Code Requirements, Anchorage and Foundation in Cold-Formed Steel Buildings

This article describes the code rules that complement the panel and stud checks in cold-formed steel buildings: calculation and distribution of the earthquake load, the share of gypsum-board panels, structural requirements, panel anchorages and foundations. For panel and stud design, see the article Cold-Formed Steel Panel and Stud Design.

1. Earthquake load: equivalent seismic load and T = 0.2 s (TBDY 10.2.6.1)

info

According to TBDY 10.2.6.1, the Equivalent Seismic Load Method is used in the earthquake calculation of cold-formed steel buildings, and the elastic design spectral acceleration is found by taking T = 0.2 s. When the building type is “cold-formed steel”, the program calculates the spectral acceleration, the seismic load reduction factor and the related quantities with T = 0.2 s; the calculated period remains for information only.

• If a mode-superposition-based method is selected as the earthquake calculation method, a “TBDY 10.2.6.1” warning and a project checklist error are given (the modes are calculated with their own periods). The same warning is given when SAP2000 modal results are used.
• Calculating the seismic load reduction factor also with T = 0.2 s is on the safe side: if the calculated period is greater than 0.2 s, the reduction factor comes out smaller and the seismic load becomes larger.

2. Panel stiffness proportional to capacity (TBDY 10.2.6.1)

According to the code, the seismic load is distributed to cold-formed steel panels according to their capacities and their positions in plan. When the shear distribution is carried out with the finite element method, the program proportions the in-plane stiffness of each cold-formed steel wall to the panel shear strength: panels with OSB, plywood or sheet-metal sheathing receive more shear than gypsum-board panels. The total cold-formed steel stiffness of the storey does not change (base shear and storey shear remain the same).

• When is it applied? When the Load Distribution to Shear Walls in Timber and Cold-Formed Steel Buildings selection (panel stiffness type) in Project General Settings is “capacity” (default) and the walls are modelled with bar elements. In the classical shear distribution, the shear is already distributed according to capacity. The selection is saved with the project; older projects that do not contain this information open with “capacity”.
• Only the in-plane lateral stiffness changes; the axial and out-of-plane stiffness and the sharing of vertical load remain the same. In a storey with a single type of sheathing the factor is ≈ 1.
• A panel that cannot participate in shear (h/l > 4 or l < 0.30 m) and a panel unsheathed on both faces practically receive no shear; if the analysis shear is less than one thousandth of the panel shear of the storey in the same direction, the panel is not considered “inadequate” and the report says “ihmal edildi, panel yalnız düşey yük için tasarlandı” (neglected, the panel was designed for vertical load only); if it exceeds one thousandth, an error is given. If no panel with shear capacity remains at all in a direction, the analysis additionally gives a storey–direction error (see Cold-Formed Steel Panel and Stud Design, Section 3.2).
• Short panels between doors/windows may receive shear below their capacity shares in the analysis. For the panel, end stud and anchorage demands, the program raises the earthquake shear and moment of each panel to at least its capacity share; the report says “kapasite payı tabanıyla ×r büyütüldü” (magnified ×r with the capacity share as the floor) (safe side).
• This scaling is not applied in the shell wall model; for distribution according to capacity, use the bar wall model or the classical shear distribution.

3. Earthquake share of gypsum-board panels (TBDY 10.2.7.2)

info

According to the code, gypsum-board panels cannot alone resist the seismic demand. For each cold-formed steel storey and direction (X/Y), the program calculates the share of the gypsum board in the panel shear strength and compares it with the limit of TBDY Table 10.3: 80% in a 1-storey building; in a 2-storey building 80% on the upper storey and 60% on the lower storey; in a 3-storey building 80% / 60% / 40% from top to bottom.

• Which face is counted as gypsum? If the sheathing type in the Cold-Formed Steel Panel tab is “Gypsum board”, it is gypsum; if it is OSB, Plywood or User kφ, it is not gypsum. If the type is “Undefined”, a face with shear strength vc ≤ 3.4 kN/m is counted as gypsum. The default for new walls is 11 mm OSB (vc = 10.2 kN/m); in older projects, panels whose type is undefined and that have 2.7 kN/m are counted as gypsum board. A panel unsheathed on both faces is not included in the gypsum share calculation.
• Mixed sheathing: If the two faces act together (2·smaller ≥ larger), each face carries half of the force; if the larger face governs alone, the gypsum has no share.
• If the limit is exceeded: A “alçı levha payı” (gypsum board share) error appears in the analysis warnings and in the project checklist. Solution: use OSB, plywood or sheet-metal sheathing in some of the panels in that direction (by entering vc and the sheathing type).

