Masonry Wall Bracing. A Simplified Approach To Bracing Masonry Walls Under Construction. Masonry Bracing Task Force.

Transcription

1 Masonry Wall Bracing A Simplified Approach To Bracing Masonry Walls Under Construction Produced by the

2 Masonry Wall Bracing A Simplified Approach to Bracing Masonry Walls Under Construction Produced by the 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

3 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.

4 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.

5 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.

6 To Achieve the Stated Goals: The 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.

7 Wall Bracing Test Program Purpose Test Wall Design Testing Procedures Summary of Test Results Conclusion Field Application

8 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.

9 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

10 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

11 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.

12 Brace Connection: 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. The Coil Loop Insert

13 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.

14 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.

15 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.

16 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.

17 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

18 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

19 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.

20 Deflection Measurement Points Test wall elevation showing location of measurement points Source: Masonry Wall Bracing, Figure A3

21 Wall Deflection Graph of wall deflection vs. wind speed developed for each test wall 50 Wall Deflection vs. Wind Speed Wind Speed (mph) North Upper Center Upper South Upper Top Average Example: Wall Test # Wall Deflection (in) Source: Masonry Wall Bracing, Figure A13

22 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.

23 Test #1

24 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.

25 Test #2

26 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.

27 Test #3

28 Test Series Test #4 Free standing wall Test performed on a 12 hour grouted wall that was 16 ft. tall with no bracing.

29 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.

30 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.

31 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.

32 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.

33 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.

34 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.

35 The End