Designing for Skew Effects

Also refer to the skew discussion on the Physical dimensions dialog input here for a detailed discussion of skew input requirements: Physical Dimensions

AASHTO Standard Specification for Highway Bridges (2002)

In the Standard Specification, NYSDOT has a blue page, 3.24.3.3, that states the following:

When the culvert is skewed relative to the over roadway, the distribution width (E), shall be reduced by multiplying “E” by the cosine of the skew angle.  In no instance shall the distribution width exceed 7 feet nor the section length of the precast unit.

The equation for distribution width is as follows:

E = 4 + 0.06 S, where E is the distribution width in feet, S is the perpendicular span length in feet.  Note that this width is per one wheel and can be double to calculate width per lane.

In summary the distribution width was reduced if there was a skew to account for multiple lanes on one unit.

AASHTO LRFD Specification (2007)

Bottom Slab of box culvert

Sections 12.11.2.3 and 12.11.2.4 state that for a bottom slab of a box culvert, wheel distribution due to skew does not need to be corrected for.

Top slab and sides of culverts/3 sided frame

Section 12.14.5.3 states that for culvert elements with skews greater than 15 degrees, the effect of skew shall be considered in the analysis.

Comparison of LRFD No Skew to 90 degree Skew

 The following is a table comparing the distribution width “E” between the two cases in LRFD, traffic parallel to span (no skew), and traffic perpendicular to span (90 degree skew).  Looking at the skew effect column, it can be seen that once the span length is greater than or equal to 30 feet, then the skew effect disappears.  However it should be noted that for spans greater than 24 feet more than one lane loaded will have to be considered for the skewed bridge.  Even though the skew effect disappears, the design is not unconservative because more lanes loaded have to be considered.

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NYSDOT Recommendation

For culverts with less than 2 feet of fill:

Distribution of wheel loads for box culverts and three sided frames with a skew of less than or equal to 15 degrees shall follow article 4.6.2.10.2 – Case 1: Traffic Travels Parallel to Span.  When designing for the Strength II limit state, only a single lane loaded along with a multiple presence factor of 1.2 shall be considered.

Distribution of wheel loads for the top slab and sides of box culverts and three sided frames with a skew of more than 15 degrees shall follow article 4.6.2.10.3 – Case 2: Traffic Travels Perpendicular to Span, unless a grid-type analysis is preformed.  When traffic is traveling perpendicular to the span, two or more trucks can be on the same design strip at the same time.  When designing for the Strength I limit state this effect must be considered along with multiple presence. When designing for the Strength II limit state, only a single lane loaded along with a multiple presence factor of 1.2 shall be considered.  The width of primary strip, E, given in Table 4.6.2.1.3-1 for negative moment, shall also be used for shear and thrust.

For culverts with more than two feet of fill:

It is stated in the code that where several wheels overlap, the total load shall be uniformly distributed over this area.  This statement already accounts for skew.  When designing for the Strength II limit state, only a single lane loaded along with a multiple presence factor of 1.2 shall be considered.