Structural Design of Light Gauge Steel / Cold-formed Steel

-For simple residential homes, the International Residential Code, IRC, provide a prescriptive Method for light gauge steel/ cold formed steel.

Section R603.1 General:
The provisions of this section shall control the construction of exterior steel wall framing and interior load-bearing steel framing for buildings not more than 60 feet long perpendicular to the joists or truss span, not more than 40 feet wide parallel to the joist or truss span, and not more than two stories in height. All exterior walls installed in accordance with the provisions of this section shall be considered as load-bearing walls. Steel walls constructed in accordance with the provision of this section shall be limited to sites subjected to a maximum design wind speed of 110 miles per hour, Exposure A, B, or C and a maximum ground snow load of 70 psf.

For big residential homes or commercial, The International Building code, call for the Cold-Formed Steel Design, AISI Manual, by American Iron and Steel Institute

7-50 GENERAL

a- There is free Technical Assistance from the Steel Framing Alliance. 1140 Connecticut Avenue, NW, Suite 705, Washington, D.C. 20036 www.cfsei.org

7-52-Light Gauge Steel connections.

a- Simpson has a special catalog for light gauge steel, please use it. Do not use the Simpson wood catalog for light gauge steel unless you call the technical support of Simpson company and they say it will work.

7-53 -Light Gauge Steel studs.

• The top of wall track in a light gauge steel wall cannot support loads therefore wall studs MUST be lined up with joist/rafters/trusses or any load from above AT ALL FLOORS.
• Most contractors like the spacing between the studs to be 24 inches not 16 inches. This is also good because it matches the trusses spacing which is usually spaced at 24 inches on center.
• To increase capacity of studs:
  • Increase gauge of studs.
  • Double studs @ spacing required by framing members above. For example: If the floor spacing is 24″ O.C. and stud spacing is required to be 12″ O.C. to achieve the loads required you cannot space studs @ 12″ O.C. instead you use double studs @ 24″o.c. to match framing members above.
• If spacing of studs does not match framing members above use an appropriate header such as L-Shape header to transfer the load to studs.(see header section of Light Gauge Steel pink binder)
• Multiple studs can be used as a column to support loads from beams or girder trusses above provided:
  • If the multiple studs are stacked parallel to the length of the beam or the length of the truss, the length of the beam or the truss must be extended to span over the entire number of studs needed to support the beam or the truss.
  • If the multiple studs are stacked perpendicular to the length of the beam or the length of the truss, the width of the beam or truss can not be more the total width of the beam or the total width of the truss.
  • To calculate the capacity of multiple studs, just use the capacity of single studs multiplied by the number of studs. Do not use the combined moment of Inertia to count for the composite section. Just simple calculation.

    If the above conditions cannot be met, use PACO column or steel tube column instead of multiple studs.

• When you are using the light gauge straps as bracing, Studs lateral bracing shall be provided by bridging only. From the basic details, keep the bridging details only and remove the blocking/strapping and the sheathing details that are used to laterally brace the studs.

7-54 Showing up wall size on plane:

Wall shall be shown by a line with two arrows specifying the start and end of wall. The line shall have a wall designation mark such as W1 or W5. A table showing wall designation with the size of the studs shall be shown on the floor plan. See project from John hanson.

Be Careful. If you use Pre-engineered products such as Hardy panel or PACO steel or other products, You cannot use the entire wall length at garages. This is because the rough openings for garage doors are 4 inches wider than the garage door size shown on the architectural drawings.

7.56 Number of braces per wall ( one side or two sides of the wall:

For the following buildings; For single story house, Two story house, Shops, and Two story apartment complexes and hotels: Brace on exterior side of the wall only ( two total per wall in a cross configuration) is preferred by most of contractors. This is especially possible for area of low seismic load and or when wind control

This is because the inside brace interfere with the sheet rock on the inside face of the wall. The current PSE criteria is that if horizontal force can be resisted by a single 3 inches wide gauge 16 strap, then use one strap on the exterior side of the wall only.

