This article will discuss engineering, exterior wall sizing, floor systems, and wall assemblies for steel framed mid-rise buildings. Light-gauge steel (LGS) also known as steel framing is a cost-effective and durable solution for building up to 10 floors. It is more affordable than constructing with post-tension concrete slabs, red-iron, and CMU masonry.
This article frequently references metal stud nomenclature; refer to this link to understand the metal stud naming convention.
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Why Build with Metal Framing?
- When compared with traditional wood framing, developers choose steel because it offers better insurance rates and makes buildings easier to maintain and refurbish.
- When compared to concrete or CMU blocks, steel framing is more cost-effective while still providing the same building longevity. Additionally, steel framing results in significantly lower building weight, which can lead to cost savings in foundation design.
Exterior Load-Bearing Steel Framing Wall
Load-bearing walls are the most expensive structural components of a mid-rise building. It is crucial to size them properly. They typically consist of 6″ studs with the gauge varying depending on the floor.
Sizing the exterior walls will be the responsibility of your structural engineer. The stud gauge and flange size typically decreases as you go up each floor. In a typical wall for a 4-story building, you might have 6″ studs at 16″ O.C. framing. You can expect the first floor to use 14 gauge (600S162-68), the second and third floors to use 16 gauge (600S162-54), and the fourth floor to use 18 gauge (600S162-43). Here are some examples of wall schedules.
In this case, they’re using 8″ studs, which allows for a lighter gauge. They are also varying the stud flange size, enabling them to use 16 gauge on level 2 with a 2″ flange, 18 gauge on level 3 with a 2.5″ flange, 18 gauge on level 4 with a 2″ flange, and 18 gauge on level 5 with a 1.62″ flange.
Here’s another blueprint for an 8-floor construction project. They’re also using a variation in stud flange size and gauge to attain the necessary load-bearing strength, beginning with a 3 1/2″ flange (600S350-68) and gradually reducing it to a 2″ flange (600S200-54).
Load-Bearing Corridor / Demising Walls
In mid-rise buildings, the corridor and demising walls are typically load-bearing as well. Their dimensions are usually identical to those of the exterior load-bearing wall studs in order to support the weight of the floor system and the walls above. Typically, either the corridor is load-bearing, or the walls between the rooms or apartments are load-bearing. In the cross-section image below, the corridor walls bear the load to support the joists. Optimization occurs when you use the shortest span for your joists or floor trusses.
In the apartment complex below, it’s more advantageous to have load-bearing walls separating the apartments to support the weight of the trusses.
Load-Bearing Wall Connection Details
Below are some examples of exterior loading walls and interior load-bearing walls with their connection details.
Interior Non-Load Bearing Walls
After you build the load-bearing walls, the rest of the walls are interior and non load-bearing. These walls are typically made of 20 gauge E.Q studs. The metal is usually 23 mils to 18 mils or 24 gauge to 26 gauge. These walls are a lot thinner, easier work with and are significantly cheaper than their structural counterparts. 3 5/8″ is typical for regular walls and 6″ is typical for walls with plumbing pipes. Interior studs are less expensive in terms of material and labor.
Architects must size for STC RATINGS (sound penetration) and UL FIRE RATING (usually 1 or 2-hour fire walls). Below is a non-load-bearing partition designed with an STC of 55 and a 1-hour rating. The elements used to reduce sound penetration are double layers of 5/8-inch drywall and sound attenuation batts.
The below is a detail of a interior wall with 1 HR fire rating and a STC rating of 58. They achieve this by using mineral wool insulation, Resilient channel on one side, and double drywall on other. Another way to reduce sound transmission is by using sound proofing sheets.
Light Gauge Steel Floor Systems
For 100% light-gauge systems, there are two ways to construct floors: joists and/or flat trusses. Often, both methods are used together. Trusses are better for longer spans. Using trusses helps avoid the need for a drop ceiling because they can accommodate air ducts and other MEPs (Mechanical, Electrical, and Plumbing systems). However, many hotel developers choose to have individual AC units in rooms, which allows them to bypass the need for large MEP openings. This, in turn, enables builders to install joists instead of trusses.
The second common floor system is joists. These are more cost-effective and generally easier to work with. Joists have a shorter profile, typically measuring 6″ to 12″ and are typically used in conjunction with a drop ceiling. In most apartments or bathroom units, a drop ceiling is installed in the kitchen and bathroom (to accommodate air ducts), while a higher ceiling is present in the regular room.
Additional padding is often added to reduce sound transmission. There are a few layers to a steel framing joist system. USG outlines a building plan with a two our fire rating on their website.
- Layer 1: Desired Flooring System
- Layer 2: 3/4 Plywood
- Layer 3: 1/4 Sound Mat
- Layer 4: Metal Studs C Joists ranging from 6″ to 14″
- Layer 5: 3-5/8″ Thick Glass Fiber Batt Insulation
- Layer 6: Resilient Channel
- Layer 7: 5/8″ Sheet Rock
Most hotels and apartments feature flat roofs with a parapet. This is a standard design choice because it is cost-effective and provides an easy housing solution for large HVAC units on the roof. Below is a typical example in a 100% light-gauge steel mid-rise construction.
A floor system typically consists of:
- Layer 1: Waterproof Membrane
- Layer 2: 3/4 Plywood or Gypsum Fiber Roof Board
- Layer 3: Insulation Panel
- Layer 4: Optional layer of Gypsum-Fiber Roof Board
- layer 5: Steel Decking
- Layer 6: Joist or Flat Truss