Have you ever had to chip a thick layer of ice off a car to get in, only to then have to run it for an hour with the heat on full blast to get the windshield cleared? If you’re from a colder climate, you’ve probably experienced freezing rain at least a few times in your life. Roads turn into ice rinks (serious, I ice skated down the road to my friend’s house in my Wisconsin town as a kid), doors freeze shut, and absolutely everything gets encased in a layer of beautiful ice.
One of the most often overlooked loads in all of structural engineering is Ice Load, specifically from “atmospheric icing”, which is formed from freezing rain, freezing fog, or sleet. For certain structures, specifically, those that are normally light and airy, the weight of a design-level ice storm is hands-down the controlling load case.
What is Ice Load?
Ice Load is the weight of ice accretions on a structure. Ice is frozen water, so somewhat heavy for lightweight structures, weighing in at a minimum density of 56 lbs/ft³, per ASCE 7-16 §10.4.1.
Scope of ASCE 7-16 Ice Loads
Ice can build up on structures from a wide variety of sources, including freezing sea spray, freezing rain, and freezing water from a burst pipe. Some ice-climbing fanatics (it’s an extreme sport) even go so far as to intentionally ice silos and other structures to create artificial ice-climbing challenges, like those in the video below.
The scope of ASCE 7-16’s Chapter 10 “Ice Loads – Atmospheric Icing” is limited though. Subject to a few other exclusions listed below, ASCE 7-16 covers ice loading from freezing rain, snow, and in-cloud icing, and does not account for local effects in mountainous terrain.
For sea spray and other oceanic effects, publications from organizations like the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) can help. The CRREL also has publications detailing the anticipated action of floating waterway ice on piers and other coastal structures, which can be devastating.
Other exclusions from ASCE 7-16’s ice load scope include:
- Structural loads caused by hoarfrost
- Any site in Alaska requires site-specific studies
- Certain “Special Icing Regions”, as indicated on the ice thickness maps
- Dynamic loads caused or enhanced by ice accretion
- Any structure which is already subject to other design standards around icing, including:
- Electric Transmission Systems
- Communications Towers and Masts
- Other structures for which national design standards exist
What are “Ice-Sensitive Structures”?
The ASCE 7-16 defines Ice-Sensitive Structures as “Structures for which the effect of an atmospheric icing load governs the design of part or all of the structure.” This is a pretty useless definition, effectively saying “consider ice loads where they matter”, but they do go on to provide some example structures.
ASCE 7-16 examples of Ice-Sensitive Structures include:
- Lattice Structures
- Guyed Masts
- Overhead Lines
- Light Suspension and Cable-Stayed Bridges
- Aerial Cable Systems, such as Ski Lifts and Logging Systems
- Amusement Rides
- Open Catwalks and Platforms
- Flagpoles
- Signs
The commentary goes on to provide a bit more explanation, stating that many “open structures” are efficient ice collectors, and the size and number of structural members included can both play a role.
Practically, this means components that are ordinarily lightweight, like thin-walled metallic or slender wood members, tend to be more at-risk of having ice load control a design, especially in certain configurations.
Ice-Sensitive Structure Example: Trellises on Curtainwall
From my own design experiences in building facades, one of the worst ice-loading situations is when we have a trellis, open louver system, or perforated panel projected off the face of the glass of our curtainwall system. There’s then almost always a catwalk between this element and the glass, so maintenance can get in to wash the exterior of the windows.
Usually these denser grids of lightweight components don’t weigh very much themselves, but they present huge surfaces for ice to build up on in a storm, growing to several times their size and weight.
All that weight projected several feet off the face of an aluminum system becomes a real challenge for a material that doesn’t love to make moment connections, in a small space that makes large bolt patterns difficult to achieve.
How do you Calculate Ice Load in ASCE 7-16?
