Some houses are simple boxes; others can be very complex with varying wall heights, angled walls, great rooms, tall window walls, stepped walls, etc. Complexity creates a need for structural engineering.

The building codes allow building officials to issue permits without engineering if structures meet certain prescriptive rules. In general, a house has to be relatively small and regular for this to happen. Increasingly, building officials are requesting structural engineering for residences because few houses are simple any more.

Also, structures that are "unconventional" have to be engineered. These include

1. Concrete masonry
2. ICF (concrete walls cast between foam forms)
3. AAC (lightweight autoclaved concrete)
4. Steel (load-bearing steel studs, steel moment frames)
5. Retaining walls.

Experienced designers of high-end houses work with structural engineers throughout the design process. Owners who buy stock plans and many of those who have their plans drawn by architects or designers first take the plans into the building permit department. The plans examiner then determines if engineering is required. We recommend, of course, that to protect yourself and your investment you should always have your house structurally engineered. If you bring your plans to us for engineering, we like to see one full set of drawings, and a truss layout plan from the truss supplier.

Our engineering usually includes design for vertical (gravity), and lateral (wind and seismic) loads. We also may be asked to engineer retaining walls in basements, retaining walls in the landscaping, clips for wind uplift on trusses, out-of-plane loads on tall walls, suspended garage floors, and perhaps some special details such as stairs. Gravity design includes the sizing of the beams, columns, footings and connections. Lateral design involves selecting a restraint system that best fits the architecture, then proportioning shearwalls, shearwall holddowns, other restraint elements, and drag struts so that the structure behaves in a predictable way.

The local code, IBC-2003, specifies that we design for a major subduction earthquake that has a recurrence period of about 300 years. The design process involves assessing the horizontal acceleration, then discounting its effect on the structure depending on how responsive and ductile the type of framing is. We have accumulated knowledge about structural behavior in earthquakes from studies of recent events (Loma Prieta, CA, Northridge, CA, San Fernando Valley, CA, Kobe, Japan, Anchorage, AK). We know that well-proportioned sheathed wood-frame houses exhibit good performance because they are light, redundant, and energy-absorbing. In contrast, unreinforced masonry is a hazard.

When we engineer commercial structures, we normally provide a full set of construction drawings. For residential structures it has been normal practice for many years to issue engineering instructions in the form of small-scale sketch plans, schedules, tables, performance specifications and generic details. Here we rely upon the experience of the contractor and the expertise of the inspectors. Also, residential contractors have a long history of creating their own preferred framing details and sequences from simple outline architectural drawings. However, there is a trend toward more complex residential structures, and correspondingly toward permit agencies requiring full-size (24" x 36") and well-detailed drawings similar to those for commercial buildings. Engineering drawings consume a lot of office time and can substantially increase the engineering fee. Among local agencies, full-size engineering drawings are usually required in Clark County, the City of Vancouver, and the City of Portland.

What is a shearwall?

A shearwall is a wall panel that is designed to restrain relative lateral movement between floor levels by taking advantage of its in-plane strength and rigidity. Shearwalls are useful for structural engineers because most structures have walls anyway, and without much embarrassment to the architect they can be made part of the structural system. They can be constructed from sheathed wood studs, poured or precast concrete, concrete or brick masonry, or AAC.

Shearwalls are proportioned to prevent failure in four different ways:

1. Shear failure through the paneling material. Shear stresses in plywood, concrete, or masonry are kept within permissible levels by selecting plywood thickness or steel reinforcing in concrete.
2. Failure of sheathing fasteners. In wood walls, the nail sizes and spacings are specified.
3. Failure by sliding. Walls may slide along their base supports. Wood walls are nailed to plates that have steel anchor bolts embedded into concrete foundations. Concrete walls have embedded reinforcing dowels. Precast concrete panels may have welded steel connection embedments.
4. Failure by overturning. The horizontal shear load acting at the top of the wall will push the wall over unless there is sufficient dead weight or mechanical fastening in resistance. Many wood walls have "holddowns" attached to their end studs for this purpose. The shorter the shearwall length, the more likely is the need for a holddown. Concrete walls use vertical steel reinforcing at their ends to resist overturning.

In residential wood-frame engineering, we specify the sheathing (OSB or plywood), the edge and field nailing, the base anchor bolt spacing, and the holddown for each shearwall.

What is a drag strut?

Drag struts (or "collectors") gather the horizontal loads in the “diaphragms” (the floor or roof systems) and bring them to the shearwalls. Shearwalls do not help if they are not connected in a rational way to the floor or roof levels they are meant to be restraining.

Often the top plates of the wall lines are used as drag struts, and sometimes beams serve this purpose. When necessary, we specify connection straps. Sometimes long steel straps have to be installed.

Hardware for drag struts often has to be located under beams and trusses, so it is important that the contractor installs the drag strut hardware in the proper sequence, and not wait until framing is completed.