Masonry Wall Bracing

concretecakeUrban and Civil

Nov 29, 2013 (3 years and 6 months ago)

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Masonry Wall Bracing


A Simplified Approach To Bracing

Masonry Walls Under Construction

Produced by the

Masonry Bracing Task Force

Masonry Bracing Task Force

Masonry Wall Bracing

A Simplified Approach to Bracing Masonry
Walls Under Construction

Produced by the Masonry Bracing Task Force

Contributing Members:


Howard Jacobs Masonry


Assoc Gen. Contractors

Masonry Institute of Oregon

SD Deacon Gen. Cont.

Milne Masonry



Baugh Construction

Degenkolb Engineers


Smith Masonry

Carlson Testing

Masonry Bracing Task Force

Introduction

Current OSHA regulations require “All masonry
walls over eight feet in height shall be
adequately braced to prevent overturning and
to prevent collapse unless the wall is
adequately supported so that it will not
overturn or collapse.”

Masonry Bracing Task Force

Defining “Adequately Braced”

OSHA does not indicate what level of bracing is
required to achieve an adequately braced
wall condition.


OSHA, however, places the responsibility of
providing proper support for the wall and
ultimately the safety of workers within the
vicinity of the wall, squarely on the shoulders
of the contractor.

Masonry Bracing Task Force

Goals of This Report

To provide a simplified and practical approach
to wall bracing that achieves the following:



Outline bracing procedures in a document that is easily
understood by workers, contractors and inspectors.


Provide life safety protection for all workers in the vicinity
of masonry walls at the jobsite.


Devise a method of adequately bracing structural
masonry walls over 8 ft. in height while constructing up
to 8 ft. of additional ungrouted wall.

Masonry Bracing Task Force


The Masonry Bracing Task Force obtained a
grant from Oregon OSHA to sponsor a
testing program.


Testing Program Developed


Testing program was developed utilizing common
construction materials and methods.


The testing program established bracing procedures
that can be used in the field by qualified personnel.


Submitted recommendations to OSHA for
statewide adoption.







To Achieve the Stated Goals:

Masonry Bracing Task Force

Wall Bracing Test Program



Purpose



Test Wall Design



Testing Procedures



Summary of Test Results



Conclusion



Field Application

Masonry Bracing Task Force

Purpose

A testing program was developed to establish
the steps required by a contractor to
construct a reinforced and partially grouted, 8
inch cmu wall to 24 ft. in height while
maintaining the stated goals of this report.

The intent of this report is that qualified
personnel may extrapolate upon this
information when designing wall bracing for
different wall heights and sizes.

Masonry Bracing Task Force

Test Wall Design

Test walls were designed and constructed to meet
the following minimum requirements as per 1997
UBC, Working Stress Design, Seismic Zone 3:


8 inch lightweight CMU


F’m 1,500 psi


2,000 psi grout


Type S mortar


60 ksi rebar


2250 lb SWL (safe working load) tension straight
coil loops & coil bolts with concrete tilt braces


High lift grouting

Masonry Bracing Task Force

Test Wall Configuration
-

Plan View


Two braces were spaced 20 ft. apart, located
10 ft. horizontally from each end of a 40 ft.
long test wall. Openings in the reaction wall
allowed connection of braces to test wall when
testing for tension.

Source: Masonry Wall Bracing, Figure A1

Masonry Bracing Task Force

Test Wall Configuration



Metal concrete tilt braces
were attached to the test
wall at approximately 60
degree angle from
horizontal.



Braces were anchored to
a 3400 lb. concrete
deadman.



Inflatable air bags placed
between test & reaction
walls were used to simulate
wind pressure.

Masonry Bracing Task Force

Brace Connection:




The Coil Loop Insert



Braces were attached to
wall via a straight coil loop
insert placed around
ungrouted vertical rebar at
a bond beam location.




Connection takes
advantage of existing
vertical rebar in reinforced
cmu walls.




Masonry Bracing Task Force

Coil Loop Insert



Coil loop insert must be
attached around rebar
located at a bond beam.



Make sure at least one
perpendicular bar is
located between attached
bar and face that receives
brace.



Braces are now attached
to an internal structure
within the wall that resists
the forces of tension &
compression produced by
strong winds.


Masonry Bracing Task Force

The Life Safety Advantage of


Straight Coil Loop Inserts:


Past industry practice of setting brace anchor bolts
in grout required a minimum cure time.


This created a window of exposure for workers in
the vicinity of the wall.


Straight coil loop inserts allow braces to be attached
to the reinforcement of an ungrouted wall. This
braced attachment imparts added strength to the
wall to resist the forces of wind.


Workers are now protected from the overturning and
collapse of walls even before grout is poured.



Masonry Bracing Task Force

Testing & Construction Procedure


A series of three tests were performed to follow
the construction sequencing of a 20 ft. high cmu
wall.


All three test walls were subjected to a minimum
35 mph simulated wind force.


A fourth test was performed on a minimum
grouted 16 ft. high wall without bracing and
where the grout was less than 12

hours old
.

Masonry Bracing Task Force

Testing & Construction Procedure

Throughout the testing
procedure, mortar and
grouted masonry prism
samples were taken and
tested per ASTM C 780 and
ASTM C 1314 respectively.
This information was plotted
on a timeline in order to
follow each test wall
condition.

