CIEG 301:

Structural Analysis

Loads, continued

Load Transfer and

Load Distribution

Considered a typical building framing plan

Work from top down

Determine tributary widths and tributary areas as

appropriate

Applicable for dead load and live load

Similar approach used for all load types

Work from point of load application to point where load is

transferred out of the structure

Load Transfer and

Load Distribution (cont’d) When members supporting the slab are oriented in one direction:

One-way slab

When members supporting the slab are oriented in both directions:

Use L2/L1 (long dimension / short dimension) to determine if “one-

way” slab or “two-way” slab

L2/L1 > 2 one-way slab, otherwise two-way slab

If two-way slab, draw lines at 45 degrees to determine tributary area

10’

30’ 20’

14’

Live Loads

“Moving loads”

Loads vary in magnitude and location

Examples In buildings:

People

Furnishings

Materials

Cranes

Automobiles

In bridges: Vehicular traffic

Live Loads, Continued

In buildings, typically applied to the

structure as a uniformly distributed load

Load is applied to all or part of the

structure to maximize load effects

Typical magnitudes of live load:

Table 1-4

Live Load Reduction

For large floor areas, there is less probability that the entire floor will be loaded

Live load reduction factors are used to reduce the applied load

In English units this factor is:

> 0.5 for members supporting one floor

> 0.4 for members supporting more than one floor

KLL = member type coefficient (for interior columns, KLL = 4)

AT = tributary area

L = LoLRF

Lo = original live load, > 100 lb/ft or reduction NA

Reduction NA for public assembly spaces, garages, and roofs

TLLRF

AK

1525.0L

Snow Load

Historical data is used to determine maximum snow depths over 50-year recurrence interval for a specific location

This gives the ground snow load, pg

pg is modified to give the roof snow load

For “flat” roofs (< 5% slope),

p = 0.7CeCtIpg

Ce = exposure factor

(0.8 for unobstructed; =1.3 for sheltered urban)

Ct = thermal factor

(1.0 for normal heat; 1.2 for unheated)

I = importance

(0.8 for agriculture / storage; 1.2 for hospitals)

Wind Load

Kinetic energy generated by wind:

KE = 0.5rV2

V = velocity (wind speed)

r = air density

Kinetic energy becomes potential energy

(pressure) when a structure blocks the air

flow

Amer. Society of Civil Engrs.

(ASCE) 7-02 Wind Map

Wind Pressure (qz)

0.5rV2 is converted into wind pressure (qz)

qz = 0.00256KzKztKdV2I [lb/ft2]

Kz = exposure coefficient (depends on structure

height and ground terrain)

Kzt = exposure topography coefficient

Kd = direction coefficient (equal to 1.0 when wind is

the only load considered)

V2 = velocity in mph

I = importance of the structure

Wind: Design Loads

The design pressure for wind loading is the difference between the external and internal pressure

p = qGCp – qh(GCpi) q = qz for the windward wall at height z

qh = qz for the leeward wall, side wall, and roof, where z = h = the mean height of the roof

G = gust effect factor = 0.85 for rigid structures

Cp = pressure coefficient

GCpi = internal pressure coefficient (+ 0.18 for fully enclosed buildings, + 0.55 for partially enclosed, 0 for open)

Handout: Selected sections of Chapter 6 (from ASCE 7-02)

Example….

Homework

Due Sept. 7th / Next Thurs.

3 Problems:

Determine wind loads acting on roof and

leeward wall of the in-class example

From chapter 1 of textbook:

1-2 (dead load)

1-10 (live load)