The Scenario
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 Calculation

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. 
Engineering Express 1 inch picket porosity calculation


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:
Engineering Express three quarter inch picket porosity calculation


Engineering Express® can help you with all your fence, railing, and gate needs using a variety of our state of the art resources.  Browse our Online Store for pre-engineered Safeguard Master Plan sheets, browse our expanding array of online calculators, or request a quote for a site specific project in any of the 21+ states to which we are licensed.


Jan 16, 2017   2476    Codes & Standards, Engineering Principles  
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