If the force need more than one 3 inches wide gauge 16 strap, then the force is big and we are worried about the studs to be twisted. In this case, use two straps, one each side wall better

B-For larger structures and /or three stories buildings, Braces on both sides of the wall ( Two on each side on the wall in a cross configuration, four braces total per wall ), is recommended. This is because brace on one side of the wall produce eccentric axial force on the studs.

If the contractor object to braces on both sides of the wall. And if there are enough walls in the building, one idea is increase the number of braces in the lower stories and less number of braces on the top story with braces on the exterior side of the walls only.

Another idea is to keep the top story braces on the exterior side of the wall only. While for the lower story the straps can be on both sides of the wall.

7-57 Roof:

7-57-20 Roof framing with rafters and purlins:

• Z section is better than C section because you can easily lap the Z section at support with the Z section face to face. If you lab the C section they have to back to back. This require the seam lines of the roof metal deck to be shifted between panel which is impossible to achieve.
• Rigid insulation above the roof deck is better and more economical than the pat insulation below the metal deck between the purlins and or rafters.

7- 59-10 Mixed System of Red-Iron and light gauge steel.

It was very difficult if not impractical to design a connection for stacked red-iron braced frame in a floor by floor plate form fashion of construction. However, stacking light gauge steel braced frame in the upper top stories over red-iron braced frames in the lower story is promising. Be cupful, the low R factor for the light gauge steel will be applicable for both the red iron and the light gauge steel. So this mixed use of light gauge steel is doable for slab on grade project.

For project when the steel starts on concrete parking garage below, the mixed system will produce high reactions that require huge concrete beams in parking structure roof.

7-60 Connection between floors when using light gauge steel bracing.

“Connection of the diagonal bracing member, top chord splice, boundary members and collectors shall be designed to develop the full tensile strength of the member OR sigma times the otherwise prescribed seismic forces.”

However in the seismic design Manual example 3, they ignored the second option of designing for the full strength of the member which in many cases may be smaller than the Amplified seismic forces.

• The example just multiply the uplift forces by the Seismic Amplification factor. In this case, the seismic forces is at the allowable stress level.
• The example for boundary elements ( multiple studs or steel post) , used the seismic amplification factor above to increase the load. However, the example increased the allowable load the member can carry by 1.7 . I recommend this way.

For compression member, the full capacity is defined as the allowable axial load Fa multiplied by the 1.7 factor. Remember that Fa could be less than the yield strength Fy based on the buckling stress. Please refer to UBC Page 2-255. Also, refer to Steel manual ASD.

For studs the easiest way to get the allowable axial compression load is from the stud tables. For any other member such as PACO column get it from PACO Tables. For other members, you can use RISA software very easily.

For tension straps, the example used the Seismic amplification factor above to increase the load. However, they did not increase the capacity by the 1.7 factor. I recommend this way. If you cannot use the above method for any reason, instead of using the amplified seismic load, you can use the full capacity of the diagonal brace to design the strap. For tension member, the full capacity of the member is defined as area of steel multiplied by the yield strength . Then, calculate the vertical component of the diagonal brace to choose the strap.

Note that for Los Angeles, they reduce the capacity of every thing. So. Please check LARR( Los Angles research reports)

The most economical way to resist the uplift forces is to use straps. It could be used on one side only or on both sides of the wall for big forces. Also, Holdowns like S/Htt22 or S/ HD8 or 10 could be used.

If you have a wall start on the top of a steel beam at the second or third story, straps will not work and Holdowns will be the best. This is because the contractor can wild the threaded rod to the beam in the shop and connect the holdown in the field.

7-61 Chord and Chord Splices when using light gauge steel bracing. Seismic amplification factor = 2.2 for braced framed.

“Connection of the diagonal bracing member, top chord splice, boundary members and collectors shall be designed to develop the full tensile strength of the member OR sigma times the otherwise prescribed seismic forces.”

However in the seismic design Manual example 3, they ignored the second option of designing for the full strength of the member which in many cases may be smaller than the Amplified seismic forces.

• Must multiply the seismic forces by the Seismic Amplification factor In this case, the seismic forces is at the allowable stress level.
• For chord member , use the seismic amplification factor above to increase the load. However you can increase the allowable load the member can carry by 1.7
• For chord splices, Use the Seismic amplification factor above to increase the load. However, do not increase the capacity by the 1.7 factor. Or you can use the full capacity of the chord to design the splice.

