US&R Raker Shore Geometry and Wall Force

ScopeA damaged masonry wall is held with rakers, and the design is a small triangle problem with a load taken from the wall's own weight. The guides design each raker for about 10% of the weight of the wall and roof in its tributary width, 2% where no aftershock is expected. A 12 in unreinforced masonry wall at 125 psf, 14 ft tall, with rakers at 8 ft centers and an 8 by 10 ft roof area at 15 psf, weighs 15,200 lb per raker, for a design force of 1,520 lb. Inserted at 12 ft and set at 45 degrees, the raker is 203.6 in long (the field rule of 17 in per foot gives 204), stands 12 ft out from the wall, and carries 2,150 lb along its length with a 1,520 lb upward kick at the wall plate. At 60 degrees it is 166.3 in long (rule 168), its base shrinks to 7 ft, but the axial force rises to 3,040 lb and the kick to 2,633 lb. That kick is the part most often missed: a wall plate that is only nailed can be pushed up the wall. The two guide editions rate rakers differently and both are shown -- the 2021 Shoring Operations Guide at 4,000 lb of horizontal force per raker (8,000 lb for a braced pair), the 2006 Field Operations Guide at 2,500 lb. The collapse zone, 1.25 to 1.5 times the wall height, reaches 21 ft here, beyond the 45 degree base, so the crew works inside it. It does not design the plate, sole, cleats, bracing, or ground restraint, or check the wall between rakers. The US&R guides, the US&R Structures Specialist, and the incident's structural engineer govern.

Formula or tableW = wall psf x wall height x spacing + roof psf x roof depth x spacing; H = 10% of W (2% where no aftershock is expected); raker length = h_i / sin(theta) (rule 17 in per ft at 45 deg, 14 in per ft at 60); base = h_i / tan(theta); axial = H / cos(theta); vertical kick = H x tan(theta); collapse zone = 1.25 to 1.5 x wall height; H checked against 4,000 lb (SOG 2021) and 2,500 lb (FOG 2006) per raker.

Source and editionFEMA Structural Collapse Technician Module 2a (raker systems for about 10% of the wall and roof weight in their tributary area, 2% minimum); USACE US&R Shoring Operations Guide, Ed. 5.0 (2021), raker length rules, FAQ R-1 (4,000 lb horizontal per raker of No. 1 Douglas Fir), Shoring Numbers to Remember (8,000 lb for a braced pair), and the collapse-zone definition; USACE US&R Structures Specialist FOG (2006) raker rating of 2,500 lb.

Free accessRight-triangle trigonometry and a percentage of a weight; the SOG is a free USACE publication.

What governsEstimate. AHJ and licensed structural engineer govern.

API field nameswall_psf, wall_height_ft, raker_spacing_ft, roof_depth_ft, roof_psf, insertion_height_ft, raker_angle_deg, aftershock_expected, tributary_weight_lb, design_force_lb, raker_length_in, rule_length_in, base_distance_in, axial_force_lb, vertical_kick_lb, collapse_zone_min_ft, collapse_zone_max_ft

• Design force is a tenth of the tributary weight 10% of wall plus roof weight per raker, 2% minimum where no aftershock is expectedFEMA Structural Collapse Technician Module 2a
• The kick is upward at the wall plate V = H x tan(theta); 2,633 lb at 60 degrees for a 1,520 lb design forcestatics
• Collapse zone 1.25 to 1.5 times the wall heightUSACE US&R Shoring Operations Guide (2021)