HYBRID STEEL-TIMBER
FLOOR SYSTEMS
ACOUSTIC FLOOR ASSEMBLIES
The acoustic performance of hybrid steel-timber floor assemblies will vary depending on the number of layers
in the CLT panel, ply thickness, wood species, and other factors. The floor assembly may require an acoustic
mass and/or resilient underlayment to meet a specific STC or IIC criteria. Some options for acoustic floor
assemblies are included in this section, with more available in Design Guide 37 Section 4.3.
Smart Reference for Architects
Example Floor Assemblies
Rhode Island School of Design
North Hall by NADAA
Floor
finish
Acoustic
mass*
STC
IIC
Bare CLT 5-ply
42
26
Resilient
underlayment
1½ in. precast concrete,
½ in. rubber membrane
56
48
Timber
structure
10mm wood floor,
2mm closed cell foam,
1½ in. precast concrete,
½ in. rubber membrane
55
51
1½ in. precast concrete,
9mm closed cell foam, CLT 5-ply,
16mm Type X gypsum board
installed on Z channels
70
56
*common acoustic
mass options include
concrete and gypsum concrete
Refer to Sabourin (2015) for full floor assemblies table.
Fig. 4-8. STC and IIC ratings for floor assemblies (Sabourin, 2015).
TYPICAL BAY AND FLOOR ASSEMBLY
BUILDING
A RELIABLE LCA
Hybrid steel-timber systems take advantage of two different structural materials’
characteristics to optimize structural performance and aesthetics—and that starts with
a steel frame.
Longer beam spans: Typical mass timber
construction spans are 20 ft to 25 ft. Adding
a steel frame allows 30- to 45-ft spans while
maintaining a shallower structural depth.
Smaller column sizes: Structural steel
reduces column profiles and preserves
valuable leasable space.
Circular materials: Steel
members produced domestically
are made of 92% recycled scrap—
and they’re infinitely recyclable,
without loss of properties.
Improved vibration performance:
Hybrid systems that use steel framing are
generally stiffer than all-timber buildings and
therefore are more appropriate for vibration
sensitive spaces and machinery.
Global Warming Potential
(GWP)
Steel column
Finished floor
Sound
isolation
(as required)
3-ply CLT (4½ in.)
Steel framing
Unit of measure for greenhouse
gas emissions in carbon dioxide
equivalent, or kg CO2 eq. Structural
steel GWP is expressed in kg CO2
eq. per kg steel.
Environmental Product
Declarations (EPDs)
Third-party verified documents
that provide data on the GWP of
various materials and products.
Most material EPDs are cradle-togate, capturing the raw materials,
transportation, and manufacturing,
but not necessarily end-of-life
considerations.
Finding EPDs
Architects must use a combination of structural
steel and CLT EPDs to calculate the GWP of a
hybrid
steel-timber
system.
Fig.
4-8. STC
and IIC ratings
for floor assemblies (Sabourin, 2015).
5-ply
CLT
W24
Greenhouse gas emissions
associated with a building
material or component, including
manufacturing, transportation,
installation, maintenance, and
disposal of that building material.
It’s important to check EPDs to
ensure that all phases are included in
calculations.
3 in.
concrete
topping
W24
Embodied Carbon
Floor Assembly
Fig. 4-8. STC and IIC ratings for floor assemblies (Sabourin, 2015).
Fig. 4-8. STC and IIC ratings for floor assemblies (Sabourin, 2015).
Structural steel: aisc.org/epds offers a
variety of resources including third-party-vetted,
industry-average EPDs for products like hotAISC DESIGN
37 / HYBRID
STEEL FRAMES WITH WOOD FLOORS / 43
rolled sections,
plate,GUIDE
and hollow
structural
AISC DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 43
sections (HSS)
as wellGUIDE
product-specific
EPDs FRAMES WITH WOOD FLOORS / 43
AISC DESIGN
37 / HYBRID STEEL
from individual U.S. mills.
1 in.
acoustic
mat
377∕8"
WHY USE A HYBRID SYSTEM?
SUSTAINABILITY
TERMINOLOGY
1. reduced foundation sizes due to lower
structural weight
2. lowered embodied carbon due to lateral
system selection
3. simplified façade connections
W24
What is it? A structural gravity system composed of steel
framing and panelized cross-laminated timber (CLT),
an engineered wood product consisting of perpendicular
layers of dimensional lumber glued into a thicker panel.
