C & C – Wood Engineering

`Components and Cladding’ … or C & C or `see and see’. In earlier posts we looked at the Main Wind Force Resisting System (MWFRS) wind forces on a couple buildings, specifically, on a relatively wide, low, flat `shop’, and a not-as-wide, and taller, `house’. The MWFRS forces are the `big-picture’ forces acting on the structure, or overall-building forces. These forces tend to lift the structure off the ground, or push, slide it along the ground, lift the roof off the walls, or pry or tip the building over. These forces are used to design the main `force-resisting systems’ … the main frames, or framing system(s). Typical systems might be actual structural frames within the building, or roof-diaphragm-shear-wall systems, or `cantilever-post-and-roof-cladding’ systems. And the system(s) that attach (anchor / tie down) the building to the foundation, or ground.

Winds act on structures via air `pressures’ they exert on building surfaces. Really strong winds also `throw stuff’ at structures … parts of other buildings already torn apart by the wind, tree branches, kitchen sinks, cows, etc. We won’t talk about these `projectiles’ here … maybe later.

Winds contain `swirls’ of air. The swirls are not necessarily visible, unless made so by water vapor, smoke, other markers, or by `waves’ as they move across fields. These swirls are characteristic of turbulent flow. Water in rivers is also turbulent, and we might say that some rivers are more turbulent than others. Serious white-water rafters like really turbulent flow. These swirls of air are in all different sizes – look at clouds – small swirls within bigger ones. As these swirls hit and move across, or are blocked by, a building, there are parts of the swirls that exert higher (or lower) pressures than others. Some parts of the swirls have higher (or lower) speed air particles than others. The highest and lowest pressures tend to act on relatively smaller amounts of building surface area. Alternately speaking, the pressures are smeared out over larger areas and as the swirls move across the building surfaces. These areas of relatively high or low pressure are smaller than the amounts of surface we looked at for the MWFRS wind forces … they are on the scale of individual `components’ of the building surface … thus we call these pressures `Components and Cladding’ pressures. Whereas before we looked at the whole building lifting off the ground, or the whole roof, or the whole house being slid across the ground, now we are going to look at the pressures and forces that tend to rip pieces off the structure. It is important, of course, for the components and cladding sake that we address these pressures and resulting forces, to keep them from coming off the building, resulting in local damage, or flying debris, or allowing water penetration to the building. It is also important to keep these components intact so that the wind doesn’t penetrate the building, causing it to `inflate’, and then explode, or cause to roof to start, and continue, to unzip, allowing the roof to peel off.

Okay, let’s do it! … ASCE 7-22 Chapter 30. And let’s return to the `shop’ we have been examining. We’re dealing with a low-rise building, partially enclosed (garage door open at one end), and so on; the Steps are provided in Figure 30.3-1.

Step 1: Risk category … I

Step 2: Basic wind speed … V = 96.5 mph, for the location of interest, Risk Category I

Step 3: Determine the wind load parameters:

… Kd = 0.85 … from before

… Exposure C, ditto

… Kzt = 1.00, ditto

… Ground elevation factor 0.915, ditto

… Enclosure classification … partially enclosed, from before

… Internal pressure coefficient(s) … +/- 0.55

Step 4: Kh = 0.85, from before

Step 5: Determine the velocity pressure, qh … from before, 18.5 psf

… and now …

Step 6: Determine the external pressure coefficient, GCp … for gable roofs … we use Figure 30.3-2B … and / but it points also to Section 30.7.

For the top surface we use Figure 30.3-2B.

We’ll assume that the bottom surface of the roof (plywood sheathing) is open to below (as per the actual structure that inspired these posts). The bottom of the plywood outside the exterior wall will be subject to the wall pressure, pw, of Figure 30.3-1, and the bottom of the plywood within the exterior wall will be subject to the internal pressure, GCpi.

Consider a 4 ft x 4 ft piece of plywood, installed at a corner.

The area of the component is 4 ft x 4 ft = 16 ft².

Going back and getting h, … h = 12.3 ft

And a = 4 ft ( … from our exam of `Corners’)

This 4 x 4 piece falls entirely in Zone 2 …

From Fig. 30.3-2B … GCp = ~ -2.4.

And let’s use a rake of 1.5 ft, equal to the eave overhang.

From Figure 3.3-1, pw = + 1, -1.4 (Zone 4 and 5).

From Equation 30.3-1 the total pressure on the `rake’ and `eave’ portions of the plywood component is … p = qh Kd (GCp – GCpi) = 18.5 psf (0.85)( -2.4 – 1.0) = 53 psf. Whoa!

The total pressure for the plywood component within the exterior wall boundary is …

p = 18.5 psf (0.85)( -2.4 – (0.55)) = 46 psf.

The area of the plywood exposed to the interior of the building below is (4 ft – 1.5 ft) x (4 ft – 1.5 ft) = 9.75 ft². The area of plywood exposed underneath to the `outside air’ is … 16 ft² – 9.75 ft² = 6.25 ft².

The total uplift force on the 4 x 4 piece of plywood is … 53 psf x 6.75 ft² + 46 psf x 9.75 ft² = 331 lb + 449 lb = 780 lb. CRAZY! This would be no place for the fasteners attaching the plywood to the framing below to miss the framing! It also is a good place to point out that the framing members will also need to be attached (held down, tied down) … as they will hold down individual components as well as larger areas of the rest of the roof.

Note that 53 psf is equivalent to the weight of almost a foot of standing water, except upward! … or 6 or 8 feet of snow! These wind forces are very significant! In a particular windstorm (or tornado, or hurricane), it might very well be that just a single component or cladding that gets ripped off the building, … or, often, more and bigger pieces follow, or the whole roof, or the whole structure.