4. HCBTİE Chapter 6 and Table 1.2 structural requirements

After the analysis, the following conditions are checked for cold-formed steel panels; violations are shown in the analysis warnings (with the names of the walls concerned), in the project checklist and in the footnote of the stud report. The capacity calculation does not change.

• Stud spacing ≤ 625 mm (if exceeded, the sheathing cannot be counted as structural: panel shear capacity and kφ are invalid).
• Structural stud C/Σ section, wall thickness ≥ 0.9 mm (≥ 2 mm if the building is higher than 10.5 m), web ≥ 70 mm, lip ≥ 9 mm.
• If stud height / web depth > 50, a row of bridging must be provided at mid-height (warning; not given for a panel in which bridging is defined, see Section 6 of this article).
• Panel h/l > 4 or l < 30 cm: does not contribute to the panel shear capacity; 2 ≤ h/l ≤ 4: 2l/h reduction (information).
• In the cold-formed steel building type, R cannot exceed 3 for limited ductility and 4 for high ductility; D = 2; the building height cannot exceed the 7 / 10.5 / 14 m limit according to the earthquake design class and ductility level (HCBTİE Table 1.2).
• The report note additionally reminds that the panel tracks must be U/C with t ≥ 0.8 mm and that cabinet loads hung on the wall must be taken as at least 0.5 kN/m.

5. Panel anchorage and connections (HCBTİE Chapter 8, TBDY 10.4.1.1)

In the Wall Properties window, next to the Cold-Formed Steel Panel tab, there is the Cold-Formed Steel Anchorage tab (visible only for cold-formed steel panel walls).

• Tension (hold-down) anchorage: Device design strength and anchor rod/concrete/storey connection strength (manufacturer or ETA values), number of device–stud screws (0 = the program calculates the required number), screw diameter, device plate thickness and tensile (rupture) strength, screw shank shear strength.
• Shear anchorage: Connection type (bolt: foundation/concrete; screw: storey slab), number (0 = from spacing) and spacing, diameter, bolt grade (4.6–10.9), concrete or substrate strength per anchor, bottom track thickness and, for screwed connections, substrate thickness. If a bottom track profile is selected in the Cold-Formed Steel Track tab, the thickness is taken from that profile (see Section 6 of this article).

At the end of the analysis, for each panel the uplift force per end (magnified by D, with the governing combination), the device / rod–concrete / screw / stud tension ratios, the shear per anchor and the bolt / bearing / concrete ratios, and the panel shear ratio (TBDY 10.3.2.1) are calculated. The device strength is the design value of the selected design method; the anchor rod/concrete and concrete shear strengths are entered as LRFD design values in both methods. Inadequacies are shown in the analysis warnings and in the project checklist, and strengths that were not entered are shown in a collective information warning. The stud report contains the rows for tension anchorage demand, tension capacities, shear anchorage, panel shear and assumptions.

6. Panel bottom and top tracks and bridging (TBDY 10.3.1.3, 10.3.1.4, 10.4.1.1(a); HCBTİE Chapter 6)

Next to the Cold-Formed Steel Anchorage tab, the Wall Properties window has the Cold-Formed Steel Track tab (visible only in the Ultimate edition). A field left blank or set to 0 is treated as not entered, and the default is used:

• Bottom / top track profile: U, C or C+ is selected from the catalogue. If none is selected, the U profile closest to the stud web height is taken automatically, with the track wall thickness of the anchorage tab (the stud wall thickness if that is not entered), and “otomatik” (automatic) is written in the report. You are advised to select the actual track profile: the wall thickness of the selected profile is also used in the shear anchorage checks (bolt bearing, screw-type storey connection) and in the floor–top track screw check; the bottom track thickness in the anchorage tab is then not taken into account.
• Double top track: Two top tracks; the tension and compression strengths are taken as double.
• First connection at the end e1 (mm): Distance of the shear anchor closest to the panel end (at the top track, of the floor screw); 0 = a/2.
• Bridging: Number of bridging rows (0–3), bridging wall thickness t, strap width b, screw diameter d (No.8 = 4.17 mm), number of screws per connection, tab length, screw edge distance and screw spacing (mm).
• Long edge of the sheathing: Vertical or horizontal. With horizontal use, horizontal bridging that transfers the shear into the panel is required (TBDY 10.3.1.4).

