The question comes up from time to time regarding requirements for a fence or railing to be 75% open (such as in the Lighthouse Point FL Building Code Amendments). This is the case for example in Florida when overlooking the Intracoastal in Broward County. This porosity also affects the design calculations for the components and foundation as affected by wind.
The porosity of a barrier system varies with the components used. For the condition of the requirement of a barrier to be designed such that it resists a 4″ sphere, two scenarios below are offered for consideration.
The first uses a 1″ picket and standard components to develop the porosity calculation. You’ll see by the formula presented that the porosity is 71%, below the 75% requirement which is typically used in our calculation for member and foundation integrity, and thus labeled on our design plans as the critical design factor.
Switching the pickets to 3/4″ which is also permitted on our plans yield the desired 75% porosity calculation with other standard dimensions and extrusions:
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The porosity of open structures has been better defined in recent ASCE codes under ‘open signs’. It explains that something porous isn’t always equivalent to the calculated porosity as per the above. One must also consider accumulation of ice & snow on the surface in question when thinking of porosity for wind. The actual porosity should always be determined by a design professional.
Take ASCE 7-16 figure 29.3-1 note 2: Signs less than 30% open shall be considered solid signs.
Force coefficients for solid signs with openings shall be permitted to be multiplied by the reduction factor (1-(1-∈)^1.5)
where ∈ = the ratio of solid area to gross area. We believe this to be the more appropriate method of determining most porosities if less than something like a fence or rail that’s mostly open.
There are more considerations which we encourage seeking professional assistance on in this porosity determination such as where the resultant force acts normal to the face or normal at a distance from the geometric center toward the windward edge (note 3 case A, B) or case C which goes further into analysis.
Contact us to discuss your design needs and work through porosity considerations for your product or project.
Last Update: April 9, 2021