Industry standard factor of safety for ballast design

Question

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 2010 load combinations for design of the limit state considering wind uplift^* are as follows^**:

7. ASD: $~~~~~~~~~~~~~~~~0.6D+1.0W_{working~stress/service~state}^{***}$
8. Strength design: $0.9D+1.67W_{working~stress/service~state}^{***}$

However, the problem with this approach is twofold.

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

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

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 (though in some cases it may be effective in restraining lateral seismic acceleration via friction).

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^{* _{Note that in ASCE 7 2010/IBC 2012 forward, wind loads are factored loads (factored up by 1.6); prior to 2010, wind loads were working stress and the ASD load factor was 1.0 (LRFD load factor of 1.6).}}

^{**_{The actual ASCE 7 load combinations utilize the ASCE 7 2010, factored wind uplift load, i.e.:}}

^{_{$~~~~$7. ASD: $~~~~~~~~~~~~~~~~0.6D+0.6W_{ASCE~7~2010}$}}

^{_{$~~~~$6. Strength design: $0.9D+1.0W_{ASCE~7~2010}$}}

^{*** _{Due to the above notes, $W_{working~stress/service~state}=0.6W_{ASCE~7~2010}$.}}

Answer 1

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).

A machine where ballast can be used to counteract forces is simpler than you think - it happens frequently in tank / pressure vessel design. The liquid inside the tank is a ballasting force that could counteract a wide variety of loads:

1. Vacuum pressure inside the vessel causing caving in a flat bottom
2. External flooding for buoyancy causing the bottom to cave in
3. External flooding for buoyancy causing the vessel to float
4. Wind forces from a wind event

The ultra conservative ASME (specifically ASME RTP-1 - I have not verified for other sections of ASME code) will not allow the liquid inside to be used to counteract any of these forces. However, they will allow the maximum allowable strain to be doubled during these events.

ASTM D3982 will allow the liquid inside to counteract the first loading for tanks with a vacuum loading less than .217 psi - or 6" of liquid.

In practice however, before a hurricane reaches land everyone fills their storage tanks before evacuating, regardless of how well they were designed.

Answer 2

I don't think the method you used to calculate the Factor of Safety for Structural Stability is correct, especially in the case of using the factored loads from the strength design method. In general, the factor of safety is defined as:

$FS = \dfrac {\sum Resisting Load}{\sum Applied Load}$ or $\dfrac {\sum Stabilizing Force}{\sum Overturning Force}$

In the past, the allowable Stress/Strength Method (ASD) was mainly used in structural stability calculation with the non-factored load combinations, and a factor of safety of 1.5 should be maintained for the normal conditions. The factor could be lowered down to 1.0 for extreme loading conditions, which were usually defined by the respective industrial authorities, or trade organizations.

After the strength design method (factored load method, limit state design method) has gained wide acceptance and adoption, the code authorities (ASCE, IBC) have decided to streamline/unify the design process and dropped the factor of safety recommendations from the structural design code, instead, the margin of safety is built into the load factors and load combinations without the explicit requirement for separate stability checks, except in the case of foundation overturning stability, for which a safety factor of 1.5 is still in effect if ASD load combination is used.

In my opinion, it is prudent to always check the structural stability against overturning, sliding, uplift, or buoyancy using the defined method stated in the beginning of this answer.