RFI on a Slip-Critical Brace Splice: Oversized Holes and an Untested Primer
A diagonal brace in a new two-story chevron-braced frame splices at mid-height between two 1/2 in A36 gusset plates using 24 ASTM A325 3/4 in high-strength bolts in standard holes, one shear plane. The connection is slip-critical at the service load of 195 kips. Faying surfaces are blast-cleaned and coated with a zinc-rich primer that has never been tested for slip resistance. The fabricator issues an RFI asking to (1) use 13/16 in oversized holes to improve fit-up, (2) use the same primer thinned for spray application, and (3) convert the joint to a bearing-type connection to save installation time. The engineer of record specified slip-critical connections at this splice because the connection must not slip under design-level frame deformation.
1. Compute the slip-critical capacity as designed and with oversized holes, and evaluate both against the 195 kip service demand.
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Slip-critical resistance per bolt: R_n = μ · D_u · h_f · T_b · N_s
- μ = 0.30 (Class A - the only value that can be claimed for an unqualified coating)
- D_u = 1.13
- T_b = 28 kips for a 3/4 in A325 bolt
- N_s = 1 shear plane
- h_f = 1.00 for standard holes, 0.85 for oversized holes
As designed (standard holes): R_n = 0.30 × 1.13 × 1.00 × 28 × 1 = 9.49 kips/bolt 24 bolts → 228 kips ≥ 195 kips → OK, margin ≈ 17%.
With 13/16 in oversized holes: R_n = 0.30 × 1.13 × 0.85 × 28 × 1 = 8.07 kips/bolt 24 bolts → 194 kips < 195 kips → NG.
The oversized-hole request consumes the entire margin and puts the connection just below demand. It cannot be granted with the present bolt count.
The primer is the larger issue: an untested coating cannot be credited as Class B (μ = 0.50). At Class A the design is already on the edge. If the primer were qualified by slip testing to Class B, capacity would rise to about 380 kips and the oversized holes would become viable - but that requires test data, not a manufacturer's claim.
2. Evaluate the requested conversion to a bearing-type connection.
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Do not approve it. The reasons are behavioral, not just numerical:
- In a bearing connection the plates slip at service load before the bolts go into bearing. In a braced frame, that slip is a permanent, unanalysed story drift and a change in the frame's lateral stiffness at exactly the load level where the design assumed elastic, non-slipping behavior.
- Slip also redistributes force within the brace group unevenly, and in a chevron configuration the brace forces are sensitive to relative movement at the splice.
- The engineer of record specified slip-critical precisely because the connection must stay put under design-level deformation. Bearing-type is not an equivalent substitution; it is a different connection with a different performance assumption.
If the fabricator's complaint is installation time, the correct answers are: match drilling and pre-assembly to eliminate fit-up problems, and consider tension-control (TC) bolts or turn-of-nut, which satisfy the same pretension requirement without the calibrations that make the crew slow. Time savings come from the installation method, not from downgrading the joint.
3. Recommend an acceptable resolution and the installation/QA requirements that go with it.
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Resolution options (in order of preference):
- Keep standard holes and require slip testing of the primer per the AISC Specification Appendix A. If it qualifies at Class B (μ = 0.50), capacity roughly doubles and there is ample margin even if oversized holes are later needed.
- Add bolts: with oversized holes at 8.07 kips/bolt, 195/8.07 = 24.2 → 26 bolts minimum (verify edge distance, bolt spacing and gusset net section for the added row).
- Upsize to 7/8 in A325 (T_b = 39 kips): R_n = 0.30 × 1.13 × 0.85 × 39 = 11.2 kips/bolt, 24 bolts = 270 kips → OK, but recheck hole size vs. plate width, edge distance and bearing/tear-out on the 1/2 in gussets.
Installation and QA requirements:
- All bolts fully pretensioned; verify with a Skidmore-Wilhelm at the start of each shift and per lot.
- Hardened washers over oversized holes on both faces, per the Specification.
- Faying surfaces free of oil, paint beyond the qualified coating, and mill scale; the qualified coating must be applied at the tested dry-film thickness - thinning for spray application voids the qualification unless retested.
- Verify hole size and hole quality (no punched holes in primary connections) at the shop, and inspect a sample of tightened bolts by the selected method (turn-of-nut match marks or TC washer gaps).
- Respond to the RFI in writing, stating that the splice remains slip-critical and which resolution was selected; do not let a field substitution be approved verbally.