Ballast for Temporary Structures
Continuing my series of posts based on some of my areas of work and not least the typical questions I get from my clients, this post will try to answer typical questions regarding Wind load and Ballast calculations, if you plan to raise a temporary structure in Denmark.
And pls. do note, this post, although it will be quite long, will only briefly scratch the surface of the vast area of the dynamics of wind, regulations, etc.
Why do calculations of Wind load differ (across the EU)?
The possible influence of wind on structures depends primarily on the size of the affected surface area, the shape and height of the structure, (the applied code and calculation standard) and the terrain of the area of the installation.
The Eurocode divides the terrain into 5 categories, named 0, I, II, III, IV. In Denmark, the Danish national annex to DS/EN 1991-1-4 decreases the number of selectable terrain categories to 4 - by renaming EC category 0 to category I, and combining the EC categories originally named I, II and III in to two categories named category II and category III. (ill. in the figure to the right).
To calculate wind loads the Eurocode uses the basic wind velocity, a parameter, which in most cases is dependent on the specific country, the code is applied to. In Denmark a value of 24 m/s is generally applied, unless the structure is placed near to, or directly on the western coast of Jutland (the most western part of Denmark), requiring a basic velocity of 27 m/s. Pls. refer to the Danish National Annex for a detailed description of the application and geographic location of the two Danish wind zones.
Despite other European countries relying on the simplified calculations stated in DS/EN 13814 and DS/EN 13782 for the calculation of the influence of Wind load on temporary structures. In Denmark, despite of DS/EN 13782 actually being stated as part of the Danish Building regulations (BR18), wind load must be calculated to the relevant parts of the Eurocode applying the relevant Danish national annexes. (Trying to keep this paragraph short - the requirement originates from the Danish national annex to DS/EN 13782)
How is the structures resistance against wind load proven?
Wind load causes a range of structural challenges and especially when looking at temporary structures, vibrations are commonly overlooked, eg. the vibrations sustained to a guy wire can easily loosen turn-buckles and should be checked especially if the structure is installed for a longer period. Another typical effect often seen on structures caused by the horizontal forces of wind load is "racking", when the wind load forces the structure to lean over "rack" to one side. Though, in my effort to keep this post short, we will concentrate our effort on the wind loads mediating the risk of sliding, lifting and overturning of the structure.
Sliding - A matter of friction
When calculating the structures resistance against sliding, four parameters will determine the stability, the horizontal wind load (W), the structural self-weight (G), the weight of added ballast (B) and the frictional coefficient between the structure and the site (μ), dependent of the surface and the material the structure is made of. Disregarding partial safety factors dependent on the specific code and applicable standard, the structure is resistant to sliding, when the weight of the structure and ballast multiplied with the frictional coefficient, is larger or equal to the horizontal load, or simply: W ≤ μ · (G + B)
An increase of the coefficient of friction between the structure and the surface is by far the easiest way to decrease the amount of ballast needed, eg. adding rubber pads beneath spindle jacks.
Overturning
The resistance against overturning of the structure primarily depends on the dimensional parameters - try to imaging something you would want to knock over - if its high with a small footprint, you´ll likely succeed without any effort - if instead the item had a large footprint or was low - you would only succeed if additional force was applied. Applying some simplified math, will give us a simplified equation, balancing the standing moment of the structure with the overturning moment: M = W · h ≤ G · ½ · d + B · ½ · d
Lifting
Depending on the type and shape of the structure, it may be at risk of either, a part, or the entire structure, being lifted by the wind load. This uplift (vertical forces) is caused by the wind flowing over the structure, similar to the way airplanes are lifted from the ground. Keeping the structure to the ground will require the sum of the self-weight of the structure (G) and the applied ballast (B) to be equal or greater than the applied vertical forces (V). or essentially V ≤ B + G
How do I estimate the amount of ballast needed?
Combining the formulas from the above sections, will give a single equation (neglecting most partial safety factors, eccentricity of loads, etc) which can be used to estimate the ballast needed. B = γ₁ · (W/μ + V) - γ₂ · G
Applying a safety factor (γ₁), essentially increasing the design values of the sum of the horizontal load (W) divided by the applicable coefficient of friction (μ), and the vertical (V) loads, substracted the structural self-weight (G) reduced by a safety factor (γ₂) - basically reducing the design value of the structural self-weight.
Can I use water as ballast?
Water should only be used as ballast, if a thorough risk assessment has been performed, especially the risk of the sudden reduction of ballast must be assessed and mitigated – do apply worst case scenarios, eg. a fork lift used at the site hits and penetrates one of the ballast containers making the water flow from it, decreasing the amount of ballast, potentially decreasing the load capacity of the site surface and causes short circuiting of power cables.
I hope this post, answered some of your wind load, and ballast related questions.
