What’s under your feet and over your head? Understanding vertical loads.

Summer is officially here and the weather has finally turned hospitable. You decide to take advantage of the nice weather and throw a small gathering on your back deck. You invite a few friends who in turn invite a few friends. Before you know it, there is a roaring party going on with people dancing on the deck, waiting on the burgers to finish cooking on the grill! Good times, right?

Well, if you’re a regular reader of this blog, you know this is likely not going to end well. If you guessed a grill associated fire, you are off this time. If you guessed deck collapse, then you are already in tune with the theme of this article: Loads!

People tend to take the floor (or roof for that matter) for granted. However, floors are designed and built for certain uses and the floor’s capacity is built around that intended use. Now you may intuitively understand what a load capacity is, simply from the words themselves or their context in the above sentence. But let us be specific. Load capacity is the amount of weight per square foot that can be placed on a given building level or platform without causing failure or collapse.

There are many types of loads, but this article is only going to cover enough to get the reader thinking and appreciating their significance. To keep it simple, think of vertical loads in two broad categories: Dead or Alive! Although it is helpful to remember them that way, let us again be accurate and call them dead or live; the latter of which can be either static or dynamic

Dead things do not have a reputation for moving around much and the same applies for dead loads. Dead loads are primarily made up of the weight of the structure itself. So, in our deck example, the deck floor would be a dead load as would the supporting structural members underneath the deck. For that matter, anything permanently installed such as an integrated grill or tile cemented on the deck floor would also fall into this category.

Live loads differ from dead ones in that they are usually temporary or moving loads. There are various ways a load can become dynamic and thus live. In addition to placing a temporary load onto the structure, the structure can also be subject to impacts, vibrations, the sloshing of fluids, etc. So, in our deck example, a couch put out for people to sit on during the party is a live load (it is not always there). The people sitting on it are live (we hope). People jumping up and down to music, are creating an impact load effect. A permanently installed hot tub would be a dead load, but the people using it and the water sloshing inside it would be considered a dynamic or live load. You get the idea.

In the winter time, the deck may be exposed to a snow load. Snow loads are a particular concern for roofs and have their own calculations which are affected by region, elevation, slope of the roof as well as what material the snow is resting on. Snow can also be blown around by the wind into uneven drifts, which can produce critical loading areas.

A generator put on the deck to provide power during a winter outage would produce vibration fatigue. Vibration creates a series of cyclic loading, which directly affects the durability of a structure.

Obviously, a schoolhouse will have different structural needs than a grain silo. So when a structure is designed, all the above concerns are considered. The environment, location, occupancy and intended use are factors that go into load calculations. Fortunately, there are building codes that have been developed to help anticipate the loads a structure is likely to experience in its lifetime and provide a minimum of compliance to reasonably accommodate them. Proper designing and managing of loads will keep a solid floor under your feet and a sturdy roof over your head.

However, excess loading can happen and it can be sneaky. Sometimes excess loading arises and causes noticeable deformations in a structure that cost money to repair but at least provide a warning sign. The real danger comes when excess loading is invisible until a critical failure brings it into sharp focus. When you take a ride in an elevator, you are often staring at a sign that expressly spells out how many people that particular elevator has a capacity for. This sign forces the user at that moment to be aware of loading. How many people are you inviting to your deck party?

The structural engineers at CED can read the signs of overloading and will know which building code is appropriate to compare the structure to. Please contact CED for more information or if you think you may have a claim or case involving loading.

Featured Engineer: Michael P. Tracey, M.S.C.E., P.E.  Structural/Civil Engineer

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