Maximum Span Length
Regarding span factors with and without water are based on bending stress and deflection. Larger diameter pipe spans require saddles for stability. Many standards that require pipes to be filled with water are based on bending and shear stresses not to exceed 1,500 psi and a deflection between supports not exceed 0.1 inches. Other factors to be considered are as follows:
- Pipe is assumed to have standard W.T.
- No concentrated loads i.e. valves
- No changes of direction
- Horizontal plane
- Maximum deflection of span is limited 1 inch at 100 feet (25.4 mm at 30 meters)
- Stress intensification factors of components not considered
The Pipeline Toolbox calculates the Hoop Stress, Maximum Allowable Bending Stress, Maximum Bending Moment, Maximum Span Length due to deflection with and without water using a factor of 10. This statement was added to address the issues of deflection.
The question related what value represents or what number to use is as follows:
- Deflection L/360 (Default) is the most conservative factor if related information is unknown.
- Deflection L/270 is used when this information is known. Example residual stresses from construction activities are often present. These stresses are often difficult to evaluate accurately but can be disregarded in most cases. However, it is the engineer’s responsibility to determine whether such stresses should be evaluated.
- Deflection Other is used when other factors need to be applied.
- Pipe Hydrotest will reduce the span length calculations due to the additional weight
- Hydrostatic Deflection Factor is based on the pipe being exposed.
- Hoop stress
- Bending Moment
- Length Bending
- Length Deflection
- Total Length of Span
Note 1: Exposure to a failure of a buried pipe poses less consequence than from exposed piping.
Note 2: Bending deflection “L/240” or “L/360” are in terms of vertical length caused by uniform linear weight causing vertical deflection along a span. This appears in section 1604.3 of the IBC. L/180 is a common deflection limit for beams in IBC, L/240 is common for piping to avoid liquid pooling.