--------------------- | ------ | --------------- | -------------------------------------------- | | End plate thickness | tp | 5/8 to 1-1/2 in | Governed by yield line analysis | | End plate width | bp | 7 to 12 in | Usually matches or exceeds beam flange width | | Bolt diameter | d_bolt | 3/4 to 1-1/4 in | A325 or A490 per AISC Table J3.2 | | Bolt gage | g | 3.5 to 6 in | Horizontal distance between bolt columns | | Bolt pitch | p | 2 to 4 in | Vertical distance between bolt rows | | Plate-to-flange distance | a | 1 to 2 in | Distance from outer bolt row to beam flange | | Prying parameter | alpha | 0 to 1+ | Less than 1 means no prying (thick plate) | | Beam flange yield | Fy | 50 ksi | ASTM A992 default | | Plate yield | Fyp | 50 ksi | ASTM A572 Gr. 50 typical |

Common Design Errors

Ignoring prying action. The most common error in hand calculations is neglecting prying forces. For plates less than 1 inch thick with standard bolt gages, prying can add 20% or more to the bolt tension. Always check prying per AISC Design Guide 4 or AISC Manual Part 9.

Insufficient plate width. The end plate must be wide enough to develop the full yield line mechanism and provide adequate edge distance for the bolts. A plate that is too narrow forces the bolts too close to the plate edge, reducing bearing capacity and potentially causing edge tear-out.

Omitting column-side checks. Engineers sometimes focus on the beam-side design (bolts and end plate) but neglect the column flange bending and web checks. The column is often the limiting element, especially when a relatively light column receives a heavy beam moment.

Incorrect lever arm. For extended end plates, the lever arm between the tension bolt rows and the compression flange includes the plate thickness. Using the beam depth alone (without the plate thickness) underestimates the lever arm and overestimates the bolt tension, which may seem conservative but can lead to selecting an unnecessarily heavy connection.

Not checking panel zone shear. In moment frames, the column panel zone shear can exceed the web capacity, requiring doubler plates. This check is separate from the end plate and column flange checks.

Additional Design Considerations

Weld design: The weld between the beam and the end plate must develop the full capacity of the beam flanges and web. Complete joint penetration (CJP) groove welds are typical for the beam flanges to ensure the full flange strength is developed. The web weld may be fillet welded for shear transfer.

Stiffener detailing: When continuity plates are required, they should be sized to match the beam flange thickness and width. The stiffener-to-column flange weld must transfer the excess force not resisted by the column flange or web. Fillet welds on both sides of the stiffener are typical.

Erection considerations: End plate connections require tight fabrication tolerances. The end plate must be perpendicular to the beam web, and the bolt holes must align between the end plate and column flange. Shims are sometimes required to accommodate column out-of-plumbness. Specify acceptable gaps in the construction documents (typically 1/16 inch for bearing-type connections).

Fatigue: For structures with cyclic loading (crane buildings, bridges), the end plate connection detail must be checked per AISC 360 Appendix 3. The weld at the beam flange-to-end plate interface is typically Category B or C. The bolt hole at the end plate edge is Category D. The allowable stress range decreases with the number of cycles.

Inputs and outputs

Typical inputs: beam section, column section, end plate width and thickness, bolt diameter and grade, bolt layout (gage, pitch, number of rows), steel grades for plate and members, and factored moment and shear demands.

Typical outputs: bolt tension including prying, end plate bending capacity, column flange bending capacity, stiffener requirements (continuity plates, doubler plates), and overall connection moment capacity.

Computation approach

The calculator follows the AISC Design Guide 4 procedure. For extended end plates, the bolt force including prying is computed using the Kennedy method or the simplified AISC approach. The end plate thickness is checked against yield line mechanisms. Column-side checks include flange bending (using the equivalent T-stub model), web yielding, web crippling, and web compression buckling. If any column-side check fails, the required stiffener size is computed.

Worked Example

Problem: Design a flush end-plate moment connection for a W18x50 beam to a W14x90 column flange. Beam moment demand Mu = 200 kip-ft. Use A992 steel, A325 bolts.

Given:

Solution:

Step 1 -- Try 4 bolts in tension (2 rows x 2 columns), gage g = 4.5 in., pitch p = 3 in.:

Bolt row 1 distance from beam compression flange = 18.0 - 1.5 = 16.5 in.
Bolt row 2 distance = 16.5 - 3.0 = 13.5 in.