If you can not use the above method for any reason. instead of using the amplified seismic load, you can use the full capacity of the chord to design the chord splice. For tension member, the full capacity of the chord is defined as area of steel multiplied by the yield strength factor.

Note that for Los Angeles, they reduce the capacity of every thing. So. Please check LARR( Los Angles research reports)

7-62 Connection at foundation when using light gauge steel bracing. Seismic amplification factor = 2.2 for braced framed.

“Connection of the diagonal bracing member, top chord splice, boundary members and collectors shall be designed to develop the full tensile strength of the member OR sigma times the otherwise prescribed seismic forces.”

However in the seismic design Manual example 3, they ignored the second option of designing for the full strength of the member which in many cases may be smaller than the Amplified seismic forces. Must multiply the uplift forces from wind or seismic by the Seismic Amplification factor In this case the seismic forces is at the allowable stress level.

For Holdowns,

• When seismic control, use the Seismic amplification factor above to increase the seismic uplift forces. However, do not increase the capacity of the holdown by the 1.7 factor.
• When wind control, use the Seismic amplification factor above to increase the wind uplift force. Also, increase the capacity of the holdowns by the 1.7 factor.
• Remember to add a holdown at each corner of the building even if the calculation does not require it.
• Also, add a note on plan that “At braced frame provide anchor bolts per braced frame schedule. At other locations where wall is on concrete provide 5/8″ diameter anchor bolt at 6′-” on center and within one feet of the beginning and the end of wall.”

7-70 Modeling the building on RISA when using light gauge straps (please see models created for KF204-369)

7-70-1 General:

I have created a worksheet to guide you through the calculations required to design a “light gage steel tension only braced frame”. The worksheet covers most any situation you may encounter when designing this type of system. You can find the worksheet in the project package / Engineering worksheets / LA Braced Frame Worksheet (final version). A sample copy is also attached in this chapter’s appendix.

We had some difficulty with running the RISA model. We shared these difficulties with RISA technical staff through several e-mails. Their answers to these challenges are collected in the Pink Binder titled “RISA Technical Support”. Please refer to this binder before you star modeling on RISA so you do not faces these tough challenges.

7-70-10 Short walls at garages:

There are three options at garages doors.

• If you have four feet of wall or so, use light gauge steel brace.
• If you have shorter walls, you have two options:
  • PSCO frame, or
  • Hardy panel

Our experience showed that PACO frame work better in RISA model.

Be careful. If you use Pre-engineered products such as Hardy panel or PACO steel or other products, You cannot use the entire wall length at garages. This is because the rough openings for garage doors are 4 inches wider than the garage door size shown on the architectural drawings.

This section is explanation for the building department.

7-100 Strap Bracing for lateral loads, wind and Seismic for Light gauge steel wall construction.

7-100-1 General:

Uniform Building Code sections 2219 and 2220

• Light gauge steel building is a wall system not a braced frame system. That is why the two stories limit of the braced frame with tension member only of UBC 2213.8.2.3 and UBC 2213.8.5 does not apply.
• Building shall not be over five story height. UBC.
• R, Over strength/ Ductility factor, = 2.8

International Building Code:

• Building height limitation is 65 feet.
• R, Over strength/ Ductility factor, = 4.0

7-100-10 Diagonal Bracing Design

The Code section will be mentioned followed by comments by PSE Consulting Engineers, Inc.

UBC 2220 Special Requirements In seismic zones 3 and 4 ( Diagonal strap, stud or cables)

1- “The L/r of the brace may exceed 200 and is unlimited.”

Comments: Any thickness and width that satisfy the force requirements will be used. We usually use gauge 16.

“All boundary members, chords and collectors shall be designed and detailed to transmit the induced axial forces.” 4- See connection section below.

“Both flanges of studs in a bracing panel shall be braced to prevent lateral tensional buckling. Wire tied bridging shall not be considered to provide such restraint.”

Comments: Bridging screwed to the metal studs are provided at 4’-0″.

