When designing a house, or any structure, two directions of loading must be considered, that being gravity and lateral loads. If gravity loads (such as the weight of the structure, snow loads, live loads due to human occupancy) are not correctly designed for it can lead to excess deflection in beams and joists, leading to cracked ceilings, squeaky floors, or in extreme cases failure of the structural member/collapse of the building. Lateral loads, such as wind, must be designed for to ensure the building does not overturn. Designing for lateral loads is critical, especially in coastal areas such as Eastern Long Island where waterfront residential structures can be exposed to higher wind speeds. This article will focus on the design aspects of the main lateral force resisting systems (a.k.a MWFRS) starting with shear walls.

Lateral Force Analysis

Before designing for wind loads, a lateral force analysis to determine the actual force, and where the force is applied on the structure. The forces are determined from pressures outlined in ASCE 7 Chapter 28 and the design engineer must identify which walls will aid in resisting the applied pressure, not all walls will be designed to resist the lateral loads just like not all walls are “load bearing walls” in the gravity system. Once the wind-resisting “shear walls” are determined the design wind pressure can be distributed to each wall to determine how much force each wall must resist to ensure the entire structure will stand.

Design of Lateral Force Resisting Systems

There are a few different systems that can be implemented to resist lateral loads, these systems differ in cost and ease of construction. The three main systems that we at DiLandro Andrews Engineering uses, in order of preference, are Wood Frame Shear wall as per AWC SDPWS, Simpson Strongtie “Strong Walls”, and steel moment frames over openings.

Wood Frame Shear Walls per AWC SDPWS

This is the most optimal solution, from both a cost to the client and ease of construction for the contractor standpoint. This system relies on attaching wood sheathing (plywood) to the 2x stud walls, both interior and exterior walls can be used. The American Wood Council outlines strength per linear foot of different walls assemblies, varying in size, spacing, and penetration depth of fasteners (nails) for many different thicknesses of plywood. There are two different walls to design these walls:

Perforated Wall System- A perforated wall system is a way to design a wall to continuously resist the lateral force along the entire length of the wall, while decreasing the strength per foot specified by AWC, to account for window and door openings that may be contained within the wall. The benefit of this method is to decrease the number of holdowns and reduces the tension felt by the holdowns, allowing for smaller less expensive Simpson Strongtie holdowns.

Segmented Wall System- This method requires the engineer to “break up” a wall whenever there is a door, window, or full discontinuity between walls in the same line parallel to the wind force. This may allow the engineer to use multiple walls that may not be connected but are in the same, or close to the same line of action. Both ends of each wall require holdowns, thus if you broke up a wall into two walls due to an opening, you would need 4 total holdowns; rather than 2 holdowns if you were to design the wall as perforated, with a reduced strength per foot.

The wood frame shear wall is the easiest & most cost-effective method but has limitations with walls that have many openings. That is where we must use two alternative methods to obtain the required strength.

Simpson Strong-Wall Shear Walls

One of the limiting attributes of the wood shear wall design is the “aspect ratio requirement”. An aspect ratio is the ratio of the horizontal length of a wall to the height of the wall, as you can imagine a 30ft tall wall that is only 1 foot long would be very flimsy in a high wind scenario. AWC requires a 3.5/1 aspect ratio for the simple wood shear wall method, therefore if the ceiling height of a wall is 9ft then the minimum length of wall required would be 2.7 ft. Now if you are designing an exterior wall that has 2-4 large glass sliding doors or windows, with only 1ft-2.5ft walls in between you cannot use those walls as per the AWC wood shear wall method. That is why Simpson Strongtie engineered a product call a Strong Wall. These are smaller products, 12”-24” wide, that can be used with heights from 7ft-20ft. This method can be used in conjunction with wood shear walls when the capacity of the wood shear wall is slightly lower than the design demand and there are a few smaller walls not being used.

Steel Moment Frames

Steel moment frames are used as a last resort due to cost to the project. When the engineer is left with a situation where there are not many (if any) wood framed walls, due to many windows and doors, along a line of shear they are left with no options other than a steel moment frame. These frames utilize moment connections, either using bolts or welds of the top and bottom flanges of the beam to the supporting columns. The length and height of the frame effect the strength against the imposed lateral load. Increasing the beam size helps with some with the stiffness of the frame but the biggest factor is the column depth parallel to the lateral force. Typically, these frames are designed with “pinned” bases to avoid a-typical baseplate connections to the foundation but may be designed as fixed bases which greatly increase the stiffness of the frame but will result in additional detail for the engineer and labor on the part of the contractor during construction.

Dilandro Andrews Engineering’s structural engineering team has extensive experience in designing lateral force resisting systems including shear walls in coastal wood frame residences. However, many of the high end residential projects we’ve worked on require use of more advanced systems such as Simpson Strong-Walls and steel moment frames. This is especially true in custom homes featuring a modern architectural style featuring narrow wall sections accompanied by large glass walls, windows and doors such as the Surfside Residence.

References:

• https://awc.org/publications/2015-sdpws
• https://awc.org/wp-content/uploads/2021/12/WDF-2002-PerforatedShearWall-Line-0206.pdf
• https://www.strongtie.com/strongwallshearwalls_lateralsystems/landingpage