40'-0"
Josh Horner Photography
This office bay is designed for 80 psf live load and 20 psf superimposed dead load.
Refer to Design Guide 37 Section 2.4.2 for more details.
Life cycle AISC
assessments
(LCAs)
for
buildings
DESIGN GUIDE 37 / HYBRID STEEL FRAMES WITH WOOD FLOORS / 43
compare the GWP against a baseline value.
Early LCAs for bay studies are a useful tool
Typical Bay
W27
for comparing hybrid systems against other
options, but will not capture:
Approx. 67∕8 in.
5-ply CLT
Non-composite
beams. Composite
action requires
additional
detailing
and design
considerations.
W27
30'-0"
CLT: Industry-average EPDs are not available at
the time of publication. Contact manufacturers
for product-specific EPD information.
FIRE PROTECTION
Although structural steel is classified as
a non-combustible material, timber is
combustible and is subject to limitations
that vary by jurisdiction. Check with
local authorities for the latest on timber
protection. Refer to IBC Type IV-A, IV-B,
and IV-C for model building code fire
protection requirements. Design Guide 19,
Fire Resistance of Structural Steel Framing,
and aisc.org/fireprotection provide more
guidance on the fire protection of steel.
1 in. noncombustible covering
CLT panel (all)
Steel
beam
(all)
Intumescent
paint (all)
1. Fully Exposed CLT
(Type IV-C Construction)
2 layers of 5∕8 in.
Type X board
2. Partially Exposed CLT
(Type IV-B Construction)
3 layers
of 5∕8 in.
Type X board
3. Enclosed CLT
(Type IV-A Construction)
The design team was inspired by the building typology of
turn of the century warehouses that expressed the structural
system. The hybrid steel-frame structure with CLT floors
HYBRID STEEL-TIMBER
FLOOR SYSTEMS
matched this intended design aesthetic. Leaving the timber
Smart Reference for Architects
structure exposed created a warm palette that was intended
to be memorable and desirable for creative office tenants.
CONSTRUCTABILITY
The project team credits
this choice in material with supporting faster leasing and higher lease rates.
The leasing requirements of this office project demanded
Collaboration
Design-assist collaboration strategies
to hybridcolumn
steel-timber
45 ftare
× well-suited
30 ft structural
bays, which is traditional for
projects. Early team involvement simplifies trade coordination during the
many
office
floor
plates.
Post
and
design phase. See Design Assist: Collaborative Design Approach Guidelines beam glulam construction
for the Fabricated Structural Steel
Industry,
publishedbut
by AISC
and AIA
was
considered
would
have required very deep glulam
Contract Documents in 2024, for further guidance at aisc.org/designassist
beams to adequately achieve the spans. The hybrid steelAt the time of publication, thereframe
are no standardized
tolerance
definitions
system was
chosen
to utilize the strength and flexibilfor CLT panels. The contracting team should establish mutually acceptable
ity
of
the
steel
structure.
This
system allowed for shallower
tolerances at the outset.
beam profiles that could be penetrated to accommodate MEP
systems.
Reinforced CLT Opening (Plan
View) The LFRS for the project is steel braced frames sur3×3 reinforcing
rounding the interior service
angles cores.
either
4 in. min to 8 in. max
side of
opening
The 5-ply, 6d in. CLT
panels
span approximately 9 ft
steel beams. The CLT panels were SPF with a
4 ft-0 in. between
min
douglas fir (DF) bottom layer. The panels were
planed rather
Castellated or
than sanded which provided a more rustic cellular
appearance
for a
beams can
alsocan
easebe
integration
6 in. o.c. system
reduced cost. The hybrid structural
seen in
of building systems
typ.
Figure 2-8.
while further reducing
the weight
of the structure.
Given the project height and3 area,
is Type
IIIin. o.c. the building
> 4 in. diameter
and
A, which requires a 1-hour fire-resistance rating (FRR) for
≤ 8 in. diameter
the primary structural frame and floors andPrefabrication
a 2-hour FRR for
the shafts. The project was designed and built under the 2012
IBC (ICC, 2012), which did not recognize CLT as a building
material. The project team worked with the city of Austin
using existing test data and analytical approaches to justify CASE STUDY:
HOUSTON
ENDOWMENT HEADQUARTERS
the 1-hour FRR of the CLT panels as a floor
system. The
project team utilized conventional steel and gypsum wall
shaft construction to achieve a 2-hour FRR for the shaft. The
exposed steel was protected with intumescent paint.