In addition: For exterior wall, the exterior side of the studs are laterally braced by the wire mesh screwed to the studs and the interior face of the studs are laterally brace by the sheetrock that is screwed to the studs. for interior walls, both sides of the studs are laterally brace by the sheetrock that is screwed to the studs.

7-100-11 Brace Connections:

The Code section will be mentioned followed by comments by PSE Consulting Engineers, Inc., inc. UBC, 2001, Section 2220

1. See Section 7=100-10 above
2. See Section 7=100-10 above
3. “Connection of the diagonal bracing member, top chord splice, boundary members and collectors shall be designed to develop the full tensile strength of the member or sigma times the otherwise prescribed seismic forces.”

   Comments:

   • All connections are designed to develop the maximum capacity of the brace. In addition we add a few more screws.
   • Capacity of the screws are obtained from Part IV of the American Iron and Steel Institute. Please see copy attached with this calculation.
4. Vertical and diagonal members of the braced bay shall be anchored so the bottom tack is not required to resist uplift forces by bending or track web.
   Comments: The Brace force is transferred to the Gusset plate then to studs and the holdowns without transferring any force to wall track. Please refer to section 7-100-30 for full descriptions of load transfer.
5. See Section 7=100-10 above.
6. Screws shall not be used to resist lateral forces by pullout resistance.”
   Comments: Screws are only used in shear.
7. Provision shall be made to pre-tensioning or other methods of installation of tension bracing to guard against loose diagonal straps.”
   Comments: This project is design build. Our contractor does apply pre-tension on the strap. We will make sure of that during our inspection.

2220.2 Boundary Members and Anchorage.

“Boundary members and the uplift anchorage thereto shall have the strength to resist the forces determined by the load combination in section 2213.51”

UBC 2213.5. Column Strength:

“———————- In addition , in seismic Zones 3 and 4, columns in frames shall have the strength to resist the axial loads resulting from the load combination in Items 1 and:”

Axial compression: 1. -Pdl + o.7 Pll + Aumiga Pe

Comments: This formula is used to check the Two- double studs back to back (four studs total at each end of the brace subjected to exception 1 below).

Axial tension: 1. Pdl +- Aumiga Pe

Comments: This formula is used to check the Two- double studs back to back( four studs total at each end of the brace subjected to exception 1 below).

“Exception:

The load combination as outlined in Item 1 and 2 above:

Need not exceed either the maximum force that can be transferred to the column, by elements of the structure, or the limit as determined by the overturning uplift which the foundation is capable of resisting.”

7-100-15 Lateral Forces, Wind and Seismic:

Lateral forces are calculated according to the Uniform building Code or International Building Code based on the Jurisdiction where the building is located.

7-100-16 Vertical distributions:

Vertical distributions of lateral forces between floors is done according to the Uniform building Code or International Building Code based on the Jurisdiction where the building is located.

7-100-17 Horizontal distribution of lateral forces at each floor:

The distribution is done using the tributary width. Assuming the wood floor diaphragms is flexible.

7-100-18. Distribution of lateral force among braces on the same grid line, same vertical plane, on the same floor:

The distribution is done according to the stiffness of each brace. Since all braces are axial tension members and have the same cross section at each grid line, the stiffness is directly proportion to the brace cosine of the angle with the horizontal.

Hi= T cosine A, where

Hi is the Horizontal force to be resisted by brace number i.
T is Tension on the brace
A is cosine of the brace angle with the horizontal.

So the total horizontal force at specific grid line is divided by the summations of the cosines of angles of all braces.

Then multiplied by the cosine of the angle of the particular brace.

Hi= Ht x Cosine Ai / Sum ( Cosine Ai)

Where i= 1 to n ( n is number of braces along the same grid line) Ht is the total horizontal force along this gird line

Special Case, Pre-engineered

Hardy Panel

The stiffness of Hardy Panel is calculated as a cantilever. Them the distributions are done as above.

1- Hardy braced frame.

The distribution is done according the each member stiffness using RISA computer model

2- Paco Frame:

The distribution is done according the each member stiffness using RISA computer model

7-100-30 Force transfer between structural elements when using tension straps:

At roof level

At second or third floor level:

Special case for discourteous brace on top of floor beam:

At foundation:

7-100 Source of information, Software, Details and Sample Projection

7-100-50 Shear wall systems:

a-When seismic control, only structural sheathing such as OSB or Plywood can be used. If wind control, gypsum boards could be used.

