Steel deck diaphragm

One zone of a bare fluted roof or form deck working as a diaphragm between two reaction lines: support and sidelap fasteners, panel stability, wind uplift at the fasteners, stiffness and deflection. To AISI S310-20 with Supplement 1 (2022), with AISI S100-16 for the fasteners.

Gravity on the same deck is the steel deck (gravity) calculation. This one takes only in-plane shear.

Inputs of the deck diaphragm #

• Diaphragm: span between the lateral supports, depth B along the reaction line. Shear is S = w × span / (2 B).
• Deck: 1.5PLB or 3.0PLN, Vulcraft's, with depth, flats, gross moment of inertia and warping constants from its 2016 catalog, or other deck. The catalog marks its dimensions approximate: check them against your product. Other deck takes depth, pitch, half the bottom flat e, top flat f, cover width, thickness and Ixg from your manufacturer. The web length and angle follow from those. All are prefilled with 1.5PLB at 22 gage, not a generic deck.
• Warping constant D: the catalog's for a listed deck and pattern. Otherwise S310 Appendix 1 Section 1.4 from the flats and the valley spacing at the panel ends, or entered.
• Steel: Fy and Fu of the deck. 50 ksi is the catalog's minimum, and 62 ksi the value its connection tables state. Yours may differ.
• Deck span and spans in a panel: S310 Table 1.3-1 sets the warping factor by spans in a panel, and 7 or more take one value. The catalog's tables assume 3 spans.
• Fastener pattern: 36/7, 36/4 on 1.5PLB and 24/4 on 3.0PLN, as Vulcraft draws them, or your own positions from the panel centerline, one per fastener, up to 30. Pattern constants come from the positions. Listed patterns put the sidelap fastener 1/2 in. in from the panel edge, which is what reproduces the catalog's printed factors.
• Support connection at the sidelap: one each side, or one through both sheets. S310 Commentary D1 gives N for each: both are counted, or A is left out. The catalog's strength tables work out with one each side. It matters: N sets the corner limit, which usually governs.
• Support fastener: No. 12 or No. 14 screws (S100 J4.3), 3/4 in. arc spot welds or others (S100 J2.2.2.1), or a strength entered from tests or a fastener's report (S310 D1.1.3, D1.1.5). S310 limits Eq. D5.2.2-1 to these two screws. For a thin support the flexibility follows S310 Commentary Figure C-D5.2.2-1.
• Screw strengths Pnss and Pnts: S310 asks for your manufacturer's. 0 takes the value printed in the SSMA Guide.
• Sidelap fastener: PunchLok II by the catalog's equations (Pns and Ss), screws (S100 J4.3, S310 Eq. D5.2.2-2), arc spot welds (S100 J2.2.2.2), button punches (S310 D1.2.6, Eq. D5.2.5-1, and no strength above 3 in. depth), or entered from tests with both Pns and Ss (S310 D1.2.7, D5.2.6).
• Sidelap and edge fasteners per deck span: counted over the panel for ns and ne. Edge fasteners go into the edge support at a reaction line: your detail.
• Deck web: bearing at the exterior support, inside bend radius, web flat. The catalog prints no radius: 3/16 in. is prefilled so you enter your deck's. It enters only Snℓ.
• Uplift: a net pressure at the required level for your method, with a reaction coefficient, or a force per fastener. T = coefficient × pressure × deck span × the widest tributary at a fastener. 1.1 is the interior reaction of three equal spans. Yours depends on your layout. 0 applies no uplift, and the calculation says so.
• Pull-over diameter: S100 J4.4.2 for your screw head and washer, at least 5/16 in. in tension.
• Pnft from tests: S310 D3.1. Required for an entered support fastener when there is uplift.
• Deflection: a load factor on the strength-level line load, and span / n. S310 C3 leaves both to you.
• Loads: wind and seismic line loads along the span at strength level. Each load kind takes its own factors.

Auto-design varies the gage, the pattern across the panel (listed patterns), and sidelap and edge fasteners per span. The deck, its span, the diaphragm, the kind of fastener and the loads are kept.

Checks performed #

Wind uplift lowers Sni, Snc and Sne through Pnft, as S310 D3.1 states. D3.1 does not name Snp. This calculation lowers it too, on the safe side. The edge fastener Pnfs is not reduced, as D3.1 permits.

S310 D3.1.2.1 applies its pull-over and pull-out equations only within the limits of S100 J4.5.1 and J4.5.2, and asks for tests outside them. A No. 12 screw into a support thicker than 0.0724 in. is outside J4.5.2, which is most joists and beams. Outside the limits this calculation uses the linear Eqs. D3.1.2.1-13 to -18 with the S100 J4 factors, and a note says so. S310 states those equations for strengths from small-scale tests, so this is your determination. Enter Pnft from tests if you have it.

An entered support fastener with uplift reads NG, with a row saying the interaction is not checked, until Pnft is entered. S310 D3.1.3 gives its interaction by tests, or from the fastener's pull-over and pull-out strengths.

Equations with an anomaly: S310's User Notes and Commentary say Eqs. D3.1.2.1-2, -4, -6, -8 reduce the shear strength by 6 to 23 percent as the tension approaches zero. The calculation applies them as written once there is any uplift.

Limitations of the deck diaphragm #

• One zone. Zone the span with a calculation for each zone.
• The edge panel takes the interior panel's patterns and width. S310 asks for both panels to be checked if they differ.
• Where shear reaches a line from two sides, S310 compares Sne with the reaction per length. Here it is compared with the shear entered.
• Not covered: concrete fill (D4), cellular, acoustic and standing seam panels, arc seam welds, top arc seam sidelap welds, wood supports, insulation between panel and support, weld washers.
• Not designed: chords, collectors, edge supports, their connections, or fastener edge distances. The drawing says so.
• Deflection is the shear of the deck only. Chord flexure, chord splice slip and framing are yours.
• Support and uplift forces are taken at the widest tributary, on one fastener. S100 J2.2.3's 70 percent for a weld at a sidelap is taken whenever the pattern has one, on the safe side.
• A manufacturer's evaluation report may modify these equations, as IAPMO ER-2018 does for Verco's decks. This calculation follows S310 as written.
• Older catalog tables use the factors of AISI S100 Table D5. Do not expect this calculation to match them. Its #12 screw strength from S100 J4.3 is also lower than the older manual value in the catalog's Table 1.

Verification examples #

Verification status: 66 values compared in 39 examples (34 published, 5 hand calculations from the cited equations): 63 within tolerance, 3 recorded deviations. Code basis: the provisions it uses were read in the target edition.

Compared with the Vulcraft PunchLok II catalog (December 2016) and AISI S310-20 Commentary Table C-1.2. Strength compares nominal values, because the catalog's factors are the older ones: the catalog's allowable value times its own factor, against Sn before any factor.

Differences stated plainly. The catalog's worked G' of about 98 kip/in. multiplies by 0.90, its own factor for three spans, on four spans. This calculation uses S310's 0.80 and gets 101 kip/in. The catalog's weld tension values are 9 percent below S100 Eq. J2.2.3-2 at the steel its table states, for a reason it does not give. The catalog's wind tables with uplift use an older interaction, so they are not a check of S310 D3.1.2.1.

Hand calculations, marked as such: Pnf of a screw, the factors of Table B1.1-1 on Snf, Snp, Sne.

Not verified against a published value: Snℓ, Sni apart from Snc, the uplift interaction under S310's factors, a full ASD or LRFD diaphragm worked to S310-20 from end to end. The current SDI Diaphragm Design Manual is not used by this calculation.