Load tracing - University of Massachusetts Amherst

Load tracing

Loads travel along load paths Load path analysis is known as load tracing

How a Light-Frame Building Resists Loads (Building Construction by Mehta et al., 2008)

Ground Reaction

Why do load tracing?

Load tracing determines how much load is apportioned to individual members When we know how much load each member carries, we can analyze the member by using:

______ ______ diagrams and equations of _____________

Tributary dimensions tributary width: the distance between midway of adjacent similar members

tributary area: the area that is assumed to load any given member tributary area = tributary width x span

Tributary width of joist A = _____

Tributary width of joist C = _____

Tributary area of joist D =______

Tributary dimensions of a beam or column

Tributary width for Beam A = ______________

Tributary area for Column C = ______________

Column C

Load paths

Load (plf)

Reactions (lb)

Loads (lb)

Ground

reactions (lb)

joist

column

Area loads from roof and floor become line loads on trusses and joists Truss and joist reactions become point loads on studs below

Units!

• Area loads:

• Line loads:

• Point loads:

Calculating line loads from area loads

Line load (plf) = area load (psf) x trib. width (ft)

**Find FBD of G1 given

area load = 100psf

Calculating point loads (reactions) from line loads (statics review)

Example: simply supported beam with uniform load

10 ft

Beam

Column

below

1000 plf

Column

below

Calculating reactions from line loads

For a general case, solve for reactions using equations of equilibrium. (statics review)

15 ft

Beam

Column

below

1000 plf

3 ft

Column

below

Classwork to hand-in – load tracing practice BCT 540 Design of Wood Structures

1. Given the joists are 2ft o/c, area load is 30psf and the RTU weighs 400lb, find the

line and point loads on joist A.

2. Draw the Free Body Diagram.

3. Find the reactions on the girders.

Girder

Girder

Double joist A

Roof top unit (RTU) at

midspan on sleepers 20’

Homework – load tracing practice BCT 540 Design of Wood Structures

Given the area load is 50psf, find the line loads and point load on joist A above.

That is, put the magnitudes on the free body diagram shown.

Joist A

Open

to

below

2’ o/c

Similar framing in 3D