Lecture 3, page 2

Diaphragms and Shear Walls
Lecture 3; Page 2

Arch 721, Structural Design for Dynamic Loads, University of Virginia
Copyright © 1996-2006 Kirk Martini. 18-Aug-2006 16:30

Recall:


Vertical systems cantilever from the ground
	Braced frames are like trusses, walls act like deep beams.

Walls acting to resist lateral loads are called shear walls
	Buildings that carry gravity loads using bearing walls typically also use the walls as shear walls. The walls must be design to serve both duties.

Reactions: lateral loads only	+	Reactions: gravity only
	=	Reactions: lateral + gravity


The lateral loads induce two types of motion: tipping and sliding.
	Tipping is rotation and sliding is translation.

Vertical reactions counteract tipping
	The reactions form a moment, resisting the rotation, note the downward tension reaction.

The gravity loads also counteract tipping
	The wall can be viewed as a pre-stressed beam. The pre-stressing effect of gravity is generally beneficial since it is usually costly to make foundations that can resist tension uplift. The combined compression of gravity plus overturning can be very high.

The lateral forces create an overturning moment, while the gravity loads create a resisting moment
	To avoid uplift forces on the foundation, the resisting moment must be larger than the overturning moment. The resisting moment typically accounts only for dead loads.

Horizontal reactions counteract sliding
	The sum of the reactions for the wall are called the wall's base shear

Look at vertical normal stresses.

Vertical stress: lateral loads only	+	Vertical stress: gravity only
	=	Vertical stress: lateral + gravity


Gravity increases compression stresses and reduce tension stresses
	For gravity and lateral acting together, the distribution of stresses is asymmetric, with most of the wall acting in compression. The compressive effect of gravity increases the compression and decreases the tension.

Look at shear stresses:

Shear stress: lateral loads only


The stresses increase moving down the wall
	Each story resists the lateral load on all the stories above.


It is good that shear walls also be bearing walls
	The gravity loads help resist overturning and reduce tension. This often requires careful arrangement of the architectural plan and the gravity framing.

The walls must be designed for combined compression due to gravity and overturning.
	This is particularly important for concrete and masonry construction, where high compression can cause sudden failure.

Shear and overturning effects are greatest at the base of the wall.
	The bottom story of the building and the connection to the foundation are critical areas.