Structural Sheathings:

There is a very good concrete non-compatible sheathing produced by Fortacrete Structural Panels.

Please use it whenever you can. It can resist both vertical loads as well as diaphragm shear forces.

We have their web and brochure/ binder Web: fortacrete.com

7-100-80-50 Hold-downs:

• Must provide hold down at end of wall.
• Also, add hold downs at building corners any way, typical.

50- Connections:

• Minimum number of screws is 5 – # 10.
• Increase number of screws by 25 % to account for rotations that may arise because we assumed the diagonal joint to be pure hinge and it is not.

7-400 Buildings with Standing Seam roof such as storage facilities:

7-400-1 General

7-400-10 Top wall connection to roof:

Since Standing seam roof has diaphragm shear value of Zero, The wall must be supported by other means such as: The bond beam at the top of the wall could provide horizontal support to the wall. The bond beam shall span between partition walls.

• For long wall, Design a horizontal strap truss inside the roof the act as a beam to support the top of the wall. In this case, the bond beam has to span between the truss points. Angle at mid span to connect to the wall is used
• At parapet provide “cap channel” at roof elevation. Usually this is a Z on it’s side fastened the masonry wall using sleeve anchors or Kwik bolts at 1’- 6” to 2”-0” O.C. and screwed to the supporting columns. The roof deck is then fastened to the cap channel.
• At partition wall, provide 2-3/4 inch diameter expansive bolt to tie the masonry wall to the per perpendicular metal shear wall. Refer to detail 8/S4.2.

7-400-20 Z purlin design:

• Since the standing seam does not have a shear capacity it does not provide lateral bracing for the purlin design.
• Provide strap cross bracing at half or one third points to provide lateral bracing. Refer to the AAA Secured storage pictures for an example. Also, refer to KF205-1158 Pear Tree self Storage.

Box headers:

• Contractors prefer box headers because it is easier to build than back to back. Also, it does not require blocking.
• Box beam or columns could be wider than the two flanges of the studs. By using bigger track, the two studs could be separated apart by 2 or 4 inches to form a stronger box.
• Options for two box headers:
  • side by side at the same elevation: You can install two box headers side by side if you install the screws of the header to the studs on the second box header from the inside instead of from the out side.
  • Side by side with one shifted higher above the other.
  • On top of each other.

Amount of Steel Required for light gauge/ cold-formed steel homes

The attached documents include a material estimate for Model B and Model C of the Paulani Estates in Hawaii. These Estimates include:

• Trusses
• Wall studs
• Exterior wall sheathing (outside only)
• Roof Sheathing
• Floor Joists (if applicable)
• Floor Sheathing (if applicable)
• Glulam beams
• Headers

Anything not listed above is not included in the estimate.

Nabil Taha, Ph.D., P.E.

Executive Summary for Model C, (Second story)

Total Steel used:

Length
470 ft of 16 gauge
428 ft of 18 gauge
4016 ft of 20 gauge

Weight
664 lb of 16 gauge
484 lb of 18 gauge
3555 lb of 20 gauge
Total: 4703 lbs Steel per sq. ft of second story: 8.4 lb/ft2

Beams Still used:
5 1/8”x 10 1/2” Glulam – 42 FT
3 1/8” x 10 1/2” Glulam- 28 FT
3 1/8” x 12 1/2” Glulam- 16 FT
5 1/8” x 16 1/2” Glulam- 8 FT
4 x 12 DF-L- 13 FT

Roof Sheathing Used:
1,135 ft2 of sheathing

Floor Sheathing Used:
600 ft2 of sheathing

Exterior Wall Sheathing (outside) Used:
873 ft2 of sheathing

Executive Summary for Model C, (First story)

Total Steel used:

Length
105 ft of 16 gauge
53 ft of 18 gauge
3876 ft of 20 gauge

Weight
149 lb of 16 gauge
60 lb of 18 gauge
3431 lb of 20 gauge
Total: 3640 lbs

Steel per sq. ft of first story: 3.12 lb/ft2

Roof Sheathing Used:
1256 ft2 of sheathing

Exterior Wall Sheathing (outside) Used:
1164 ft2 of sheathing

Executive Summary for Model C, (First and Second story)