ASCE 7-16 provides a whole chapter, Chapter 10, detailing the intricacies of calculating ice loads on structures, but the brief overview is as follows:
- Look up Input Values
- Ice Importance Factor, I.i, from Table 1.5-2, based on the building’s Risk Category
- Check out my article on How Risk Categories Are Assigned
- Mapped Nominal Ice Thickness, t, from Figs. 10.4-2 through 10.4-6
- Topographic Factor, K.zt, as defined in Chapter 26
- Ice Importance Factor, I.i, from Table 1.5-2, based on the building’s Risk Category
- Calculate the Height Factor, f.z, per Equation 10.4-4
- Calculate the Design Ice Thickness for Freezin Rain, t.d, per Equation 10.4-5
- Apply the Design Ice Thickness to either the Prismatic Shapes or Flat Plates equations to compute a design volume of ice
- Multiply the design ice volume by the density of ice, code minimum of 56 pcf
The major inputs for these calculations are the Ice Importance Factor (which adjusts for the building Risk Category), the Mapped Nominal Ice Thickness (from Figs. 10.4-2 through 10.4-6), the height on the building of the component in question, and the Topographic Factor, which is a bundle of joy to calculate in its own right.
| Risk Category | Ice Importance Factor – Thickness, I.i |
|---|---|
| I | 0.80 |
| II | 1.00 |
| III | 1.15 |
| IV | 1.25 |
Note that there are special icing regions in Figs. 10.4-2 through 10.4-6 which do not provide a nominal ice thickness. For those regions, local climactic data will be needed, which can often be had from the building inspector in larger municipalities.
Like the ASCE 7 wind speed maps, the ice thickness maps are all based on a standardized component height of 33 feet (10 meters) off the ground, though the load intensity does depend on component height. To account for this dependence, we use Equation 10.4-4 to calculate a Height Factor, f.z, as a function of elevation, z:
Now that we have all our inputs, we can calculate the Design Ice Thickness, t.d, in accordance with §10.4.6:
Armed with our design ice thickness, we then go on to apply that thickness per one of the two sections below, and finally multiply the design ice volume by the ice density, which is mandated as at least 56 lbs/ft³ to finally get the ice load.
Ice Loads on Prismatic Shapes
For structural shapes, prismatic members, and anything else similar (think guy wires and the like), we use Equation 10.4-1 to calculate the ice volume.
The only new variable here is D.c, the Characteristic Dimension, which is the largest diagonal dimension of the shape. Those familiar with extrusion design will know this dimension as the “circle size” of a particular die. Six examples of that dimension are diagrammed in Figure 10.4-1.
One the Area of Ice is known, simply multiply by the design ice density, minimum of 56 lbs/ft³, and you’ll have an ice load in lbs/ft of the prismatic member.
Ice Loads on Flat Plates and Surfaces
On flat plates and large, three-dimensional objects, we calculate the volume of ice per Equation 10.4-2, which just multiplies pi and the design ice thickness times the surface area of the plate or object, A.s:
We’re given two reductions that are allowed for vertical or horizontal flat plates:
- Ice Volume may be multiplied by 0.8 for vertical plates
- Ice Volume may be multiplied by 0.6 for horizontal plates
With the ice volume in hand, we just multiply by the design ice density, minimum of 56 lbs/ft³, and end up with the total weight of ice on the structure. This can be divided out by the surface area to get a weight per square foot, it that is needed for design.
Limitations of the ASCE 7-16 Ice Load Approach
Ice load is one of the least understood and least-documented areas of the entire ASCE 7-16 standard. If you start reading through the commentary, Chapter C10, you’ll quickly find some rather disturbing statements.
“No standardized method” was used to collect the design ice thicknesses utilized for the maps, and in fact, they’re largely based on historic newspaper reports of ice thicknesses measured by people of a variety of backgrounds on the telephone and electrical cables in their areas.
Also, the entire model for prismatic sections is based around circular cross-sections, as the actual distribution of ice on structural shapes under an arbitrary combination of wind direction and intensity with atmospheric conditions and rainfall intensity on even the simplest of shapes becomes practically unknowable. This approach is fine for getting something close to a worst-case scenario for estimating the total weight of ice deposited, but cannot help predict the actual non-uniform distribution. See “The application of a uniform radial ice thickness to structural sections” (2006 Jones & Peabody) for more info on this topic.
More research is clearly needed in this area, but for now, we do have code-mandated equations to work with and are legally bound to use them.
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