It seems every time a big wind storm comes through, or the snow gets deep, or rains make soils heavy, a few buildings collapse. When big events like strong hurricanes, earthquakes, or tornados strike, whole towns can be leveled.
Who’s job is it to prevent this, and how do they know how hard the wind will blow or the ground will shake?
Structural Engineers strive to design economical buildings that are safe for their inhabitants, and they rely heavily on the ASCE 7 “Minimum Loads for Buildings and Other Structures” to help inform them of what loads and magnitudes they need to design against.
Load Combinations
Many different types of loads are outlined below, but it should be noted that not all of these loads are combined in design.
Engineers recognize that the probability of having the hottest day ever (most thermal expansion), with the most overloaded occupancy loads, biggest snow load, thickest ice accretion, biggest wind storm, plugged drains during the biggest rain storm, along with a huge earthquake all at once are practically zero.
Instead, both the International Code Council’s “International Building Code” (the model code all US states draw on) and the American Society of Civil Engineers’ “Minimum Loads for Buildings and Other Structures” (which the IBC references extensively) contain load combinations.
Load combinations are probability-based combinations of various load types, each with its own “load factor”, scaling its influence on the component in question.
One example load combination from the LRFD load combinations in ASCE 7-10 is 1.2*D + 1.6*L + 0.5*S, or 120% of the expected Dead Load plus 160% of the expected Live Load plus 50% of the expected Snow Load.
Load Types & Intensities
Loads can be broken into two overarching categories: vertical, or gravity loads; and lateral loads.
Gravity loads, like the weight of building materials, occupants, snow, ice, and rain on a structure, are those that primarily act downwards, towards the earth. Lateral loads “primarily” act horizontally.
I throw the air quotes on primarily, as things like wind and seismic actually have components that act in both vertical and lateral directions, but effectively we’re splitting these into loads that act only vertically, and those that act in the horizontal direction as well.
Risk Categories and their Impacts
No one wants their building to collapse, but constructing buildings to withstand overly-harsh loading conditions can make them so expensive that they never get built in the first place.
“Any idiot can build a bridge that stands, but it takes an engineer to build a bridge that barely stands.”
– Unknown
Not every building needs to be perfectly operable after the largest earthquake, but critical functions do need to be maintained. In order to help ensure that small storage buildings are not held to the same standards as a nuclear power plant or Level I trauma center, engineers and architects work together to define Risk Categories for new structures.
Risk Categories vary from I up to IV, with more important buildings getting higher category numbers. These categories then drive the magnitude of the loads used in design, whether directly multiplying the load’s intensity through an “Importance Factor“, or by changing the underlying drivers.
Wind load, for example, is calculated from wind speed maps provided in the ASCE code, or viewable for any given location on the ASCE 7 Hazard Tool. The wind speed used for a particular building depends on the historic wind speed data for the building site, as well as the risk category of the building.
- Risk Category I buildings use the mapped 3-second wind gust with a “mean recurrence interval” (MRI) of 300 years, corresponding to a 15% probability of exceedance in an assumed 50-year design life of a given building.
- Risk Category II buildings use a 700-year MRI, for a 7% probability of exceedance in 50 years
- Risk Category III buildings use a 1700-year MRI, for a 3% probability of exceedance in 50 years
- Risk Category IV buildings use a 3000-year MRI, for a 1.6% probability of exceedance in 50 years
For more information on what drives Risk Category selection, check out my article on Risk Categories (coming soon!).
Gravity Loads
All loads that act exclusively in the gravity direction. Interestingly, for the sake of safety, many design standards require that even structures that have no lateral loading (those in low-seismic regions and built within another structure, say a stage apparatus in a football stadium or similar) still end up being subjected to a so-called “notional load”, a minimum required percentage of the other loadings on the structure, applied horizontally throughout the structure.
Dead Load
The dead load, so-called because it doesn’t move, consists of everything permanently in the structure. This is the weight of the building materials (beams, columns, slabs, flooring, walls, insulation, plumbing, etc), permanent equipment, and anything else heavy that’s meant to be there all the time. Most engineers will call it “anything bolted down”. Things like permanent equipment and heavy water tanks can sometimes be forgotten in this category, and cause problems later on.
Some examples of dead load sources include:
- Weight of building materials (there’s an extensive list of these weights for standard materials in the Commentary of ASCE 7, Chapter C3)
- Weight of permanent equipment, like HVAC, water heaters, solar panels, etc.
- Weight of soil and other materials in planters and green roofs
- Weight of anything else permanently there, “anything bolted down”
Live Load
Live load is the name for transient, occupancy loads. People, desks, bookshelves, books, even cubicle dividers and the like. Chapter 4 of ASCE 7 provides a large table of both uniform and point load intensities for live loads in all manner of buildings and occupancies, from cell blocks in penal institutions (40 psf) to heavy manufacturing floors (250 psf).
