COMPONENTS AND CLADDING 1

[Zulak]

In reference to wind loads on parts of buildings, what exactly is considered a "component and cladding". Specifically, would you consider a non-loadbearing masonry exterior wall (or a precast concrete panel) a "component and cladding"? By strict definition, it would be yes, but in the spirit of the design approach, I would hesitate to classify it in the same category as a metal stud wall. Any thoughts would be appreciated.

 

A building structure member that is not part of the "main wind force resisting system" is "components and cladding." For a steel or reinforced concrete frame building with infill concrete block walls, the block walls are components and cladding per ASCE 7 loads determination. If, on the other hand, the block wall is made to be an integral part of the lateral force resisting system and is tied to the frame to provide such function, then the "main wind force resisting system" loads apply.

 

[JAE]

hoolie is right on track. Just another slant on the differentiation between the two:

When wind is directed upon a building structure, there are vast amounts of chaos and randomness in the intensity of the pressure. At any one spot on the building exterior, the wind pressure could vary from negative 40 psf to 1 psf to positive 40 psf (or more). These variations occur over small areas across the building.

Therefore, at any one small piece of area, the potential,....or statistical probability, that the pressure will reach a certain high level is higher than if you look at a large area where the variations across the surface "average out" to a lesser magnitude.

So the codes are written to take this into account. A single steel stud in a wall supports a small area. There is a high chance that, due to the wide variations in wind pressure, the stud would receive a very high stress.

However, if you look at a horizontal wall girt that supports a full 30 ft + bay, the tributary area is much larger and the "chance" that the wind pressure would AVERAGE a high value just isn't there.

For a main wind force resisting system, the tributary area is huge...therefore the wind pressures are less than components.

ASCE 7-95 actually has a footnote in Table 6-1 that states "Major structural components supporting tributary areas greater than 700 sf shall be permitted to be designed using the provisions for main wind-force resisting systems".

So the definition sort of turns on the trib. area.

 

[alfred]

I think using the 700 square foot area limit to define a main wind force system element is valid and also seems to carry on the tradition of other codes and previous versions. However, the 2000 IBC does not specifically state this limit so there is the possibility that you could get in trouble if your design came under scrutiny. Some people read the code verbatim and don't allow any interpretation or judgment. If only we could just use 15 psf (like the old days) for all wind loads we could save a lot of time! We still get similar pressures for low rise buildings.

 

[JAE]

alfred: Actually, the way I read it, the IBC 2000 simply references the ASCE 7 which allows the area limit to differentiate between the two.

 

[Ron]

In my simple minded approach, the "Components and Cladding" are independent structural elements, typically on the exterior of a building, that transfer load to the "Main Wind Force Resisting System" whether they be exterior or interior items. They are the "replaceable" items on the exterior, that if removed would not constitute a structural detriment for the short term (e.g..."non-load bearing in the vertical sense&quot.

As JAE noted, the loading and interaction are essentially undefined except on an average basis, which is where the codes address them. To do otherwise would be a practicable impossibility, as conditions vary with locale, constituents, construction, materials, geometry, orientation, and a host of other variables.

I read IBC 2000 the same as JAE.

 

[chichuck]

There is a definition of components and cladding in the commentary to ASCE 7-95. It says that cladding recieves wind loads directly. Examples would be roof deck and metal wall panels. Components receive load from cladding. Examples of components are girts & purlins, fasteners. An additional point I learned at one of the ASCE seminars is that components and cladding get load from one surface only. (for example, the panels, girts and fasteners on the windward wall receive loa only from that wall, actually from both the internal and the external surface of that wall, but only from that wall. These are components and cladding.) Main wind force resisting elements are things like the columns, girders, vertical bracing, horizontal bracing. These parts of the "global" system for resisting lateral wind forces receive loads from windward wall, leeward wall, and roof of the building.

Sometimes, the girts, wall panels and fasteners of the roof or of the sidewalls are designed to be a diaphragm to resist lateral wind loads, for those cases, the forces in those elements would be calculted using the coefficients in ASCE 7 for main wind force resisting system.

But you need to recognize that for one load condition, those elements may be a MWFRS, for another, they may be classed as components and cladding. All possible load cases must be accounted for.

Masonry walls, whether or not they have grouted cells, are similar to those sidewall panels. For one load case they are main structural bracing, and thus MWFRS, and for another they are components and cladding.