Step 2 -- No prying action check (thick plate assumption). Bolt tension per bolt row:

Sum of distances: d1 = 16.5 in., d2 = 13.5 in.
Sum of d^2: 16.5^2 + 13.5^2 = 272.25 + 182.25 = 454.5 in^2

Tu_row1 = Mu * d1 / sum(d^2) = 200 * 12 * 16.5 / 454.5 = 87.1 kips (2 bolts)
Tu_row2 = 200 * 12 * 13.5 / 454.5 = 71.3 kips (2 bolts)

Step 3 -- Bolt tension capacity per AISC 360-22 Table J3.2:

rn = Fnt * Ab = 90 * 0.442 = 39.8 kips per bolt (nominal)
phi*rn = 0.75 * 39.8 = 29.8 kips per bolt (design)

Row 1: 87.1 / 2 = 43.6 kips/bolt > 29.8  NO GOOD

Step 4 -- Increase to 6 bolts (3 rows x 2 columns), d3 = 10.5 in.:

Sum of d^2: 16.5^2 + 13.5^2 + 10.5^2 = 272.25 + 182.25 + 110.25 = 564.75

Tu_row1 = 2400 * 16.5 / 564.75 = 70.1 kips / 2 bolts = 35.1 kips/bolt (> 29.8, still high)

Step 5 -- Upgrade to 7/8 in. dia. A325 bolts:

Ab = 0.601 in^2
rn = 90 * 0.601 = 54.1 kips
phi*rn = 0.75 * 54.1 = 40.6 kips > 35.1 kips  OK

Result: Use 6 -- 7/8 in. dia. A325 bolts (3 rows x 2 columns), 3/4 in. thick end plate, A992 steel. Verify prying action per AISC 360-22 Section J3.7 and check column flange bending per Part 9 of the AISC Manual.

Frequently Asked Questions

What is prying action in bolted end plate connections? Prying action occurs when the end plate or column flange flexes under the applied tension, causing the contact point near the bolt line to act as a fulcrum. This increases the bolt tension beyond the applied tension by an additional prying force Q. The magnitude of Q depends on the plate thickness, bolt gage, and the distance from the bolt to the beam flange. Thick plates have minimal prying; thin plates can increase bolt tension by 20-40%.

What is the difference between flush and extended end plates? A flush end plate does not extend beyond the beam flanges; bolts are placed between the flanges. An extended end plate extends beyond one or both beam flanges, with bolt rows above and/or below the flanges. Extended plates have higher moment capacity because the outer bolt rows have a longer lever arm. The 4-bolt extended unstiffened (4E) and 8-bolt extended stiffened (8ES) are the most common configurations.

When are column stiffeners required? Column stiffeners (continuity plates) are required when the column flange is too thin to resist the concentrated force from the beam flange without excessive local bending, or when the column web cannot resist the compression or tension delivered by the beam flanges. The need for stiffeners depends on the column flange thickness, web thickness, column depth, and the magnitude of the beam flange forces. Stiffeners add fabrication cost, so selecting a heavier column to avoid stiffeners is often more economical.

Can end plate connections be used for seismic moment frames? End plate connections can be used in seismic moment frames, but they must be prequalified per AISC 358 or justified by testing and analysis. AISC 358 includes prequalification for several end plate configurations, including the 4E and 8ES types. The prequalification specifies limits on beam depth, column depth, plate thickness, bolt diameter, and other parameters. Connections outside these limits require project-specific qualification testing.

How do I select between A325 and A490 bolts for end plate connections? A325 bolts (Fnt = 90 ksi) are adequate for most end plate connections with moderate to high moment demands. A490 bolts (Fnt = 113 ksi) provide higher tensile capacity per bolt and are used when the bolt tension demand exceeds A325 capacity and increasing the bolt count or diameter is impractical. A490 bolts are more expensive and have stricter installation requirements. They should not be galvanized due to hydrogen embrittlement risk.

What is the minimum end plate thickness? There is no universal minimum, but AISC Design Guide 4 recommends that the end plate be at least as thick as the bolt diameter for extended unstiffened configurations to limit prying action. For 7/8-inch bolts, use at least a 7/8-inch plate for a no-prying design. Thinner plates can be used if prying is explicitly checked and the bolt capacity is verified including the prying force.

How does the panel zone doubler plate interact with the end plate connection? The doubler plate is welded to the column web to increase the panel zone shear capacity. It does not directly affect the end plate or bolt design, but it is part of the overall connection system. The doubler plate thickness is selected so that the combined web-plus-doubler shear capacity exceeds the panel zone shear demand. The doubler plate must be detailed with plug welds or fillet welds to the column web to prevent local buckling of the thin gap between the doubler and the web.

Code References

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