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What is Load?
For our purposes, a load is the force, or combination of forces, that act upon a structural system or individual member. Structural components in a building are analyzed and designed to function properly and to safely resist any loads that may be expected to affect the structure throughout its lifetime.
For wood structures, all loads are generally categorized into load application types and load classifications. It’s important to know both the type and the classification of a load in order to understand and design for the effect it will have on the structure.
A load’s application type indicates how the load is distributed over or into the member.
Commonly, more than one of these load application types affects a single member.
Design Load Classifications:
Design loads are the loads, specified by the Engineer of Record or prescribed by the building code, that a structural member must be designed to support. A load’s classification is determined not only by the source of the load (e.g.: wind, snow, etc.) but, more essentially, by the amount of time the load is expected to act on a member. This amount of time is referred to as the load’s duration.
Duration of Load:
Different load classifications affect members differently. Structural components are assigned design values, which indicate their load bearing capacity. These values are based on the component’s size and shape as well as the material (wood, steel, concrete, etc.) it is made from.
Wood is a unique material because its load bearing capacity varies depending on the amount of time the load affects the structure. Tests have shown that wood has properties that allow it to carry substantially greater maximum loads for a short duration than it can over a longer duration.
For this reason, design values of wood can be adjusted according the length of time the loads are likely to affect them. The ‘Duration of Load (Time)’graph is based on the amount of time over the life of the structure that a full design load will be applied. The load duration factor is the allowable increase for design values based on the duration of load.
Notice in the load duration factor table that each Load Duration time corresponds to a typical design load classification, which indicates the source of the load. Typical design load classifications include the following:
Dead Load: The dead load is a vertical force applied to a framing member due to the weight of all permanent structural and non-structural components.<br><br>The dead load is typically expressed in pounds per square foot (PSF) on construction documents and may include items such as joists, roofing materials, lights, fixed mechanical units, etc. <br><br>In short the dead load will include the weights that will remain in place at all times during the structure’s life.<br><br>There are several readily-available published documents that contain weights of common construction materials. Among them are the AITC Timber Construction Manual and many of the iLevel® Trus Joist® Specifier’s Guides.
Live Loads: Live load is the superimposed weight on the structural components by the use and occupancy of the building. In other words, live load is a moving or variable load such as people, furniture, objects placed on the joists temporarily during construction etc.<br><br>Generally, live loads can be broken down into floor live loads and roof live loads; each is a different load classification with a different load duration factor.
Floor: Floor live load is assumed to affect the structure for a cumulative period of ten years over the life of the structure, having a load duration factor of 1.00. Floor live loads can be either uniform of concentrated.
Roof: Roofs must also be designed for construction live loads. Such loads on a roof, however, are expected to affect the structure for a cumulative period of only seven days.<br><br>The load duration factor, therefore, is greater than that for floor live loads. The load duration factor for roofs is 1.25.<br><br>These loads typically range from 12 PSF to 20 PSF depending on the roof slope and tributary area or specified load. For more detail about roof live load requirements, see Section 1607.11.2 of the 2003 IBC, and in TABLE R301.6 of the 2006 IRC.
Snow: If a roof is in an area where it can expect to carry snow, this roof is likely to support temporary loads for about two months over the life of the structure, allowing a load duration factor of 1.15.<BR><BR>Under these conditions, the structure is designed with a design snow load instead of a roof live load. Since the magnitude of the design snow load can vary largely over a small geographical area, the International Building Code refers the designer to the local building official to determine the design snow loads.<BR><BR>Similar to live load or dead load, design snow load (or base snow load) is applied uniformly to the entire roof. But as you know, snow can also form drifts on a roof that are not uniform, or accumulate unevenly at roof overhangs. In addition to base snow load, a designer often must also consider snowdrift loads and special eaves requirements.
Wind Load, Wind Uplift, Concentrated Brace: Wind can apply load to a roof or deck member in either an upward or downward direction. As with snowdrift, the Engineer of Record calculates wind loads. As vertical loads on a member, wind loads are generally applied in two ways: as uniform wind uplift and as concentrated brace loads.
Every load applied to a wood structure is analyzed according to its load application type (how it is distributed) and its load classification (its duration). We have seen how design load classifications differ from one another:
- Dead load considers weight of permanent materials, such as roofing material, floor decking,etc.
- Floor Live load considers the weight of non-permanent items on a floor, such as maintenance workers, occupants, furniture, etc.
- Partition load is a special floor live load that considers the weight of partition walls that may be moved during the life of the structure. It is applied to commercial office buildings.
- Roof Live load considers the weight of non-permanent items on a roof in areas without snowfall.
- Snow load considers the weight of snow accumulated on a roof or deck. Design snow loads are uniform, but snow can drift to form tapered loads, or accumulate unevenly at eaves, requiring special design considerations for roof members.
- Wind load can be a uniform upward force on an entire roof or a concentrated force in either an upward or downward direction at areas where the roof is braced.
- Seismic load is an axial load resulting from an earthquake.
But we have not seen how these loads are applied, or how they are analyzed differently from one another. In the next section we will examine how loads are transferred into and though the structural components of a building, and how different load types and classifications are considered in member design.
How Are Loads Applied?
Transfer of Loads in a Structural System:
Loads, as we discussed in the previously, are applied to structural members in various ways. We will now examine how the loads are transferred through the structural members and into the structure’s foundation. All the loads applied to a structure, regardless of type or classification, are ultimately transferred through the structural components of the system to the foundation of the building.
Look at the colored areas on top of the joist in the pictures (below), Load on the Floor, Transfer Load to the Joist, and Transfer Load into Wall.