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Behavior of Normal and High Strength Reinforced Concrete Structures under Blast Loading

Translation and Other Rights For information on how to request permission to translate our work and for any other rights related query please click here. Chapter 1 - Introduction. Chapter 2 - Research Background. Chapter 3 - Experimental Program. Chapter 4 - Analytical Research Program. Column 1A2, an in.

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Col- umn 1A1, identical to Column 1A2, was tested at a larger stand- off distance than Column 1A2 and experienced only minor damage. Continuous spiral reinforcement also improved the column response to close-in blast loads as demonstrated by the performance of Column 1B.


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One finding from the exper- imental test program is that, in cases where spiral reinforce- ment is not used, discrete ties can be made to perform well if adequate anchorage is provided. The in. Thus, given the same blast loading scenario, the circular blast-detailed col- umn would be expected to perform better than the respective gravity or seismic columns. Therefore, the square column with blast- resistant reinforcing details would be expected to perform better than other columns that had less reinforcement when tested at similar loads.

Only two of the six columns 1A1 and 2A2 experienced a complete breach. Columns 2A1 and 2B experienced a significant loss of the concrete core, while the remaining two columns stayed intact. Column 2-Blast and 3A exhibited spalling of the side concrete cover, which was not initially expected. All columns tested in this experimental program fell into Design Cate- gory C i.

Columns with a small scaled standoff were exposed to a severe blast load that resulted in the formation of plastic hinges, spalling of concrete cover, and in some cases, total breach of the column. Decreasing the design threat by providing sufficient standoff distance from bridge columns is a safe alternative to increasing the design category and detailing requirements. In general, a higher scaled standoff requires less stringent detailing requirements because of the lower inten- sity of the blast loading.

Proven Technology

One of the best ways to decrease the design loads and hence the design category is to increase the standoff distance with physical deterrents such as bollards, security fences, and vehi- cle barriers. Maximizing the standoff distance is the easiest and often the least costly method to achieve the appropri- ate level of protection for a bridge. Therefore, if access to the columns is sufficiently limited, the design standoff dis- tance can be increased, which will decrease the effects of blast loads on the columns and the associated design category. When standoff distance is not available to avoid Design Cate- gory C, the design and detailing provisions described below should be met.

Cross-sectional shape affects how a blast load interacts with a column. Therefore, the use of a circular column cross-section over a square cross-section is recommended. With proper detailing, however, columns with rectangular and square cross-sections can be made to be blast resistant. The cross-sectional dimensions of a column also have a major impact on column capacity in the case of a close-in blast, and this parameter controls the onset of a breaching failure. Consequently, a minimum column diameter of 30 in.

Experimental observations show that continuous spiral re- inforcement performs better than discrete hoops with standard hooks for small standoff threats. To avoid anchorage pullouts and to improve the performance of blast-loaded columns with discrete hoops or ties, longer hook lengths than currently specified should be used. This increased level of detailing should extend over the entire col- umn height to help account for the variability associated with different threat scenarios.

The splicing of longitudinal reinforcement should be avoided when feasible. Locating splices away from contact charges can help minimize localized blast damage. As stated previously in the report, it is not possible to design all bridge columns to resist all possible threats.

Blast Resistant Highway Bridges: Design And Detailing Guidelines 2010

An acceptable level of risk must be accepted for these extreme load cases. If a large enough quantity of explosive is placed close enough to a bridge column, failure is to be expected. It is desirable to obtain additional experimental data that can be used to further validate the proposed recommen- dations. Also, more research is needed to better understand the spall and breach patterns for concrete columns subjected to close-in blasts. Spalling on the column sides was noted in this experimental program with six half-scale blast tests.

Prior to these tests, columns subjected to close-in blast loads were assumed to perform similarly to walls, with spalling on the front and back face. According to Winget et al. This added ductility and confinement allows for the concrete core to stay intact and continue providing sup- port to the superstructure once plastic hinging starts to occur.

As the amount of transverse steel increases, however, the ease of construction decreases.

Placement of reinforcing steel becomes problematic, which increases the probability of voids in the concrete. To avoid such problems, other blast-resistant design alternatives that should be considered include the use of concrete-filled tubes, fiber-reinforced concrete, mechani- cal couplers at splice locations, and external retrofits.

Concrete-filled steel tubes would eliminate the difficulties in construction seen with regular reinforced concrete columns. More research is needed, however, to better understand connections of these members to the foundation and super- structure. Bruneau et al. A concrete- filled tube may be an economical solution if labor is expensive and steel prices are low.

Fiber-reinforced concrete, in combi- nation with a typical gravity-reinforced column design, may be a viable alternative to a heavily reinforced concrete column. Fiber-reinforced concrete aides in the prevention of con- crete spall by reinforcing concrete away from the location of reinforcing bars. Additional research is needed, however, to validate the performance of fiber-reinforced concrete columns subjected to blast loads and to determine if they provide a cost- effective option for such load cases. Mechanical couplers at splice locations in reinforced con- crete columns subjected to close-in blasts may reduce the chances of column failure associated with discontinuous longitudinal reinforcement.

According to Zehrt et al.

References

External retrofits can also be employed to improve the response of reinforced concrete columns to close-in blasts. The use of fiber-reinforced polymer wraps and steel jacketing are two.