Understanding Built-up Beams and Cover Plates

Built-up beams are structural members constructed by joining together different steel sections, such as plates and angles, using riveting, bolting, or welding. This allows engineers to create beams with specific properties or to use available materials efficiently. Cover plates are additional plates attached to the flanges of a beam (like an I-beam or a built-up section) to increase its moment of inertia and bending strength, especially at locations along the span where bending moments are high.

Rivet Holes and Their Effect on Area

When rivets are used to connect cover plates to the main beam section, holes are drilled or punched through the plates and the beam flanges. These holes remove material, effectively reducing the cross-sectional area of the member at the location of the holes. For members subjected to tension (like the tension flange of a beam or a cover plate on the tension side), this reduction in area can significantly affect the strength. Therefore, structural design involves calculating the 'net area' (total area minus the area of holes) to ensure the member can safely carry the tension forces.

Allowance for Rivet Holes in Cover Plate Area Calculation

In the design of cover plates, especially when using riveting, it is standard practice to consider the impact of the rivet holes on the required area. The question asks about an allowance for rivet holes to be added to the calculated area of cover plates. This allowance accounts for the reduction in area caused by the holes.

While the net area is critical for strength checks, the design process often involves determining a required area based on stress and then making an allowance for the material removed by holes. This allowance is essentially an additional area considered in the design or material estimation to compensate for the holes.

According to standard steel design practices (particularly relevant to older methods involving riveting), an allowance is made for the rivet holes when determining the required area of tension members or tension flanges/cover plates. This allowance is typically expressed as a fraction or percentage of the calculated net area or sometimes related to the area of the holes themselves, added back to ensure the gross section can accommodate the loss due to holes while maintaining sufficient net area.

The value commonly adopted as an allowance factor for rivet holes when calculating the required area of cover plates in built-up beams is 0.13 times the calculated net area. This means that if a certain net area is required for strength, the gross area provided must be sufficient to yield that net area after deducting the holes, and the allowance factor helps relate the required net area to the gross area or the material needed.

Therefore, to the calculated area (often referring to the required net area), an allowance for rivet holes equivalent to a factor is added or considered in the design process.

The allowance for rivet holes to be added to the calculated area of cover plates of a built-up beam is typically taken as 0.13.

Detailed Breakdown of the Allowance

• The presence of rivet holes reduces the load-carrying capacity of the plate in tension.
• Design codes require considering the reduced 'net area' at the section with holes.
• When calculating the required area of a cover plate, an allowance is included to compensate for the area lost to holes.
• This allowance helps ensure that even with holes, the remaining material (net section) is sufficient.
• A standard allowance factor used is 0.13.

Conclusion on Rivet Hole Allowance

Based on standard engineering practice for riveted connections in steel structures, the allowance for rivet holes added to the calculated area of cover plates for a built-up beam is 0.13. This value helps ensure the designed cover plate has adequate capacity considering the material removed by the holes.

Revision Table: Built-up Beam Cover Plates

Review the key concepts related to built-up beams and cover plates:

• Built-up Beams: Structural sections made by joining standard shapes and plates.
• Cover Plates: Additional plates attached to flanges to increase bending strength.
• Rivet Holes: Holes for rivets that reduce the cross-sectional area.
• Net Area: Gross area minus the area of holes. Critical for tension members.
• Allowance for Holes: An added area or factor to compensate for area loss due to holes in design calculations.

Additional Information: Steel Design Considerations

When designing steel members with holes for fasteners (rivets or bolts), several factors are considered:

• Net Section Rupture: Failure occurring due to the tensile stress exceeding the ultimate strength on the reduced net area.
• Block Shear Failure: A failure mode involving simultaneous shear yielding/rupture along fastener lines and tension rupture across the end of the member.
• Yielding of Gross Section: Failure occurring due to the tensile stress exceeding the yield strength on the gross area (away from holes).
• Bearing Strength: The capacity of the plate material around the hole to resist the bearing force from the fastener.
• The allowance factor of 0.13 discussed is specifically related to compensating for hole area in older design methods, often impacting the determination of the required gross area based on net area calculations. Modern design codes (like Limit State Design) handle the checks more directly based on gross area yielding and net area rupture strength calculations.