Since live loads are transient, they’re listed at their maximum loads, but actually hitting this maximum loading across a large space is highly unlikely. When individual structural elements, like beams or columns, support large areas of floor space, or large “tributary areas”, engineers are permitted to reduce the intensity of some, but not all types of live loads.
Also of note, for some elements having the entire floor loaded is NOT the worst case. Engineers must consider “patterned loading” wherever having some parts loaded and others unloaded makes for the most extreme loading case on a particular element. Experience guides them in narrowing down the number of such patterns they must consider, and computer tools help speed up the analysis for those various patterns that are considered.
Some examples of live load sources include:
- Weight of people in occupiable space
- Weight of movable furniture and equipment
- Weight of any vehicles that might be allowed in an area, such as in parking garages
- Loads on handrails and grab bars
- Loads on tie points for maintenance and safety rigging
- Vehicle barriers
- Helipad loads
- Crane loads for any built-in cranes in a facility
Snow Load
As anyone who’s ever shoveled a driveway after a particularly wet and heavy snowfall can tell you, snow is heavy.
With wet and heavy snow, it tends to just stick where it falls, but lighter snow also is susceptible to drifting around. Snow drifts on flat surfaces, causing all manner of scalloping (like waves on a field), but when you introduce roof shapes that slope, angle, and step, the drifting gets even worse. Snow can even drift off of one building to land on another, lower roof nearby.
ASCE 7 Chapter 7 is all about snow, with maps of expected ground snow depths under a “design-level event”, and equations to help modify those expected loads for roofs. These equations feature factors for how warm the roof is, how slippery the material is, and how exposed the roof is to the wind.
Drift equations also feature heavily, with drifting snow adding to the total load that the roof system must be designed for. Snow can also slide off of a higher roof onto a lower one, adding load to the lower one. This condition should be avoided though, as ice chunks from the higher roof can (and often do!) damage the roof below.
In general, any building more than a couple of stories tall should not use a sloped roof, as sliding and falling snow and ice can be very dangerous to people below.
Some examples of snow load sources include:
- so-called “Balanced Snow Load”, the load expected on a roof without drifting
- Drifting Snow
- Sliding Snow
Rain Loads
Rain as a gravity load may seem a bit odd, as rain most often causes structural failures by softening the ground, leading to mudslides and basement cave-ins, but the weight of water on a flat roof is considerable, especially if the primary drainage system plugs up.
Buildings with flat roofs are required to have at least two drainage systems, a primary and some sort of secondary. Buildings that don’t have much of a lip on the edge of the roof (those without parapets) will sometimes opt to have the “secondary” drainage system just be water overtopping whatever wall stubs they have, but most buildings feature some sort of scupper system in their parapets.
Engineers must assume that the primary drainage system is clogged, and compute the water depth the roof can accumulate based on mapped rainfall intensities and the flow rate of the secondary drainage system.
Leaf litter and other foreign materials can serve to block up a drain, but regular cleaning alone isn’t enough. Animals can nest on a drain or rip off a downspout extension and flood a basement (my dog actually did this…), or ice from melting and refreezing snow can block the drains in springtime, just in time for a massive rain storm.
Ice Loads
Speaking of ice, it’s also heavy! Remember that for weather-related loads, building codes take a long view. Ordinary, Risk Category II buildings are designed for 700-year MRI wind gusts, and the ice loads are based on a similar scale.
For buildings near the coast, subject to splashing waves and stormy seas, extra research is needed on the part of engineers, and publications from organizations like the US Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) can help.
For the rest of the country, maps of freezing rain thicknesses are included in Chapter 10 of ASCE 7, along with equations to help adjust the ice thicknesses based on building risk category and height, although these equations and thicknesses are a bit lacking at the moment.
The code ice information is based almost entirely on research on wires and pipe structures, so it starts to get a bit ridiculous when applied to structural shapes like I-beams and slender rectangular bars.
Nevertheless, the code is the law, so at present, engineers are forced to design to these very large ice loads.
For more on how to actually calculate ice loads, check out my article “How to Calculated Ice Loads per ASCE 7-16”.
Lateral Loads
Lateral loads are all those which have at least some major component oriented differently from gravity. A few examples are given below, but there are of course more.
Lateral Earth Pressure
Steep dirt slopes like to collapse, and people like to build basements, trenches, ditches, and roadways with steep walls.
Engineers back to the time of Leonardo da Vinci, and likely well before, have warred against “lateral earth pressure”, the lateral thrust component that acts on retaining walls and other structures in contact with the earth.
Lateral earth pressure depends mostly on the type of soil present, as well as the depth of the water table, and it increases the deeper you dig. For basic projects in known soil types, ASCE 7 Chapter 3 has a table that can help engineers look up design values based on the Unified Soil Classification designation of their site’s soil.
