Thermal stresses

Temperature and thermal stress variations in a segmental box girder bridge arc studied. A finite element model using the general finite element software MARC is used to predict temperature and thermal stress variation, for segmental box girder bridges. The predictions are compared with actual measured temperature data of two segmental box girder bridges, instrumented with thermocouples and vibrating wire strain gages, in Davie, Florida. Continuous monitoring was carried out for two months in July-August, 1991, and for one month in January-February, 1992. Comparison are also made with the NCHRP suggested profiles. A Monte Carlo analysis is performed to accommodate the probabilistic variation of ambient temperature conditions.

Recently developed electronic packages called Overmolded Pad Array Chip Carrier (OMPAC) IC packages frequently fail at the interface between the overmold compound and the substrate. In this study, this generic type of structure was evaluated by a combination of experimental and analytical methods. Model specimens representative of OMPAC structures were designed, manufactured and tested to failure. Detailed finite element models of the specimens were developed and analyses conducted to calculate debond stresses. Analytical methods were refined to include the effect of stress singularities. Stress results were averaged over a distance of.010 in. around the stress singularities to capture the intensity of the stress. These results were used in a combined stress failure criterion to calculate interfacial strengths based on macroscopic failure loads. The interfacial strengths were found to approach, but not exceed, those of the bulk overmold compound.

A series of experiments were performed on aluminum-steel explosively welded joints. The pub of this research was to better understand the synergism, if any, between thermal heat treatments and exposure to a seawater environment on the tensile-shear strength of the explosively welded joint. The effect of three different heat treatments on the tensile-shear strength of the bond was examined. The combined effect of two heat treatments and immersion in flowing aerated, natural seawater has been studied for two different corrosion times. For comparison purposes, some samples were also immersed in stagnant, aerated 3.5% weight NaCl solution. Open circuit potential measurements and microhardness profiles were also made. It was found that a heat treatment that would anneal the aluminum without causing the formation of an intermetallic layer at the aluminum steel explosively welded interface, was the best compromise between reduced mechanical properties and improved corrosion resistance.

Reinforced and prestressed concrete bridges are subjected to non-linear temperature
variations leading to complex thermal stresses which vary continuously with time. Though
these stresses are often comparable with those produced by live and dead loads, little
guidance is given in bridge design codes on how these stresses are accurately computed.
The objective of this project is to study the response of Florida bridges in the extreme
thermal environment The project is divided into the following four tasks
i) Computer modeling of the bridge and estimation of the thermal response.
ii) Field measurements of temperatures in typical bridges.
iii) Comparison of observed and estimated data.
iv) Suggestions and/or revisions to the existing thermal stress allowances in the code.
A computer software FETAB was used to model and analyze the thermal response
of several bridge cross sections. Two single cell box girder bridges, located at the 1-595
and US-441 interchange, Fort Lauderdale, were instrumented with thermocouples and
vibrating wire strain gages. The predicted temperature variations were found to compare
fairly well with those measured in the field. Though the analytical values vary a little from
the actual field data, emphasis was given to gain insight into the problems associated with
the thermal effects in concrete bridges. Suggestions were made for revision of existing
design code provisions for improved design of bridges.