In multiple selection, fields that differ among the selected walls appear blank (no selection in the box, check box greyed) and are not changed when OK is pressed.

The following are checked in the analysis:

• Bottom track: N = v·max(a/2; e1), v = Vd/L (the shear magnified by D in the panel anchorage table); tension strength (HCBTİE 4.4, net section with the bolt hole deducted) and compression strength (EWM). a is taken from the number or spacing of the shear anchors; if not entered, from the required number of anchors.
• Top track: Axial force within the panel (with the floor connection spacing) and, if the wall lies on a cold-formed steel floor diaphragm line, the collector force (the largest axial force of the line + the force within the panel).
• Bridging: HCBTİE 6 (a)–(f) details; brace force F = 0.02·P (cumulative along the studs in a wall without sheathing); connection screw (TBDY 10.3.4) and strap tension (if the width is entered; otherwise the required area is reported).
• TBDY 10.3.1.3 and 10.3.1.4: The track must be a U or C section with t ≥ 0.8 mm; bridging must be present with horizontal sheathing. Information is given if the track web height differs from the stud web height by more than 10 mm (the stud does not seat in the track) and if the track wall thickness is smaller than that of the stud. The h/d > 50 warning is not given for a panel in which bridging is defined.

In studs without sheathing, n rows of bridging reduce the weak-axis and torsional buckling length to h/(n+1). The results appear in the ALT / ÜST BAŞLIK (bottom / top track) and KUŞAK (bridging) rows of the stud report. For the equations, see the article Theoretical Basis of Cold-Formed Steel Design.

7. Foundations: forces magnified with D (TBDY 10.4.1.2)

In a cold-formed steel building, the foundations are sized with the seismic internal forces magnified by the overstrength factor D (2 for cold-formed steel). The program applies this in the “Finite Elements - Combined Structure-Foundation Analysis” option:

• to the bending, axial force, shear and torsion design of strip foundations,
• to the soil stress of strip foundations,
• to the raft foundation.

danger

If “Rigid Assumption-Classical Analysis” is selected, the foundation calculation is made with vertical loads only and the analysis gives an error; select “Finite Elements - Combined Structure-Foundation Analysis” in Project General Settings. In user-defined load combinations, D is applied only to shear and torsion and a warning is given; use the program combinations. An information line “TBDY2018 M.10.4.1.2” appears in the project checklist.

8. Frequently asked questions

I get an “alçı levha payı” (gypsum board share) error on every cold-formed steel storey. The sheathing of the panels is counted as gypsum board. According to TBDY 10.2.7.2, gypsum-board panels cannot alone resist the seismic demand; enter OSB, plywood or sheet-metal sheathing for some of the panels in each direction (vc and sheathing type).

In the finite element solution, the shear of the OSB panels increased and that of the gypsum-board panels decreased. According to TBDY 10.2.6.1, the panel stiffnesses are proportioned to capacity; the total storey shear does not change. If the shear ratio of the OSB panel is inadequate, reduce the screw spacing or strengthen the sheathing.

The foundation reinforcement or soil stress of the cold-formed steel building is larger than expected. According to TBDY 10.4.1.2, the foundation internal forces are magnified with D (2); this magnification is applied to the bending, axial force, shear, torsion and soil stress in strip foundations and rafts.

I get a “TBDY 10.4.1.2 (temel)” (TBDY 10.4.1.2 (foundation)) error. The foundation calculation is being made through “Rigid Assumption-Classical Analysis” and does not include the earthquake. Select “Finite Elements - Combined Structure-Foundation Analysis” in Project General Settings.

Why is the earthquake load in a cold-formed steel building independent of the calculated period? According to TBDY 10.2.6.1, the spectral acceleration is calculated with T = 0.2 s. The earthquake calculation method must be the Equivalent Seismic Load Method; if mode superposition is selected, a warning is given.