Lateral Earth Pressure becomes a tag-team effort between the structural and geotechnical engineers on any large, complex project, where geotechnical teams often conduct extensive soil boring and characterization studies.
Flood & Tsunami Loads
Keeping water out of dry spaces is always tricky, but structurally it’s about more than just weather seals.
Recently the American Society of Civil Engineers has been focusing on introducing more information for designers working to make flood-resistant and tsunami / storm-surge-resistant buildings.
For floods, it’s just a simple static water pressure calculation, as well as checking that the building doesn’t become buoyant and lift out of the ground or off its foundations. With surges and tsunamis, there’s an impact component as well.
ASCE 7 Chapter 5 details Flood Load provisions, with a lot of focus on loads induced by wave action of the small waves on top of the flooded areas.
Chapter 6 – Tsunami Loads and Effects details extensive research into the effects of tsunamis on buildings. It includes various modeling and analysis methods, and many tables and maps of inputs for those models.
Wind Load
Wind loads on buildings have been one of the more rapidly-expanding areas of the ASCE 7 documentation. In ASCE 7-05, released in 2005, a single chapter (a scant 60 pages!) was devoted to all wind loads, and local building codes were no better.
As recently as 2020, Chicago (the Windy City) had a simple table that ignored everything but building height, and just read out one number for components and cladding wind loads.
Now, ASCE devotes six full chapters to wind loads, over 140 pages.
The wind load provisions are all based on the “stagnation pressure”, a concept deriving from Bernoulli’s equation of total head. This simple equation depends only on the density of the air and the speed of the wind gust.
To capture the variety of different factors influencing wind speed and the relative importance of various components of a building, engineers use a variety of different equations and adjust factors to come up with two sets of wind loads: one for the “Main Wind Force Resisting System” (the bones and muscles of the building) and one for the “Components and Cladding” (the skin of the building).
Some of the factors influencing Design Wind Loads include:
- Building Location and Risk Category (which drive the mapped wind speed)
- Surrounding Terrain Topography and Surface Roughness
- Site Elevation (check out my full article dedicated to this topic)
- Building Height
- Type of Component being designed
- Location of Component on the building (local discontinuities bump up wind loads)
I actually have a full article dedicated to an overview of the various influences on wind loads, check it out here to learn more!
Seismic Load
The other loading type which is rapidly expanding in pages devoted in the ASCE 7 specification, seismic loads are actually not so much an external load imposed as the inertia of the building and its contents when the ground moves out from under it.
Due to the inertial nature of seismic forces, basic masonry without engineered steel reinforcement fairs extremely poorly in earthquakes, while light and flexible stick-framed wood structures with very ductile nailed connections do extremely well.
Seismic loads act in all directions, but the magnitude of lateral loads is generally much greater than that of the vertical components.
Sadly, many of the United States Interstate Highway System bridges currently in operation were constructed back in the 1960s, when our knowledge of seismology was very lacking. As recently as the 1970s, proofs showing how seismic accelerations could never exceed gravity were widely-accepted, but we now know this not to be the case.
Engineers are given a variety of tools and methods in the thirteen full chapters devoted to seismic engineering and detailing within ASCE 7-16.
These methods range from quick hand calculations to approximate the maximum anticipated loads in a conservative manner, which is perfectly adequate for regions of the country with low seismicity, to advanced computer modeling of the entire building, simulating various amounts of live load being present and finding a much tighter bracket on the actual expected seismic loads, enabling engineers to design light and airy structures in very difficult areas.
Thermal Expansion & Other Self-Straining Loads
As seen in the photo above, thermal expansion can wreak absolute havoc on structures that don’t have appropriate movement joints built in.
Buckled train tracks can easily derail a fast-moving train, causing massive monetary and environmental damage. Several buildings in the early 19th century in Chicago failed to account for enough differential expansion between their granite facades and steel underlying structures, which resulted in large granite slabs tumbling onto the sidewalks below.
Thermal expansion is no joke, and can easily be forgotten about.
The forces induced by restraining a structural member that’s trying to expand are absolutely enormous, and they quickly lead to buckling or other unexpected failures.
Additional self-straining forces can originate from things like:
- Shrinkage of wood or concrete as they dry and season
- Creep of metals or concrete under sustained loading
- Differential Settlement of foundation elements
Summary
Structural Engineers have to try to anticipate every loading condition a building can see over its entire lifetime, and design it to resist each and every possibility.
Tools like the ASCE 7-16 “Minimum Design Loads and Associated Criteria for Buildings and Other Structures” and other design guides help tremendously and set the minimum legal loads to be considered, but often it still does fall to the engineering judgment of the responsible Engineer of Record to try to catch any extraordinary loading anticipated for a particular project.
Make sure you engage a responsible, experienced engineer for help on any building projects, and engineers, make sure to consult an expert whenever you venture beyond your area of expertise.
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