Structural Requirements for Existing Buildings

Last updated April 2, 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.

Existing buildings are governed by the existing building code (IEBC). 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 meet the current IBC standards for structural engineering.

We typically design any new elements to withstand the full structural loads they will be subject to, including lateral loads from an existing building that might not be capable of resisting them itself. For this reason, additions typically end up being more heavily reinforced than new construction (especially for shear walls), because they have to support themselves plus some portion of the existing structure.

5% / 10% Rule

In most cases, existing structural components do not need to be replaced or upgraded as long as their demand : capacity ratio (DCR) does not increase by more than 5% for gravity components or 10% for lateral components. This means small increases in loads (increasing demand) are acceptable, and it also means that small reductions in capacity (such as drilling a hole through the center of a post) can be OK. If you need to reinforce a structural element to support increasing loads, however, you should plan on designing the reinforced member to support the full load.

Note: 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

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 (see 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 (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 (405.2.2).

If the damage to vertical components of the lateral force resisting system is substantial, you need to complete a structural evaluation to submit to the authority having jurisdiction. 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 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 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 405.2.4. If the damage was caused by wind or seismic loads, then also see 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 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 project is either using the Prescriptive Compliance path or using the Work Areas compliance path and the work qualifies as a Level 3 alteration (more than 50% of the building is being reconfigured). See sections 503.11 / 906.2.
For other (architectural) code reasons, the project is using the Performance Compliance path. See section 1301.4.1.
A change in occupancy changes the risk category of the building. See section 1006.3.
Alterations to a large building (4 or more stories above grade and over 12,000 square feet) if the project is using the Prescriptive Compliance path or using the Work Areas compliance path and the work qualifies as a Level 2 or 3 alteration. See the Washington State amendments, sections 503.19.1 / 805.5.1.
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 (see the code sections flagged above). If reduced seismic forces are allowed, refer to section 304.3.2.

Note: under the Washington State amendments, a Level 3 alteration can be triggered by work area (50%) or by exceeding the thresholds of substantial improvement or substantial damage, as defined below. See Washington State amendments, section 604.1.

SUBSTANTIAL DAMAGE: Damage of any origin sustained by a structure whereby the cost of restoring the structure to its before damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred value determined by one of the following methods:

Values developed for property tax assessment, adjusted to the approximate market value where the land is appraised separately from the structure.
Through a professional appraiser using estimates of a structure’s actual cash value, including depreciation and improvements.
The latest building valuation data published by the International Code Council.
Qualified estimates based on the professional judgment of the building official. However, when the ratio falls between 40 and 60 percent, the building official may require the applicant to provide a detailed list of costs.

SUBSTANTIAL IMPROVEMENT: Any repair, alteration, addition, or improvement of a building or structure, the cost of which equals or exceeds 50 percent of the market value of the structure, before the improvement or repair is started. of the value determined by one of the following methods: [the options are the same as those for substantial damage, above].

If the structure has sustained substantial damage, any repairs are considered substantial improvement regardless of the actual repair work performed. The term does not, however, include either of the following:

Any project for improvement of a building required to correct existing health, sanitary or safety code violations identified by the code official and that is the minimum necessary to ensure safe living conditions.
Any alteration of a historic structure, provided that the alteration will not preclude the structure’s continued designation as a historic structure.

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 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 1006.2.

Unreinforced Masonry

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