AECT360 Lecture 38

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Lecture 38 - Page 1 of 8

Lecture 38 Lateral Force Resisting Systems Braced Frames

Building and other structures subjected to lateral loads must have some methodof being stabilized or collapse will occur. This is particularly obvious for verytall structures where the lateral forces are the most important design

consideration. There are many methods available for stabilizing structures, somewill be discussed below.

1. Moment-Resisting Connections:

This method involves constructing very rigid beam-to-column connectionsthat permit moment transfer across the joint. Monolithically-pouredreinforced concrete structures inherently have moment-resisting joints, butsteel and timber frames do not. A typical moment-resisting beam-to-column steel-framed connection involves transferring horizontal loadsthrough the beam flanges directly to the column flanges by using angles

and column web stiffener plates as shown below. The analysis of theconnection is fairly complex. It is very labor-intensive and expensive toconstruct and is not as good as other methods of stabilization.

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An elevation view of a frame analysis using moment-resisting connectionsmight look like the following:

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2. Braced Frame:

In general, a braced frame consists of diagonal members used to resistlateral loads. A braced frame may come in many varieties. Someexamples of braced frames are as follows:

X BraceLeast available space,greatest bending infloor beams

K BraceOpenings possible,least bending in floorbeams

Full Story Knee BraceLarger openings, morebending in floor beams

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ExampleGIVEN: The one-story steel-framed building below is subject to windloading as shown below. Diagonal steel rods (shown inRED) are used toresist the lateral loads from the North-South winds and act in TENSIONONLY.

REQUIRED:1) Determine the tensile load on the tension rod, Fdiag2) Determine the smallest diameter rod that can be used if the steel is

A36 (Fy = 36 KSI).

Step 1 Determine TOTAL wind pressure in North-South direction:

Total wind pressure = Trib. Area x Pressure= (70 x 13) x (29 PSF + 21 PSF)= 45,500 lbs.

13-0

Leeward wallpressure = 21PSF

Windwardwall pressure= 29 PSF

70-020-0

North

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Step 2 Determine wind pressure P acting at elbow:

P = Pressure on shaded area acting on elbow= (Total wind pressure)= (45,500 lbs.)= 11,375 lbs.= 11.4 kips

Step 3 Determine axial tensile load on diagonal brace, Fdiag:

20-0

13-0

11.4 kips

P

Elbow

Figure 1

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The force in the diagonal member, Fdiag, can be determined by similartriangles:

horz

horz

diag

diag

LF

LF =

ft

Kips

ft

Fdiag

20

4.11

9.23=

Fdiag= 13.6 kips

20-0

13-0

11.4 kips

Figure 2

20-0

13-0

11.4 kips

Figure 3

This member is incompression and isassumed to beinactive

Ldiag = 23.9 ft.

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Step 4 Determine minimum diameter of tension rod (considering yield ongross area only):

Recalling from steel design, the allowable tensile load on grossarea, Pallow:

Pallow = 0.60(Fy)(Ag)

Rearranging to solve for gross area Ag and substituting Pallow = 13.6kips:

Ag =)(60.0

6.13

yF

Kips

Ag =)36(60.0

6.13

KSI

Kips

Ag = 0.63 in2

Solving for diameter:

Acircle =2)(

4Dia

Diameter =

4

gA

Diameter =

4

63.0 2

in

Diameter = 0.896 inch

Use 1 diameter A36 steel rod 1 > 0.896

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Multi-Story Brace Analysis:

The frame of a multi-story building using diagonal lateral bracing isanalyzed as a vertically-oriented truss as shown below. Forces inthe vertical, horizontal and diagonal members are addedto forces

in those members obtained from a gravity load analysis for thedesign of the member.

Lateral load analysis Gravity load analysis

AECT360 Lecture 38

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