Code compliance in existing buildings

Nadia Kostyshyn, the former plans examiner who taught the students' code course in the same term, joins the Stanfield studio in the ninth week with a short opening statement. 'Building codes are written for new buildings,' she says. 'When you apply them to a building of 1911 word for word, almost everything fails. The stairs are too steep, the walls have no fire rating anyone can find in a table, the floors are wood, the windows are single glazed, there is one exit and an iron fire escape. If the code allowed only full compliance, nearly every historic building in the country would have to be gutted or demolished. So the codes and the officials who apply them have learned other ways to reach the same goal: a building that is reasonably safe, healthy and accessible, without destroying what makes it worth keeping.'

The basic problem

The purpose of a building code is to set minimum standards of safety and health, and its requirements are written as specific dimensions, ratings and systems that a new building can be designed to meet. An existing building was designed to an older code, or to none, and it usually meets few of today's requirements exactly. Three principles govern how codes treat it. First, an existing building that is not being changed may generally continue in use as it is, unless it poses a clear hazard; the code does not require owners to upgrade every building each time the code changes. Second, new work must generally meet the current code. Third, when a building is altered extensively, changes use, or is enlarged, the code may require parts of the existing building to be upgraded as well, in proportion to the scale and nature of the work. Converting the Stanfield from a warehouse to homes is a change of use, the most demanding trigger of all, because the risks to people who sleep in a building are very different from those in a building used for storage.

Existing buildings under the American codes

Most American jurisdictions regulate work on existing buildings through the International Existing Building Code, published by the International Code Council alongside the International Building Code. The code offers three methods of compliance, of which the designer chooses one. The prescriptive method sets requirements for repairs, alterations, additions and changes of occupancy in a single chapter. The work area method classifies the work by its scope and applies requirements in proportion to it. The performance method scores the building's safety in a set of categories, such as fire separation, egress and sprinklers, and allows deficiencies in one category to be offset by strengths in another. Minnesota adopts a version of this code for existing buildings, and its building codes are currently based on the 2018 International Building Code, with rulemaking under way to adopt the 2024 editions.

Under the work area method, alterations are classified in three levels. Level 1 alterations remove and replace existing materials, elements, equipment or fixtures with new ones serving the same purpose, such as new finishes or new windows. Level 2 alterations reconfigure space, add or remove doors and windows, or extend systems. Level 3 alterations are those in which the work area exceeds 50% of the building's aggregate area, and they carry the most extensive requirements for upgrading the building, in addition to the Level 1 and Level 2 requirements. A change of occupancy triggers its own chapter, which addresses the new use's demands for fire protection, egress, structure and accessibility. The code also includes a chapter for historic buildings, which allows certain features to be accepted or modified where full compliance would damage historic character, with the approval of the code official.

Change of use: what the Stanfield must provide

The students list what changing the Stanfield to homes requires, drawing on both codes and on Kostyshyn's experience of how officials apply them. The building must have an automatic sprinkler system throughout. This is the single most important upgrade in any conversion, because sprinklers control fires early and allow many other features of the existing building to be accepted. It must have a fire alarm and smoke alarms in every home. Each home must be separated from the others and from the corridor by fire-rated construction, typically one hour, and the stairs and elevator shafts must be enclosed in rated shafts. There must be at least two exits from every floor, separated from each other, with travel distances within the limits for a sprinklered residential building. Homes must have windows that meet the requirements for light, ventilation and, in some cases, emergency escape. The structure must carry residential loads and be checked for the effects of the new addition. Accessible routes must reach the ground floor and the homes on every floor served by elevator, and a share of the homes must be accessible.

The building already meets some of these requirements by virtue of how it was built. Its floors were designed for warehouse loads of farm machinery, several times the load of homes, so they need no strengthening for the new use, as Szymańska confirmed. Its heavy timber construction, with large posts and beams and thick plank floors, is recognized by both codes as having substantial fire resistance, more than light wood framing of the same weight. The International Building Code classifies buildings with heavy timber members of minimum sizes as Type IV-HT construction. The question is whether the existing assemblies can achieve the ratings required between homes, or whether they need added protection.

Fire separations without losing the timber

Fire resistance of the timber frame is the central technical question of the Stanfield's conversion, because the most obvious way to achieve fire ratings, covering every beam and floor with gypsum board, would hide the character-defining elements the students have worked to keep. Heavy timber resists fire because wood chars at a fairly steady rate, roughly 1 1/2 in per hour for many softwoods under standard fire exposure, and the char layer insulates the wood inside. A beam that is large enough keeps enough sound wood, after an hour of charring on its exposed faces, to carry its load. The students check the Stanfield's beams by this method.

# Residual section of a Stanfield floor beam after a one-hour fire (three faces exposed)
b, d = 10.0, 16.0            # beam width and depth, in (measured)
rate = 1.5                   # char depth in one hour, in (typical softwood)
zero_strength = 0.3          # additional layer assumed to carry no load, in
loss = rate + zero_strength
b_res = b - 2 * loss         # both sides exposed
d_res = d - loss             # bottom exposed, top protected by the floor
S0 = b * d ** 2 / 6
S1 = b_res * d_res ** 2 / 6
print(f"residual section {b_res:.1f} x {d_res:.1f} in")
print(f"section modulus retained: {S1 / S0:.0%}")
# bending demand in fire (dead + reduced live load) vs capacity at a higher fire-case stress
span, trib = 22.0, 18.0      # ft
w = (33 + 12) * trib         # lb/ft: existing floor and topping + residential live load share in fire
M = w * span ** 2 / 8 * 12   # lb-in
f_fire = 2900.0              # psi, assumed short-term bending strength in the fire case
Mr = f_fire * S1             # lb-in
print(f"demand {M / 12000:.0f} kip-ft, residual capacity {Mr / 12000:.0f} kip-ft -> {'OK' if Mr >= M else 'needs protection'}")

residual section 6.4 x 14.2 in
section modulus retained: 50%
demand 49 kip-ft, residual capacity 52 kip-ft -> OK

