Fortified For Safer Living

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1 Fortified For Safer Living Module 12: Shear Walls and Shear Resistance When the wind blows on the side of a house, it s trying to push that house over or push it off its foundations. Shear resistance is taking those forces and bringing them down into the foundation to keep the house standing upright and solidly in place. In this module we re going to look at the assemblies that do that shear walls. In addition to understanding how shear walls develop shear resistance, we ll look at some of the general rules of thumb that will help you evaluate whether a shear wall has been designed properly and whether the drawings contain the necessary information for the builder to build it right. We ll cover the basic elements of shear walls. And we ll talk about two different types of shear walls: Type I and the Type II. Then we re going to look at shear walls that are combined (subjected to combined shear and uplift) and how to make sure we ve got resistance for both. Type I vs. Type II Shear Walls The illustration below shows a shear wall, and to understand the difference between Type I and Type II, we need to understand that the hatched lines indicate where the wall would be fully sheathed, and the blue lines indicated hold-down anchors bolted to the double studs at the ends of the wall IBHS or its affiliates, All Rights Reserved Module 12, Page 1

2 In a Type I shear wall, you need hold-downs at the end of each shear wall segment. To be a true shear wall segment for this type of wall, the width of the segment has to be at least one-third the height of the segment. So typically there might be a hold-down at the end of each plywood panel (visible in the illustration where close nailing is shown at the edge of each panel), and the studs at these locations would have to be double members. Type I Shear Wall In Type II shear walls, there are only two hold-downs: one at each end of the wall, with full sheathing throughout the rest of the wall. You can assume only about half the shear resistance for the wall shown below than you could if you had hold-downs at the end of each segment (shown above). Type II Shear Wall There are sizable reductions in the shear capacity when you start having openings and you only have the hold-downs at the very ends of the shear wall and not along each segment IBHS or its affiliates, All Rights Reserved Module 12, Page 2

3 Shear Wall Requirements There are a number of ways in the prescriptive standards to determine shear wall requirements. You can either deal with the overall size of the building and determine your shear walls from that, or you can break it into boxes. We re going to look at this second approach in this portion of our module. Let s take an example: In the illustration below, we ve got a basic rectangular house with a garage sitting on one side. Our shear walls line up at different locations, and there are different ways to treat this depending on which direction the wind is coming from. When the wind s coming from the left or the right, the easiest way would be to break this into two boxes: one box that represents the rectangular house and another box that represents the garage. Interestingly, the common wall (marked at 36 long in the illustration) can be one continuous shear wall but it s got to be able to resist the loads from both of these boxes. So in cases where you do a multiple box approach, you have to make sure that common walls are designed to carry the loads from both. Now let s take a look at a house with a bit more complex situation. This next illustration (next page) is a typical floor plan from a house in Louisiana. How you determine the shear resistance on this building depends on the direction from which you re looking. If you re looking at winds coming from the north and south (from the top and bottom), you ve got a nice shear wall on the left side of the house and on the right side of the house, so you can treat this thing as a single box from that direction IBHS or its affiliates, All Rights Reserved Module 12, Page 3

4 However, when your winds come in from the east and west (or from the left and right), you ve got to look at the walls that you ve got available at the top and bottom of the illustration. The general rule is if these walls are out of alignment more than 4 feet, then you ve got to treat them as separate shear walls. You can t tie them together and consider them a single shear wall if the offset is more than 4 feet. Clearly, we ve got to deal with things a little bit differently on the side of the house at the bottom of the illustration because we have some pretty big offsets there. In the illustration at left we have the roof plan of the same house, which gives you an idea of some of the very steep roofs in Louisiana. This raises some additional loading issues. When we re looking at winds coming in from the sides, we would deal with the shear by breaking the house into two boxes. There s so much offset between the different walls at the bottom of the illustrations that you really have to treat it as separate boxes and make sure you ve got enough shear resistance in each box IBHS or its affiliates, All Rights Reserved Module 12, Page 4

5 To simplify the analysis, let s look at a similar floor plan (below). We have lined up the back wall (at top), so we can treat it basically as one shear wall across the back end. If we hadn t gotten rid of this offset, we would have had two separate shear walls at the back. As with the other floor plan, if the winds are coming from the north or the south (or the top or bottom of the illustration), then we can treat it as a single box with sufficient shear wall on the left and right sides (depicted in yellow). But when we consider winds coming from the east/west directions (right), we would break the house into separate boxes. And because of the number of openings in the lower common wall, we have some other issues. The blue bar depicts what we call a drag strut, which allows us to engage the first shear wall with the other shear wall. Typically this is accomplished with strapping and blocking through the ceiling plane that transfers loads from one shear wall to the next. Note that we have a shear wall at the front of the garage (lower left box). We would need a hold-down at each end of this shear wall and at the garage because we have such short returns here. So we re going to need two hold-downs (shown as green dots) at each side of the garage in order to get any kind of shear resistance out of that front area IBHS or its affiliates, All Rights Reserved Module 12, Page 5

6 We ve also got to have a shear wall at the front of the bedroom area (lower right box in the illustration), which also requires two hold-downs. At the back of the bedroom and the garage, however, we can treat this as a Type II shear wall with the drag strut. Then we can get by with just a hold-down at the end of shear: just two across the entire span of the house. Design Choices for Shear Resistance and Code Compliance We simplified the floor plan in the previous example for a reason. Doing the shear walls with the first floor plan would have been very difficult and more expensive. In areas where the high-wind codes haven t been used much, we re finding that people were just drawing out a shape they liked. But with some fairly minor modifications to that shape, such as moving walls around a little bit, you can still keep the same size rooms but will make the structure work out a lot better while at the same time reducing the cost and the difficulty of construction. So we need designers to begin to understand the importance of tweaking the design and layout of the house to get the strength that s needed without having to pay a premium for the shape of the house. When you design to and have to build to a complex shape, you re typically going to end up having an engineer do the design, and the prescriptive methods won t work. But if you simplify the structure a bit, then you can use the prescriptive methods without having to pay for the engineering. That s really one of the biggest differences in the cost. If designers are willing to make some changes, such as eliminating shear wall offsets and limiting garage door sizes, then there can be a balance of cost versus difficulty versus the design that they want to build. There has to be some give and take. Summary Let s review the key elements of effective shear wall construction. We need to anchor to the floor and to the foundation below. We need to look at the nail size and spacing of the shear walls because that affects how well you re using the sheathing and how strong that sheathing s going to be in terms of resisting shear. The use of sheathing or straps in combination to transfer uplift is another important issue. There s the basic overall shear capacity that you need. If you ve got a lot of wall and very few openings, it s pretty easy to get the shear resistance you need. If you ve got a lot of openings, then it becomes much more difficult to get the shear resistance you need. We re going to be looking for hold-downs at the ends of the shear walls and we re going to look for strapping across floors. If there s a hold-down required on an upper floor, you re going to have to match it with 2008 IBHS or its affiliates, All Rights Reserved Module 12, Page 6

7 a hold-down on the lower floor because you ve got a concentrated load that s coming in and you ve got to be able to transmit that on down to the foundation. So in some cases, if you ve got a shorter top floor and you ve got hold-downs at the ends of those walls, you re going to have to carry that hold-down down even though it s in the middle of the wall below IBHS or its affiliates, All Rights Reserved Module 12, Page 7