Structure for mid-rise buildings

Marta Szymańska, the structural engineer who helped the housing studio choose between wood and concrete last term, spreads three drawings on the table in week seven: a section through the studio building framed in concrete, one in steel and one with mass timber floors and columns above a concrete podium. 'The building is the same in all three,' she says. 'Same uses, same floor area, same site. But the floor heights are different, the weight on the clay under the site is different, the time on site is different, the carbon is different and the rooms feel different. And in every version, something interesting happens at the top of the second floor, where the grid of the shops and workshops meets the grid of the homes.' This lesson is about structural systems for mid-rise mixed-use buildings and how architects choose among them.

What makes mid-rise structure different

A mid-rise building, roughly five to twelve stories, sits between the light frame of a house or walk-up and the heavy, wind-dominated structure of a tower. Its gravity loads are significant: eight floors of homes, a workshop floor with heavy machines and a ground floor with a grocery's cold rooms. Its lateral loads, from wind and in some regions earthquakes, are moderate but must be resisted by walls, braced frames or rigid frames. Fire resistance becomes a major factor: codes require the structure of taller buildings to survive a fire for one to two hours, which limits the use of combustible materials or requires them to be protected.

Mixed use adds a further complication. Ground floor shops and the second-floor workshop want wide, open spans, about 27 to 30 ft between columns, and tall floors. Homes above want shorter spans, thinner floors and walls that align with the partitions between units. If the column grids of the commercial floors and the residential floors do not match, the loads from the upper columns must be carried sideways at the podium by deep transfer beams or slabs, which are expensive and eat into the floor height.

Loads and the grid

The first step is a grid that suits both parts of the building. In the avenue bar, the residential floors are double-loaded, about 66 ft deep: homes of about 30 ft on each side of a corridor of about 6 ft. That suggests column lines at the two facades and on both sides of the corridor. Along the bar, apartments are typically 21 to 26 ft wide, so a column spacing of about 25 ft would put a column line on every second or third party wall, depending on unit width.

Szymańska shows how the same grid can serve the podium: bays of 25 ft along the avenue suit the grocery and the café, and across the depth of the bar the columns at 30 and 6 ft spacing are awkward for the grocery, whose aisles want clear space. She suggests leaving out the corridor columns at the ground and second floors and carrying the corridor lines on a deeper beam over a 36 ft span: a partial transfer, but a simple one, repeated along the bar.

# Gravity load takedown for one interior column of the avenue bar (studio values, unfactored)
trib = 25 * 18                       # tributary area per floor (ft2)
floors = {
    # floor: (dead load, live load) per unit area, psf
    "roof": (65, 40),
    "residential (6 floors)": (95, 40),
    "maker space": (105, 100),
    "ground floor": (125, 100),
}
counts = {"roof": 1, "residential (6 floors)": 6, "maker space": 1, "ground floor": 0}   # the ground slab bears on grade
total_d = total_l = 0.0
for name, (d, l) in floors.items():
    n = counts[name]
    total_d += n * d * trib / 1000; total_l += n * l * trib / 1000
    print(f"{name:24s} x{n}  dead {n * d * trib / 1000:8.1f} kips  live {n * l * trib / 1000:7.1f} kips")
factored = 1.2 * total_d + 1.6 * total_l
print(f"tributary area {trib:.1f} ft2; unfactored dead {total_d:.0f} kips, live {total_l:.0f} kips")
print(f"factored load at the base of the column ({1.2} D + {1.6} L): {factored:.0f} kips")

roof                     x1  dead     29.2 kips  live    18.0 kips
residential (6 floors)   x6  dead    256.5 kips  live   108.0 kips
maker space              x1  dead     47.2 kips  live    45.0 kips
ground floor             x0  dead      0.0 kips  live     0.0 kips
tributary area 450.0 ft2; unfactored dead 333 kips, live 171 kips
factored load at the base of the column (1.2 D + 1.6 L): 673 kips

The column at the base of the avenue bar carries roughly 670 kips of factored load, about a fifth of it from the single workshop floor, which is designed for heavier live loads than the homes. The numbers are approximate, the live loads can be reduced for large tributary areas, and the code's load combinations are more detailed than this simple one, but the result is enough to size a column roughly and to remind the students that the workshop floor matters.

The rest of the load path follows from earlier courses. Loads travel from floors to beams or slabs, to columns and walls, to the foundations and into the ground. Under the studio site, as under much of downtown Minneapolis, the soils need investigation; the studio assumes glacial soils over limestone bedrock, with piles or caissons under the columns. Heavier buildings need bigger foundations, so the weight of the structure itself affects cost below ground.

Three systems

Szymańska compares three systems that are common for buildings of this size.

• Concrete flat plate: a reinforced concrete slab, about 8 to 9 in thick, spanning directly between columns without beams, sometimes post-tensioned with high-strength steel tendons to span farther with a thinner slab. Concrete is fire-resistant, stiff and quiet, and the thin slab without beams keeps floor-to-floor heights low, about 9 ft 6 in to 10 ft for homes. It is heavy, it is slower to build than prefabricated systems, and its embodied carbon is high.
• Steel frame with composite deck: steel beams and columns with a concrete topping on a metal deck. It is light and fast to erect, spans well and adapts easily to transfers, but needs fire protection, deeper floor zones that add height, and careful acoustic design for homes.
• Mass timber over a concrete podium: cross-laminated timber floor panels on glulam beams and columns, above a concrete ground and second floor. Mass timber is prefabricated, light and quick to erect, stores carbon from the trees, and can be left partly visible inside homes. It needs fire protection, attention to acoustics and moisture during construction, and a grid that suits panel sizes.

