domingo, 24 de julho de 2011


Concentrated Loads

Loads from beams, girders, trusses and other concentrated loads that frame into the wall must be applied to the RBM beam in the appropriate manner. 

Concentrated loads may be assumed to be distributed over a wall length equal to the base of a trapezoid whose top is at the point of load application and whose sides make an angle of 60 degrees with the horizontal.  In FIG. 4, the portion of the concentrated load carried by the beam is distributed over the length indicated as a uniform load.  The distributed load, wp, on the RBM beam is computed by the following equation:
                            wp = P/(b + 2ytan 30)                  Eq. 1 
wp= design uniform distributed load, 1b/ft  (kg/m)
P= design concentrated load, 1b (kg)
b= length of bearing plate, ft (m)
y= distance from top of beam to bearing plate, ft (m)

 This is approximately 0.866 times P divided by y.  Because the apex of the 45 degree triangle is above the top of the wall in this example, the RBM beam should be designed assuming no arching action occurs.
The designer should check the stress condition at bearing points for RBM beams.  This applies to loads on the beam and to the beam's reaction on the wall. 

The MSJC Code limits the bearing stress to 0.25 fm, where fm is the specified compressive strength of masonry.  A rule-of-thumb recommended for many years is to provide a minimum of 4 in. (100 mm) of bearing length for masonry beams.  The masonry directly beneath a bearing point should be constructed with solid brick or with solidly grouted hollow brick. 

Concentrated loads should not bear directly on ungrouted hollow brick masonry because of the potential for localized cracking or crushing of the face shells.

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