Walls and Slabs

This example will look at a multi-story shear wall design using the perforated method. This method considers the entire length of the shear wall as a single panel, while the segmented method looks at the shear wall divided into multiple segments based on relative stiffness. The perforated method is a more conservative approach, however an advantage is that holdowns are only needed at the very ends of the shear walls, rather than at each of the openings. In ClearCalcs, another advantage to using the perforated method is that it allows specifying different openings on each story.

Background Information

• Two story building located in Portland, OR
• The story height is 10 ft
• The total wall length of 70 ft
• Six openings, with three windows on the second floor and two windows plus a door on the first floor
• Applied loads and location of wall openings can be seen in the diagram below

Entering Your Design Criteria

Our wall self weight is the weight of the sheathed wall. We can use a value of 12 psf, corresponding to a 2x6 exterior stud wall spaced at 16”.

Next we can input our dead loads that counteract the overturning loads, transient loads (snow or live) that will contribute to compressive loads in the chords, as well as ultimate wind shear load and seismic loads which impacts the lateral loading. Our loads are specified at the story level, so we will calculate each load. We will assume we have 10 ft long joists framing into the wall so we can use a dead load of 10 psf. For this, we can calculate 10 psf multiplied by half the joist length (50 plf). We can do the same for a 20 psf transient live load on the roof, resulting in 100 plf, and a 40 psf transient live load on the first floor.

Our ultimate wind and seismic shear loads should be inputted at the ultimate level, as ClearCalcs will automatically factor them by 0.6 and 0.7 for wind and seismic loads, respectively. To determine these values a lateral load analysis should be completed, which can be done using our wind and seismic load calculators, in combination with our diaphragm analysis tool. For this example the final values are outlined in the background information. Inputting seismic loads will also prompt you to include a design spectral response acceleration at short periods. This SDS value can be found using our Seismic Analysis calculator. It is used to calculate the additional vertical loads from vertical motion during an earthquake. For this building in Portland, we can use SDS = 0.77.

Our structure geometry can be entered based on the diagram in our background information. With the perforated method, ClearCalcs supports unique opening sizes per story! We can try it out with our larger opening on the first floor for the door, and a smaller opening above it for the window. To do so, you can specify “1” or “2” for a specific story level, or we can type “All” if the location is the same on all stories. The inputs here will automatically update our diagram in ClearCalcs under the results and will be as follows:

Optimizing Your Design

After inputting the loads and structure geometry, we can determine the required sheathing and studs required for a passing design. Our sheathing and nail pattern can be selected using the member selector, in which we can specify our sheathing material, thickness, as well as our nail size and spacing. These will govern the shear capacity of the wall. Thicker sheathing, or smaller nail spacing will result in higher shear capacity. For our design, we will select a 15/16” OSB/Plywood, 6d@2”.

We can also specify if our wall is sheathed on one or both sides. This will design the shear wall with the same sheathing and nail pattern on either side, doubling the capacity. Since our capacity is above a passing utilization, we can design this to be sheathed on both sides. Similarly with blocking at the panel edges, we can fasten the edges of the panels to studs. An unblocked shear wall will have lower shear capacity. To help with the high shear demand, we can design a blocked shear wall. We can leave our nail spacing in field of panels at 12 inches. This value doesn’t have much of an impact on the strength, but is required as per the code.

For the studs, these will be used for the chord design. The stud species will impact the shear resistance of our wall. For our design, we can select based on the studs used for the wall designs, let’s select this as a 2x6 Douglas Fir No. 2.

Our stud spacing can be left as 16 inches, but can be decreased if we find the deflection governs. We will also leave our hole size in the chords as 0.5 inches, although this value will impact tensile strength in the chords and should be considered if larger connections are used to fasten hold down anchors.

Interpreting Calculations and Results

Under the shear capacity section, we can see how the capacity is calculated per story for wind and seismic conditions. For the perforated method, an adjustment factor is calculated per story based on the total area of openings and the total wall area. This factor accounts for a shear capacity reduction due to unsheathed areas or openings. The unadjusted capacity is taken from tables in the AWC SPDWS, and is determined based on the sheathing and fastener spacing. The capacity is then adjusted with the shear capacity adjustment factor (Co), the specific gravity adjustment factor and the effective width. For perforated shear walls, an upper limit of 2435 psf is applied to the shear capacity. The story with the highest utilization governs the shear design.

Similarly, we can see how the chord loads are determined per the perforated method. The shear values taken per story are used to determine the overturning moment. From there, the tension or compression forces are factored and calculated. For multi-story shear walls, the total force considers the forces from the floor above as well.

Finally, we can scroll to the top and see a summary of the governing and allowable wind and seismic loads, as well as the tension and compression in the chords. If we find that our design is failing in shear, we can increase our sheathing thickness or nail spacing. If our chords are failing, we may need to increase our chord member size. For this example, we have a passing design using a 15/16” OSB/Plywood sheathing with 6d@2” nail patterns, with 2x6 Douglas Fir No. 2 chords.

Holdown Design

Now that we have designed our shear wall, we can select the holdowns. Holdowns are required when there are overturning conditions that cause uplift in the chords. ClearCalcs currently supports a selection of holdowns from Simpson Strong Tie. When we open the member selector, we can see the utilization for each. For this design, let's use a HDU2-SD2.5. YOu can specify an offset distance, which would be the distance from the exterior face of the chord to the center of the holdown anchor rod. This value is considered for the moment arm and calculation of the tension and compression chord forces. From there, we can see the holdown capacity check and required wood fasteners. These values are both based on the Simpson Strong Tie catalog. For the perforated method, one benefit is that holdowns are only required at the ends of the walls, rather than at each opening. For this design, two HDU2-SD2.5 holdowns will be required at either ends of the wall.