The design team engaged CLT suppliers at the early stages
of design to ensure success utilizing this innovative structural
system. The supplier was engaged in a design-assist process,
working with the design team to design panel layouts and
connections for the project. The design team and fabrication team utilized building information modeling (BIM) and
prefabrication on this project, but early trade coordination of
the MEP systems could have allowed for further leveraging
of the prefabrication abilities of mass timber and steel.
The floor assembly consisted of 3 in. concrete topping on
a 4 in. acoustic mat on a nominal 7 in., 5-ply CLT panel
to achieve the acoustic requirements of the project, as illustrated in Figure 2-9.
2.2.3
Houston Endowment Headquarters
The Houston
Endowment Headquarters
shown in FigWaugh Thistleton Architects
Kevin Daly Architects
ure 2-10 in downtown Houston, Texas, leverages the benefits
of steel framing and CLT floor panels for a cost-effective
Offsite fabrication offers strategic advantages for speed and
quality control. Shop-fabricated openings in steel beams and
CLT panels can ease MEP integration.
The Client
The Endowment is one of the largest
philanthropic organizations in the city.
The Structure
Suggested Erection
Tolerances
3
A two-story, 40,000-sq.-ft office
building that sits atop a parking
garage and is topped with a steelframed photovoltaic (PV) canopy.
greater of ½ in. or
0.25% of shortest side
The design team originally planned
the building in concrete. They
switched to a hybrid system to
reduce cost and schedule, reuse
existing building foundations, and
accommodate the feature PV canopy.
The Houston Endowment
Headquarters uses steel framing to
cantilever up to 15 ft from column
lines, with a maximum bay size of
about 30 ft by 30 ft. Three-ply CLT
panels span approximately 10 ft
between the steel beams.
surface levelness of ¼ in.
in 10’-0” MAX
Screw Layout for Beam-Panel Connections
24 in. MAX typical
spacing staggered
on each side
of web
AISC (2022), Design Guide 37: Hybrid Steel
Frames with Wood Floors, American
Institute of Steel Construction, Chicago Ill.
AISC (2023), Steel Construction Manual,
16th Edition, American Institute of Steel
Construction, Chicago, Ill.
AISC (2024), Environmental Product
Declarations, American Institute of Steel
Construction, http://aisc.org/epds
AWC (2024), National Design Specification
(NDS) for Wood Construction, American
Wood Council, Leesburg, Va.
CLF (2023), North American Materials
Baselines, Version v2, Carbon Leadership
Forum, https://carbonleadershipforum.org/
clf-material-baselines-2023
CLF (2019), Life Cycle Assessment of
Buildings (LCA): A Practice Guide,
Version 1.1, Carbon Leadership Forum,
https://carbonleadershipforum.org/
lca-practice-guide
Sabourin, I. (2015), “Measurement of
Airborne Sound Insulation of 8 Wall
Assemblies Measurement of Airborne
and Impact Sound Insulation of 29 Floor
Assemblies,” Nordic Engineered Wood
Report, No. A1-006070.10, National
Research Council of Canada,
Ottawa, Ontario.
SOM (2017), AISC Steel & Timber Research
for High Rise Residential Buildings,
Skidmore, Owings, and Merrill, LLP,
Chicago, Ill.
WoodWorks (2021), U.S. Mass Timber
Construction Manual, WoodWorks®,
Washington D.C.
The Hybrid Solution
+∕- 3 ∕8 in.
elevation
∕16 in. MAX
ADDITIONAL RESOURCES
Lesson Learned
EQ.
EQ.
EQ.
Odeh Engineers, Inc.
Fig. 2-6. CLT panels precut around columns (photo courtesy of Odeh Engineers, Inc.).
Kevin Daly Architects
Architects designing façades should
avoid protruding plates, bolts, and
other elements on the steel beam top
flange that would prevent the masstimber panels from bearing firmly
on the supporting steel framing.
Refer to AISC Design Guide 22:
Façade Attachments to Steel-Framed
Buildings, for more details.
NEED MORE INFO?
AISC Design Guide 37: Hybrid Steel Frames
with Wood Floors is a multi-disciplinary
review of the design considerations that
impact hybrid steel-framed structures.
Need your own
copy? Visit
aisc.org/
architecture or
scan the QR code.
Have questions?
Email us at
architects@aisc.org.