INTRODUCTION TO STRUCTURAL SYSTEMS,

CONSTRUCTION METHODS AND MATERIALS

MODULE #2

Prepared by

Marcia C. Belcher, P.E.

Common type of construction for low rise structures (1-2 story).

Combination of: Bearing wall: vertical structural elements

which carry loads directly to foundations

Shear wall: panel that counters the effects of lateral load acting on a structure (wind, seismic, fluid pressure, etc.)

Bearing/Shear Wall Systems

Load Path of a Bearing Wall

Load applied to lateral wall Transmitted to the diaphragm

Load Path of Shear Walls

Transmitted from diaphragm to shear wall Reactions go to top of shear wall

Lateral walls are designed to resist bending forces

Shear walls are designed to resist shear and overturning

Load is transferred from shear wall to foundation

Sliding and overturning must be considered

How Shear Walls Relay Load to Foundations

Anchorage of Shear Walls

Example: Masonry Bearing/Shear Wall Construction

Masonry walls form the “box” (bearing/shear walls)

Metal bar joist/metal deck provide the diaphragm action in

the system

Vertical steel provides vertical flexural resistance.

◦ Cells containing reinforcement must be grouted solid to make steel effective.

Horizontal bars are strategically placed to form “bond beams.”

These are integral horizontal “beams” that are placed to increase the horizontal flexural strength of the wall.

Reinforcement of Masonry Bearing/Shear Walls

Typical construction types: Wood

Masonry

Reinforced Concrete

Light Gage Metal Studs (Cold Formed Steel)

Bearing/Shear Wall Systems

Braced frames use trussing (triangulation) to resist sideways forces on buildings.

Trussing is formed by inserting diagonal members into rectangular areas of a frame.

This stabilizes the frame against sideway forces from earthquakes and wind.

Concept: Braced Frames

The frame shown is laterally unstable in the present configuration.

Example of 2D Frame

Different bracing configurations to resist the load

Diagonal X-brace Knee brace

Example of 2-D Frame

2-D Frame: Other Bracing Configurations

Common roofing systems used in both bearing/shear wall and steel-frame type construction.

Can you identify lateral force resisting frames in this photograph?

Example of lateral bracing rather than rigid connections.

Metal Bar Joist: (Open Web Steel Joist)

.

Building systems can vary:◦ Moment resisting frames◦ Bearing/shear wall system

Typical methods:◦ Precast/Prestressed Concrete◦ Post-tensioned◦ Cast-in-place reinforced concrete

Reinforced Concrete Structures

Precast concrete is cast in a form which is then cured in a controlled environment, transported to the construction site and lifted into place.

Placement and handling methods are provided by the manufacturer.◦ Storage, blocking, lifting, etc.

Precast/Prestressed Concrete

Concrete element in which compressive forces are applied prior to loading.

◦ This counteracts the effect of subsequent loads

◦ Limits cracking and deflection

◦ Can achieve longer spans than practical for ordinary reinforced concrete

◦ Can be either pre-tensioned or post-tensioned

What is Prestressed Concrete?

Pre-tensioning Concept

Wire tendons are placed in formwork and put into tension (jacking force)

Concrete is placed and cured in forms

Jacking force is released & force is transferred to the concrete through bond

Element is put into compression

Element is pre-loaded

Post-Tensioning Concept

Wire tendons in “sheaths” are place in formwork and concrete is poured/cured

Tendons are tensioned & end anchorage “locks” tendon

Element is put into compression by end anchorage

Element is loaded

Pre-tensioning is done at a precast manufacturing facility.

Precast concrete manufacturer must be certified by PCI's Plant Certification Program.

Post-tensioning is performed on the jobsite in cast-in-place applications.

Applications

Concrete that is supplemented with steel bars to resist: Tension

Compression

Shear

Cast-in-place Reinforced Concrete

Drawings & Specifications will provide:

Steel requirements:

Specified grade of reinforcing steel

Detailed drawings of size & location of bars

Lap splice location & length information

Type/location of welded splices & mechanical connections (restrictions similar to masonry)

Cast In Place Reinforced Concrete

Drawings & Specifications will provide:

Concrete requirements (per ACI):

Slump

Entrained air

Unit weight

Minimum compressive strength

Permitted admixtures

Cast In Place Reinforced Concrete