Masonry Bracing Task Force

Mortar Strength Test Timeline

Mortar samples for each wall height were tested for code
compliance per ASTM C 780

Source: Masonry Wall Bracing, Figure A10

Masonry Bracing Task Force

Grouted Prism Strength Timeline

Grouted prism samples for each wall height were tested for code
compliance per ASTM C 1314

Source: Masonry Wall Bracing, Figure A9

Masonry Bracing Task Force

Test Measurements


For braces in tension, brace force was
measured via load cells.


Brace force was not measured for braces in
compression.


Air bag pressure measured using piezometers.


Test wall deflections measured at six locations
on wall
.


Masonry Bracing Task Force

Deflection Measurement Points

Test wall elevation showing location of measurement points


Source: Masonry Wall Bracing, Figure A3

Masonry Bracing Task Force

Wall Deflection

Graph of wall deflection vs. wind speed developed for each test wall

Wall Deflection vs. Wind Speed
0
5
10
15
20
25
30
35
40
45
50
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Wall Deflection (in)
Wind Speed (mph)
North Upper
Center Upper
South Upper
Top Average
Example: Wall Test
#
3

Source: Masonry Wall Bracing, Figure A13

Masonry Bracing Task Force

Test Series

Test #1



Wall brace is in tension
connected to a 12 ft. high
wall with the last 8 ft left
ungrouted.



Brace is connected at
approximately 8 ft. above
grade.

Masonry Bracing Task Force

Test #1

Masonry Bracing Task Force

Test Series

Test #2



Wall brace is in tension
and connected to a 20 ft.
high wall with last 8 ft. left
ungrouted.



Brace is connected at
approximately16 ft. above
grade.

Masonry Bracing Task Force

Test #2

Masonry Bracing Task Force

Test Series

Test #3



Wall brace is in
compression and
connected to a 20 ft. high
wall with the last 8 ft. left
ungrouted.



Brace is connected at
approximately 16 ft. above
grade.

Masonry Bracing Task Force

Test #3

Masonry Bracing Task Force

Test Series

Test #4



Free standing wall



Test performed on a
12 hour grouted wall
that was 16 ft. tall
with no bracing.

Masonry Bracing Task Force

Summary of Test Results


All three braced test walls successfully withstood a
minimum wind force of 35 mph and remained intact


thereby meeting the stated goal of providing life safety at
the worksite.


All three braced test walls remained life safe to a wind
force of at least 45 mph.


The fourth test on a free standing, minimum grouted wall
also withstood a wind force in excess of 60 mph


demonstrating the strength that is rapidly achieved in a
reinforced, 16 ft. high wall where the grout in the top 4 ft.
is only 12 hours old.


Masonry Bracing Task Force

Summary of Test Results (cont.)


Test walls proved to be fairly elastic.


There was noticeable deflection and cracking as
loading was applied.


However, walls returned to a nearly plumb condition
after loading was removed.


What about the amount & degree of cracking?


Many of the walls observed during testing showed
some amount of joint cracking.


While cracking may have a negative affect on the
overall performance of a masonry wall it is not
considered a life safety issue.


The criteria used to judge a successful test was the
ability of the wall to stay intact and not overturn, thus
providing life safety for workers at the site.








Masonry Bracing Task Force

Conclusion


It has been established that the construction and bracing
of a 24 ft. high structural masonry wall, utilizing this
bracing system, can withstand a minimum wind force of
35 mph and remain intact, thus providing life safety.



Masonry test walls were able to rapidly gain strength and
resist moderate wind loading due to a combination of a
fixed connection between base of wall and footing, along
with the additional strengths achieved by mortar and
grout.


Masonry Bracing Task Force

Conclusion (cont.)


A qualified person may design bracing, for purposes of
life safety, for walls up to 24 ft. high when no more than
8 ft. of wall is left ungrouted.


It is reasonable to assume when using the information in
this report that walls in excess of 24 ft. may be properly
braced by increasing the number and/or reducing the
spacing of braces on the wall.


Bracing design may be modified to reflect the additional
support given by intersecting walls, corners or when
limiting grout pours to 5 ft. 4 in.

Masonry Bracing Task Force

Field Application

Important guidelines for the bracing up to a 24 ft. high wall:


Max. horizontal spacing between bracing is 20 ft.


Max. horizontal spacing from end of wall or control joint
is 10 ft.


Metal tilt braces are attached from base connection to
wall at a max. 60 degree angle from horizontal.


Metal brace connected to wall via minimum ¾ x 4 inch
diameter straight coil loop insert looped around rebar
.


Make sure perpendicular rebar is placed between
attached bar with coil loop insert and face shell to
receive brace.


Masonry Bracing Task Force

Field Application (cont.)


Brace connection to base is a minimum ¾ inch bolt,
attached to either a floor slab (4 inch min.) or deadman,
with a minimum 3400 lb. pullout strength.



No more than 8 ft. of vertical ungrouted wall will be
braced at anytime.



Max. of 4 ft. of ungrouted wall above a brace.



When moving braces to a higher point on a wall,
reposition sequentially (one
-
at
-
a
-
time) to maintain as
many brace supports to a wall at a given time.





Masonry Bracing Task Force

The End