However, nowadays curved skewed I-girder bridges are used widespread and have a specific percentage of total. Horizontal curved/skewed I-girder bridge has advantages in saving material and simplifying structure by curving along roads and reduce intermediate supports.

Pre-cast pre-stressed double-tee girders are considered as an essential element of modern infrastructure used to construct civil project worldwide. These girders can be found in under-strengthlevel because of the local damages and developed cracks as the result of collision due to transportation and mishandling. Therefore, it is very important to find out an efficient retrofitting technique in order to restore the lost capacity and/or even outperform it. The current paper presentsa field tests up to collapse forthree actual-size pre-cast pre-tensioned double-tee (DT) girders having different levels of damage retrofitted using Externally Bonded Carbon Fiber Reinforced Polymer (EB-CFRP) technique. The girder stems for two of them were strengthened in flexure using uni-directional U–shaped CFRP sheets, while all girders were shear-strengthened at the dapped-ends. Each girder was loaded incrementally up to collapse using concrete Jersey Barriers, at the meantime, the deflections of the girder and developed strains on both concrete surface and the CFRP sheets were recorded at each increment. Test results assured the adequacy of the adopted strengthening configurations from both ultimate capacity and ductility viewpoints. It addition, the flexural resistance showed reasonable results compared to the analytical design resistance recommended by ACI 440.2R-08.

Web crushing failure: In this type of failure the direct forces are transferred from the loading zone to the