Structural Requirements for Existing Buildings

Last updated April 4, 2025
By Ian Story

This article discusses the structural requirements for projects involving existing buildings. It covers repairs to damaged buildings, alterations of existing buildings, additions, and changes of use. This article focuses on residential buildings (R-2 and R-3 occupancies).

Applicable Codes

Buildings that fit within the scope of the IRC (one- and two-family dwellings with 3 or fewer stories above grade) can choose whether to use the IRC or IEBC when analyzing repairs, alterations, or additions to the building. From an architectural / building planning perspective, the IRC is generally simpler to apply and has less costly requirements (for example, significant alterations under the IEBC may require fire sprinklers, where these are typically not required under the IRC). For this reason, we will almost always stick to the IRC when planning a repair, remodel, or addition to an existing building.

For structural analysis, the IRC permits the use of accepted engineering practice to design individual structural elements (provided that these elements are compatible with the rest of the structure). The relevant code section is R301.1.3. This section allows the IBC as a basis for engineered design, but does not specifically mandate its use. Within the context of existing structures, this code language gives us freedom to decide how best to evaluate existing structural components. As a standard operating practice, we have decided as a firm to apply the structural provisions of the IEBC when analyzing existing structural elements. These are the same structural provisions that would apply if the building were analyzed under the full building code (IEBC + IBC). The remainder of this article discusses those IEBC provisions.

Existing buildings that don’t fit within the scope of the IRC are governed by the existing building code (IEBC).

General Guidelines

Generally speaking, existing structural elements can remain in their current state unless something in the IEBC specifically notes that they need to be replaced or upgraded. Any new elements need to be designed according to the current code.

We design any new elements to withstand the full structural loads they will be subject to, including any loads transferred from the existing building. This is particularly relevant for seismic loading; frequently, we find that existing buildings have few reliable shear walls along one or more gridlines. When we extend those gridlines in an addition, we design the new shear walls to pick up the slack from the existing building, which often results in additions being more heavily reinforced than standalone new construction.

5% / 10% Rule

In most cases, existing structural components only need to be replaced or upgraded if their demand / capacity ratio (D/C ratio) does not increase by more than 5% for gravity components or 10% for lateral components. This threshold can be triggered by increased loading or by decreased capacity (for example, drilling a hole through the center of an existing element). Sometimes we can keep an existing weakened element in service by finding ways to reduce the load on that element (for example, by providing a stiffer shear wall segment elsewhere). However, we typically don’t allow reinforcement of existing members to increase the capacity for this calculation; if a member needs to be reinforced, you should plan on designing it to resist the full applied loads, without the 5% / 10% discount.

To assess the existing demand on structural elements, we use the same loads we would use for new construction, given the current use of the element. This means that we will typically run two seismic analyses for an existing building: one for the building in its current state (to establish existing demand), and one for the proposed changes & additions. We use the same seismic design criteria for both cases – if the proposed structure is designed for full seismic loads, we analyze the existing structure for full seismic loads.

When applying this provision for seismic design, remember to consider the effects of any new structural irregularities caused by an addition or alteration when assessing demands on existing lateral force resisting systems.

The relevant code sections depend on the IEBC compliance method:

Prescriptive Compliance: 502.4, 502.5, 503.3, 503.4
Work Areas Compliance (Level 1 Alterations): 706.2
Work Areas Compliance (Level 2 & 3 Alterations): 805.2, 805.3

Reduced Seismic Loads

Some provisions of the IEBC allow existing structures to use reduced seismic forces. Typically we only consider these for repairs or remodels – if an addition will be adding seismic load, we design the lateral force resisting system for full seismic loads. If reduced seismic forces are allowed, refer to IEBC 304.3.2 for the calculation procedure.

Repairs

When repairing damage to structural members, first determine whether the damage constitutes substantial structural damage (see definition below). If the damage is not considered substantial, existing members can be restored to their predamage condition or replaced like for like (IEBC 405.2.1), with some exceptions:

If the damage was caused by snow loads, the damaged members need to be reinforced or replaced to carry the full loads under current codes (IEBC 405.2.1.1).
If the damage was caused by an earthquake, proceed as noted below for substantial structural damage to the lateral force resisting system (IEBC 405.2.2).

If the damage to vertical components of the lateral force resisting system is substantial, you need to complete a structural evaluation. The evaluation should determine whether the building, if repaired to its original state, would be able to withstand the full code loads (with reduced seismic loads). See IEBC 405.2.3.1. If yes, then the damaged members can be restored or replaced like for like. Otherwise, the structure will need to be upgraded (see IEBC 405.2.3.3 for nuances about the wind and seismic loads required).

If the damage to gravity components is substantial, damaged members and all supporting members need to be replaced or reinforced to carry the full gravity loads required under current codes. See IEBC 405.2.4. If the damage was caused by wind or seismic loads, then also evaluate the lateral force resisting system per the paragraph above.

SUBSTANTIAL STRUCTURAL DAMAGE: A condition where any of the following apply:

The vertical elements of the lateral force-resisting system have suffered damage such that the lateral load-carrying capacity of any story in any horizontal direction has been reduced by more than 33 percent from its predamage condition.
The capacity of any vertical component carrying gravity load, or any group of such components, that has a tributary area more than 30 percent of the total area of the structure’s floor(s) and roof(s) has been reduced more than 20 percent from its predamage condition, and the remaining capacity of such affected elements, with respect to all dead and live loads, is less than 75 percent of that required by the IBC for new buildings of similar structure, purpose, and location.
The capacity of any structural component carrying snow load, or any group of such components, that supports more than 30 percent of the roof area of similar construction has been reduced more than 20 percent from its predamage condition, and the remaining capacity with respect to dead, live, and snow loads is less than 75 percent of that required by the IBC for new buildings of similar structure, purpose, and location.

Seismic / Wind Retrofits

Upgrades to the existing lateral force resisting system are only required under the following conditions:

Alterations to the existing gravity structural system are sufficient to be classified as a substantial structural alteration (a defined term, see below), and the work qualifies as a Level 3 alteration (more than 50% of the building is being reconfigured). See sections IEBC 503.11 / 906.2.
For other (architectural) code reasons, the project is using the Performance Compliance path. See IEBC section 1301.4.1.
A change in occupancy changes the risk category of the building. See IEBC section 1006.3.
Alterations to large buildings (over 12,000 square feet) using the Prescriptive Compliance path. See Washington State amendments to the IEBC, section 503.19.1.
The City of Seattle has their own process for determining when seismic retrofits are required. See Tip #314.

If a lateral retrofit is required, you may be allowed to use reduced seismic forces when sizing the retrofits.

SUBSTANTIAL STRUCTURAL ALTERATION: An alteration in which (the gravity load-carrying structural elements altered within a 5-year period) support more than 30 percent of the total floor and roof area of the building or structure. The areas to be counted toward the 30 percent shall include mezzanines, penthouses, and in-filled courts and shafts tributary to the altered structural elements.

Live Loads

If a change in occupancy increases the live loads on existing gravity members (directly or via any load path), those members need to be reinforced to support the new, higher loads. The 5% rule still applies here. See IEBC 1006.1.

Snow and Wind Loads

If a change in occupancy changes the risk category of the building, structural members need to be evaluated based on the updated loads. See IEBC 1006.2.

Unreinforced Masonry

Many alterations require upgrades to reinforced masonry. This section is incomplete – review the IEBC (with Washington State amendments) if your project includes existing unreinforced masonry to determine what upgrades are required.