Comparing the systems

# Compare three structural systems for the six residential floors (studio values, assumed)
systems = {
    # name: (floor-to-floor ft, structure self-weight psf, embodied carbon kg CO2e per ft2 of floor,
    #        relative frame cost, weeks per floor on site)
    "concrete flat plate": (10.0, 110, 8.8, 1.00, 1.6),
    "steel and composite deck": (10.67, 63, 10.2, 1.05, 1.2),
    "mass timber on concrete podium": (10.5, 50, 4.2, 1.08, 0.8),
}
area = 53000               # residential floor area of the six upper floors (ft2)
for name, (fh, w, co2, cost, weeks) in systems.items():
    height = 30 + 6 * fh
    print(f"{name:32s} floor to floor {fh:.2f} ft, total height {height:.1f} ft, "
          f"self-weight {w} psf, frame carbon {co2 * area / 1000:5.0f} t, cost index {cost:.2f}, "
          f"about {6 * weeks:.0f} weeks for six floors")

concrete flat plate              floor to floor 10.00 ft, total height 90.0 ft, self-weight 110 psf, frame carbon   466 t, cost index 1.00, about 10 weeks for six floors
steel and composite deck         floor to floor 10.67 ft, total height 94.0 ft, self-weight 63 psf, frame carbon   541 t, cost index 1.05, about 7 weeks for six floors
mass timber on concrete podium   floor to floor 10.50 ft, total height 93.0 ft, self-weight 50 psf, frame carbon   223 t, cost index 1.08, about 5 weeks for six floors

The figures are studio estimates, not design values, and the carbon figures in particular depend on the cement, steel and timber supply, but they show the pattern that real comparisons often find. Concrete gives the thinnest floors and the lowest building, and it is the heaviest. Steel is light but adds height and carbon. Mass timber is the lightest, the fastest to build and the lowest in embodied carbon for the frame, but the most expensive in this estimate, and its floor is deeper than the concrete plate because of the acoustic topping and ceiling layers that homes need. The height difference matters on this site: with a height limit of 100 ft, a deeper floor system leaves less room for the stepped top floors and the rooftop equipment.

What the code allows

Whether a timber structure is allowed at this height depends on the building code. The 2021 and 2024 International Building Codes added three types of mass timber construction, Types IV-A, IV-B and IV-C, which allow buildings of up to nine, twelve and eighteen stories depending on how much of the timber is protected with gypsum. Minnesota's current building code is based on the 2018 IBC, which does not include those types, and the state began rulemaking in 2025 to adopt the 2024 edition; the studio assumes the new edition will be in force when the building is permitted, and notes the risk. The studio building, with six timber floors over a two-story concrete podium, falls within these limits.

In Minneapolis, T3, a seven-story office building of timber over a concrete base in the North Loop, completed in 2016, showed what mass timber could do downtown before the new code types existed. Projects like these helped build the experience and the test data behind the code changes.

The podium and the transfer

At the top of the second floor, the studio building changes. Below, the concrete podium spans wide bays for the grocery and the workshop and supports the weight of everything above. Above, the residential floors use a closer grid and a lighter system. The podium roof, the slab at the top of the second floor, does several jobs at once: it is the transfer level for the corridor columns, the fire separation between the commercial and residential uses, the acoustic barrier between the metal shop and the homes, and the floor of the residents' courtyard garden, where it must carry soil, trees, snow and waterproofing.

Szymańska recommends a thick concrete slab at that level, about 12 in, with downturned beams along the corridor lines where the upper columns land. The courtyard part of the slab carries planting soil up to 2 ft deep in places, which weighs about 200 to 250 psf when saturated, more than twice the weight of a residential floor. 'Gardens are heavy,' she tells Ingrid, who had drawn mature trees all over the courtyard. 'Put the trees over the columns.'

Lateral systems

Wind pushes on the building's broad avenue face and must be carried to the ground. In a building of this height, the stair and elevator cores, built as concrete shear walls, are the natural lateral system: they are stiff, they are needed anyway, and they run from the foundations to the roof. In a mass timber building, the cores are often concrete or cross-laminated timber walls, and the timber floors act as diaphragms, horizontal plates that collect the wind load and deliver it to the cores. The positions of the cores therefore matter structurally: two cores, at the inside corner of the L and at the Harland Building's wall, are far apart and resist twisting well. Seismic loads are low in Minneapolis, so wind governs.

Structure and architecture

Structure is not only a technical decision. It shapes the rooms, the facade and the feel of the building. A concrete flat plate gives smooth ceilings and freedom to place partitions anywhere. Exposed timber ceilings, where the code allows, give homes warmth and a sense of craft, and in the Foundry Collective's spirit, a building of wood above a base of concrete and brick echoes the timber-framed warehouses next door. Columns on the facade create the rhythm that the facade lesson will develop. The deep beam at the podium roof can be expressed on the facade as a horizontal band, like the cornice line of the older buildings.

The students also think about time. Downtown construction disrupts neighbors, sidewalks and traffic, and every week on site costs money. Prefabricated mass timber floors can be lifted into place at about a floor a week, shortening the frame schedule by months. For Riverlot, which pays interest on its construction loan every month, speed has real value.

Exercise 7: structure

This exercise takes one week.

• Draw the structural grid on the ground, second and typical residential floor plans, and mark any transfers.
• Do a gravity load takedown for one interior and one facade column.
• Compare two structural systems for the residential floors by floor-to-floor height, weight, carbon, cost and time.
• Draw a section through the podium roof at 1 in = 2 ft showing the slab, beams, waterproofing, insulation and planting soil.
• Show the lateral system and the cores in plan.

Self-critique checklist

• Do the upper columns land on the podium structure without costly transfers?
• Does my floor-to-floor height fit the height limit?
• Where are the heavy loads, and are they over columns?
• Does the structure contribute to the architecture?