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RE: SEI/ASCE 7-02 Wind Loads

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>What effective wind area should be used for concrete masonry unit wall
>design per figure 6-3 of SEI/ASCE 7-02.  Should it be vertical wall
>reinforcing spacing times wall height which would give a high
>pressure?   For a 15'-0" high wall I am contemplating installing a bond
>beam at mid-height which should increase the effective wind area and allow
>designing for a lower pressure.  Is it reasonable to assume that the joint
>reinforcing would make the wall act as a unit such that the effective wind
>area would be the wall height times its length.  Does anyone have any
>recommendations for effective wind area?
>
>James a. Cory III, P.E.
>Cory Structural Engineering, P.C.

This question came up recently at an ASCE 7 seminar with Kishor Mehta and Joe Minor.  Of course, the short answer is that for components and cladding, the effective wind area need not be less than the span times 1/3 the span where the span is 15'.  So for your example, you would not be penalized for placing steel closer than 5' o.c.  This may not be very helpful. 
For certain aspect ratios of wall width to wall height, one could make the argument that the effective wind area is much larger.  The same goes for horizontal bond beams which can also increase the effective wind area.  It may be appropriate to do a deflection compatibility analysis to see if the horizontal bond beam is effective.
In some cases the C&C pressures may not be applicable at all.  I wish ASCE 7 was more clear on this.  Even Kishor and Joe did not appear to agree on this point.  According to ASCE 7s strict definitions of C&C and MWFRS, if the wall is part of the MWFRS if does not qualify as C&C.  However, these definitions are quickly trodden under foot.  For example, the top chord of a wood truss is a necessary part of the MWFRS of the truss but it is also designed for C&C pressures because it takes out-of-plane bending loads from the plywood.  The same is true for a masonry wall in a simple diaphragm structure.  The tie beams are clearly an integral part of the MWFRS. They resist uplift, gravity, in-plane shear, in-plane and out-of-plane bending, sometimes all at the same time.  They then transfer these loads to the ground or roof/wall diaphragms by any of several primary and secondary effects.  This does not meet the description of a C&C element which must transfer wind load directly to the MWFRS, and yet most engineers would agree that masonry tie columns should be designed using C&C pressures.  I think if you put your problem into a Finite Element Analysis program and design the entire wall as a plate with appropriate loading and boundary conditions you could use MWFRS pressures exclusively.  Almost any other simplified analysis invite trouble if C&C pressures were not considered in the most conservative way possible. 
 
Anyway, I'm running out of ink.
 
Regards,
 
Christopher A. Banbury, PE
Vice President
Nicholson Engineering Associates, Inc.
PO Box 12230, Brooksville, FL 34603
7468 Horse Lake RD, Brooksville, FL 34601
(352) 799-0170 (o)
(352) 754-9167 (f)
www.nicholson-engineering.com