Practice Periodical on Structural Design and Construction

Mold is a clear and present danger that the practicing structural engineer should be aware of and equipped to handle. Although mold has been with us since the dawn of time, toxicologists have only recently trumpeted the need for concern with respect to indoor air quality and human exposure. The availability of moisture and organic matter in the building envelope are the key factors in controlling the occurrence and spread of mold. Consequently, the structural engineer must be aware of the risks that mold presents to building occupants and the importance of building design in limiting the potential for mold impacts. The danger areas for the structural engineer include; current design practices, the insurance industry’s efforts to shift responsibility for mold remediation to construction professionals and building owners, the legal profession’s proclivity to litigate, and the general public’s perception that molds are “toxic” and serious health hazards. The structural engineer is also being compelled to design tighter structures using more “green” building materials, which adds further challenge in preventing the growth of mold. Thus, the structural engineer must work closely with the architect and project design team during the engineering design phase to limit the potential for mold impact and limit the project team’s liability for mold impacts over the projected life of the new structure. A short check list prepared and presented by the writers should assist the structural engineer with this effort.

A common design/construction procedure for highway bridges over marshes, small creeks, and streams is to have the bridge superstructure supported on pile bents. The focus of this article is to compare the relative lateral stiffness and pushover capacities of X-braced three-, four-, five-, and six-pile bents under combined gravity loadings and extreme flood/scour events. The relative performances of single- and two-story X-braced bents with and without a horizontal bracing strut at the bottom of the lower X-bracing were examined. Bent pushover capacities for the various combinations of the numbers of bent piles, bent heights, and X-bracing conditions previously described are presented in this paper.

A common design/construction procedure for highway bridges that are over marshes, small creeks, and shallow bodies of water is to have the bridge superstructure supported on pile bents. During major flood events, the volume and velocity of flood waters can cause considerable scouring to occur at the bents. As the elastic buckling capacity and stability of these bents vary inversely with the square of the bent height, a scour of $4.6\mathrm{m}$ $\left(15\mathrm{ft}\right)$ at a bent that was originally $4.6\mathrm{m}$ $\left(15\mathrm{ft}\right)$ in height will reduce its elastic buckling capacity by a factor of 4. Also the correct buckling load of such bents is somewhat questionable because of the unknown degree of fixity afforded by the soil at the pile ground line. This paper summarizes an investigation of the effect of the soil subgrade modulus on bridge pile bent buckling and pushover capacity. Analysis results indicate that bent pile buckling and pushover loads are not very sensitive to the soil subgrade modulus, $k_0$ , unless the value of $k_0$ is very small, i.e., $k_0⩽1.36\mathrm{N}∕{\mathrm{cm}}^3$ $(5\mathrm{lb}∕{\mathrm{in.}}^3)$ . Also, the analyses indicate that complete pile fixity can be assumed at approximately $1.5\mathrm{m}$ $\left(5\mathrm{ft}\right)$ below the ground line unless $k_0$ is very small, i.e., $k_0⩽1.36\mathrm{N}∕{\mathrm{cm}}^3$ $(5\mathrm{lb}∕\mathrm{in}{.}^3)$ .

A common design/construction procedure for highway bridges in Alabama is the use of steel HP piles driven to a firm stratum with a length above ground up to the level of a concrete bent cap which supports the bridge superstructure. The use of three, four, five, or six such piles in a row with the two end piles battered are very common bridge pile bents. Because of the large number of bridges in the state and the tendency to use standardized designs with pile bent substructures, the Alabama Department of Transportation recently developed a “screening tool” to assist its engineers in assessing the adequacy of bridge pile bents for extreme flood/scour events. The evaluation procedure employed in the screening tool is presented in macro and microfloodchart form in this paper, and basically moves through the following sequence: (1) preliminary or general checks such as the bridge bent being located in water with scour possible, etc.; (2) checking the bent piles for possible “kick-out” or plunging failure; (3) checking the bent piles for buckling failure; and (4) checking the bent for transverse (transverse to the bridge centerline) pushover failure from combined gravity and flood water loadings. The screening tool is a dynamic tool and is currently being automated, expanded in capabilities, simplified, and improved.

A common substructure for highway bridges over streams and creeks consists of a group of steel HP piles with a length above ground up to the level of a concrete bent cap, which supports the bridge superstructure. The use of three, four, five, or six such piles in a row with the two end piles battered constitutes a very common bridge bent configuration. In streams subject to significant flooding and scour, a critical load condition for these bents can be a gravity dead and live loading in conjunction with a horizontal flood water loading acting near the top of the bent. Such a loading may cause a pushover failure of the bent and, in turn, failure of the bridge. Pushover loads for such pile bents of various heights, bracing conditions, levels of gravity loading, and levels of scour are presented in this paper. A procedure is also presented for determining the maximum applied flood water loading acting on a bent, for determining the bent’s adequacy against a pushover failure.