The calculation, which the structural engineer will repeat with the values the codes prescribe, suggests that the beams retain about half their section modulus after an hour and enough strength to carry the reduced loads assumed during a fire. The posts, which are larger, do even better. The plank floors are the weak point: a floor of planks on edge, with gaps between boards and no topping, would let fire and smoke through long before an hour. The acoustic topping the students specified in the previous lesson solves this problem too. A continuous layer of concrete or gypsum over the planks seals the gaps, adds mass and, with the planks below, can achieve the required rating, which the team will demonstrate by reference to tested assemblies or by analysis. The walls between homes are new construction and are built to rated designs. The timber remains exposed, and the sprinklers provide a further layer of protection.

Exits, stairs and the fire escape

The Stanfield has one original stair, in an open well at the north-west corner, with treads of oak worn hollow by a century of use and risers higher than any modern code allows. It has the freight elevator, and it has an external iron fire escape on the lane, rusted beyond repair. The conversion needs two enclosed exit stairs. The students keep the original stair as one of them, enclosing it in a rated shaft with a glazed wall facing the corridor so that it remains visible, and accept its steep risers on the basis that it is an existing stair, that the building is sprinklered and that a second new stair of full code geometry is provided. Kostyshyn says this is the kind of trade-off officials often accept, but only when it is argued clearly and offset by improvements elsewhere. The second stair is new, placed against the east party wall near the light court, where it removes the least fabric. The fire escape is recorded and removed. The new passenger elevator rises beside the original stair; the freight shaft becomes a service riser.

Doors, guards and handrails raise smaller questions. The original stair's wooden railing is too low for current guard heights; rather than replace it, the students add a slender steel guard rail behind it, so the old railing stays in place and the new one provides the required height, a common solution. Existing doors to be kept on rated walls, such as the paneled office doors of the mezzanine, will be upgraded with intumescent seals and closers, or kept as non-rated doors where they are not on an exit path.

Accessibility

The Stanfield has no step-free entrance; its avenue doors are three steps above the sidewalk and the loading dock on the lane is at truck height. When a building is altered, the Americans with Disabilities Act requires the altered areas to be accessible, and where a primary function area is altered, the path of travel to it must also be made accessible up to the point where the cost becomes disproportionate, defined as more than 20% of the cost of the alteration. A change to housing also brings the Fair Housing Act's requirements for covered multifamily dwellings, and the building code's requirements for accessible units. For qualified historic properties, the ADA standards allow alternative minimum requirements where full compliance would threaten or destroy historic significance. The students provide a step-free entrance on the avenue by lowering the floor of one bay of the ground floor, inside the storefront that is being replaced anyway, so that the historic steps at the main door remain. The elevator serves every floor, all common spaces are accessible, and accessible units are distributed through the building. The mezzanine's woodwork room becomes a meeting room reached through a new short bridge from the third floor.

Energy upgrades and the risk to masonry

Energy codes treat existing buildings more leniently than new ones, and many exempt historic buildings in whole or in part where compliance would compromise historic character. But the Stanfield's homes need to be comfortable and affordable to heat, and the obvious way to improve its solid brick walls, adding insulation to the inside, carries a real risk. A solid masonry wall stays dry because heat from inside warms it and lets the moisture it absorbs from rain evaporate. Insulating it on the inside makes the brick colder in winter, so it dries more slowly and spends more time near freezing while wet, which increases the risk of the freeze and thaw damage Petrenko showed the students. The risk depends on the brick, the exposure and the climate, and it can be assessed with hygrothermal modeling, computer simulation of heat and moisture movement through the wall over many years of weather, together with laboratory tests of the brick's properties.

The engineers' advice for the Stanfield is typical. Insulate the roof thoroughly, where most heat is lost and the risk is least. Repair and storm the windows. Seal air leaks, especially at the roof and around windows. Insulate the walls on the inside with a modest thickness of vapor-open insulation, such as mineral wool or wood fiber, on the sheltered lane and party walls, after modeling confirms the brick will tolerate it, and keep the exposed parapet and avenue elevation, where wetting is greatest, less insulated or protected by careful detailing of caps and sills. Above all, keep water out of the walls, because a dry wall can tolerate insulation that a wet one cannot.

Exercise 9: a code strategy

• List every code requirement triggered by converting a historic building near the school to housing, and mark which the existing building already meets.
• Calculate the residual section of a heavy timber beam after a one-hour fire and compare its capacity with the fire-case load.
• Propose a compensating measure for each deficiency you choose to accept, and explain how it achieves an equivalent level of safety.
• Identify where interior insulation can safely be added to a solid masonry wall, and where it should not.

Self-check

• Have I identified the trigger, such as a change of use or the scale of alteration, that sets the requirements?
• Have I distinguished the features the existing building already provides from those it lacks?
• Have I proposed compensating measures for accepted deficiencies, rather than asking for exemptions?
• Have I protected the character-defining elements while meeting the code's intent?