Total Steel used:

Length
575 ft of 16 gauge
481 ft of 18 gauge
7892 ft of 20 gauge

Weight
813 lb of 16 gauge
544 lb of 18 gauge
6986 lb of 20 gauge
Total: 8343 lbs

Steel per sq. ft of building footprint: 6.95 lb/ft2

Beams Still used:
5 1/8”x 10 1/2” Glulam – 42 FT
3 1/8” x 10 1/2” Glulam- 28 FT
3 1/8” x 12 1/2” Glulam- 16 FT
5 1/8” x 16 1/2” Glulam- 8 FT
4 x 12 DF-L- 13 FT

Roof Sheathing Used:
2391 ft2 of sheathing

Floor Sheathing Used:
600 ft2 of sheathing

Exterior Wall Sheathing (outside) Used:
2037 ft2 of sheathing

References:

1. Prescriptive Method for Residential Cold-Formed Steel Framing, NAHB Research Center prepared for U.S. Department of Housing and Urban Development, HUD, Co-sponsored by American Iron and Steel Institute, AISI and National Association of Home Builders.
2. Commentary on the Prescriptive Method for Residential Cold-Formed Steel Framing.
3. International Residential Code, IRC, for One and Two family dwellings.
4. Shear Resistance of walls with steel studs by American Iron and Steel Institute, AISI.
5. Steel Deck Institute Diaphragm Design Manual by the Steel deck Institute, SDI.
6. Cold-Formed Steel Design, AISI Manual, by American Iron and Steel Institute, AISI.
7. North American Specification for the Design of Cold-formed Steel Structural Members, AISA Standard, by American Iron and Steel Institute, AISI.
8. Commentary on North American Specification for the Design of Cold- formed Steel Structural Members, AISI Standard, by American Iron and Steel Institute, AISI.
9. Cold-Formed Steel Framing Design Guide, by American Iron and Steel Institute, AISI.
10. Cold-Formed Steel Connectors for Residential and Mid-Rise construction by Simpson.
11. Steel-Frame, House Construction by Tim Waite, NAHB Research Center.
12. Residential Steel Framing Handbook by Robert Scharff and the editors of Walls & Ceilings Magazine.
13. Steel Stud Brick Veneer Design Guide by American Iron and Steel Institute, AISI.
14. Floor vibration criterion for Cold-Formed C-shaped supported Residential Floor System. Prepared for U.S. Department of Housing and Urban Development, HUD.
15. Steel Framed Residential construction: Demonstration Homes, prepared for U.S. Department of Housing and Urban Development, HUD.
16. Innovative Residential floor construction: Horizontal Diaphragm Values for Cold-formed steel Framing, prepared for U.S. Department of Housing and Urban Development, HUD.
17. Design Guide for Cold-formed Steel Trusses by American Iron and Steel Institute, AISI.
18. Light Gauge Steel Engineers Association, LGSEA magazine/Newsletter.
19. North American Standard for Cold-Formed steel Framing, Truss Design, American Iron and Steel Institute, AISI.
20. North American Standard for Cold-Formed Steel Framing, Header Design, American Iron and Steel Institute, AISI.
21. X-Standard for Cold-Formed Steel Framing, Lateral Design, American Iron and Steel Institute, AISI.
22. Cold Formed Steel Design by Wei-Wen Yu.
23. Jordan Commons, Cold-Framed Steel Training Manual, by American Iron and Steel Institute , AISI, Residential Steel construction Program.
24. Field Evaluation and Recommendations for Steel Framed Homes, Jordan commons Project, prepared for U.S. Department of Housing and Urban Development, HUD.
25. Steel Framing, Erection Manual and Standard Details.
26. Vulcraft, Steel Roof and Floor deck.
27. Product Data, Dietrich Industries, Inc., www.dietrichindustries.com
28. SCAFCO Product Data.
29. Clark, Steel Framing system, www.clarksteel.com
30. Light Steel Framing Manual by Metal Framing .org, steelnetwork.com