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Industry standard factor of safety for ballast design

Industry standard factor of safety for ballast design

Industry standard factor of safety for ballast design

What design standards exist for the design of ballast against uplift forces, and what factor of safety against uplift do these design standards promulgate? I am familiar with a typical FS of roughly 1.5 against sliding forces due to active soil pressure for the design of many types of structures (including cantilevered retaining walls and gravity dams), but I am not aware of any such standard FS against uplift.

It is possible- perhaps likely- that the safety factor can vary depending on the nature of the loading. Specifically I have in mind uplift due to wind. But other forms of applied uplift may include: buoyant forces, dynamic/static pressure applied by some other fluid (e.g., water, etc.), and load transference forces (perhaps in a machine where ballast is used to counteract forces applied to some arm/member).

The way I have done this in the past is to use standard load combination equations in design codes. For example, the ASCE 7 load combinations for design of the limit state considering wind uplift are as follows:

# 7 ASD: 0.6D + 1.0Wworking_stress/service~state (note that in ASCE 7 2010, W = 1.67 Wworking_stress)
# 6 Strength design: 0.9D + 1.67 Wworking_stress/service~state

However, the problem with this approach is twofold.

First, using these load combinations directly leads two different, inconsistent effective factors of safety:

7. Effective FS_{ASD~Method} = 1.0/0.6 = 1.67
6. Effective FS {Strength~Method} = 1.67/0.9 = 1.85

Secondly, the intention of a design code such as ASCE 7 is to provide guidance for the *load side* of the design equation. The *resistance side* of the design equation is usually left to the various engineering groups/societies who publish standards for different materials used to resist loads, e.g. the American Concrete Institute (ACI), or the American Institute of Steel Construction (AISC). These groups provide standard factors of safety (ASD design) and resistance factors (LRFD design). However as far as I am aware, there is no American Institute of Ballast.

Also note that for this question I am not considering uplift due to seismic forces, because seismic events occur in the form of a spectral acceleration which *results* in a force, and that force gets higher as the mass of the ballast increases. Therefore ballast is not effective on its own in preventing uplift due to seismic acceleration.

RE: Industry standard factor of safety for ballast design

USACE has criteria for a flotation stability safety factor. See ETL 1110-2-307 FLOTATION STABILITY CRITERIA FOR CONCRETE HYDRAULIC STRUCTURES. this is intended to apply primarily to dams, locks, pumping stations, conduits, canals, spillways etc.

RE: Industry standard factor of safety for ballast design

Very helpful; 1.5 seems in line with the typical sliding safety factor as well. Purely subjectively, 1.5 also just feels about right. But I still wonder if 1.5 would be appropriate for the case of wind uplift.

RE: Industry standard factor of safety for ballast design

I would stick with ASCE 7 for buildings, roofs, etc. for large civil structures such as dams and floodwalls, wind might be ignored.

Quote (USACE EM 1110-2-2100)

e. Wind. Wind loads are usually small in comparison to other forces, which act on civil works structures. Therefore, wind loads should usually be ignored. For structures such as coastal flood walls where wind might cause instability, or for structures under construction, wind pressures should be based on the requirements of ASCE 7

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