Load Paths – Tributary Area Concepts

Load Paths

An extremely important structural concept in the design and construction of a building, yet one that is often underappreciated, is the confirmation or creation of a proper load path.  All applied loads including Live Loads (LL) such as wind, occupancy, snow, etc. and Dead Loads (DL) such as building material self-weights must have a clearly defined path to the foundation which in turn distributes the loads to the earth. 

A load path can be as simple as a single member such as a flag pole which carries its own weight directly down the pole in the vertical direction to the ground.  At the same time, lateral wind loads are resisted and channeled to the base primarily through bending of the pole. 

“All buildings need a proper load path to get the forces to the ground. 

If you don’t provide it, Mother Nature and gravity will do it for you.”

Professor M. Kev

Most buildings are far more complicated than a flag pole but the concept remains the same.  Lateral loads must get to the ground through the use of moment frames, shear walls, braced frames or a combination of components.  At the same time,  occupancy loads (Live Loads) and building material weights (Dead Loads) are collected by horizontal members such as floor decks, beams and trusses ultimately traveling vertically to the earth through components such as bearing walls and columns.  Refer to the Building Failures Forum post Loads and Codes for additional discussion on Dead Loads (DL) and Live Loads (LL).

Students demonstrate concepts of Load Path and Tributary Width / Area by painting directional arrows on an actual structure in one of the Engineering Units located at Penn State.

This all may sound like common sense and in many ways it is.  You don’t have to be a structural engineer to trace the load paths through most buildings.  You will, however, likely need to consult a structural engineer to make sure the various elements of the load path are continuous and to ensure the proper design of the connections between structural members.

 Tributary Area Examples

An excellent example of tracing load paths combined with an illustration of tributary width / area concepts is containted in a power point presentation titled “Load Paths and Tributary Area Examples from A Beginner’s Guide to Structural Mechanics/Analysis by T Bartlett Quimby.  Mr. Quimby uses actual photos from the Alaska State Fairgrounds Farm Exhibits Building which was constructed with an exposed structure in most areas.

 This post been prepared in conjunction with AE 210, Introduction to Architectural Structures as a summary review to be used in conjunction with the student’s lecture and practicum notes as well as related information contained on the Angel course management website. 

 

 

 

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