📺Understanding the behaviour and design of portal frames [WEBINAR RECORDING]

Posted on December 12th, 2022 in Webinars

Summary

This MasterSeries Software webinar provides a comprehensive guide to steel portal frame building design, utilizing both British Standards and Eurocodes. The session delves into the anatomy, structural behavior, and stability of portal frames, covering crucial aspects like typical dimensions, various load types (dead, live, snow, and wind), and the assessment of second-order effects due to frame flexibility. Participants also gain insight into practical design details such as haunch and apex connections, as well as the important consideration of serviceability limits for deflections. The latter part of the webinar offers a demonstration of the MasterPort software, illustrating how to efficiently generate, analyze, and design 2D and 3D portal frame structures while automatically handling complex calculations and load combinations.

Key Concepts

• Portal Frame design
• Structural Analysis
• Loading Consideration
• Masterseries Software
• Design Details

Description

This presentation, titled "Understanding the Behaviour and Design of Portal Frames with British Standards and Eurocodes," focuses on the design of steel portal frame buildings using MasterPort software from MasterSeries. The webinar is presented by Andre Survey and Patrick McGinley and covers an overview of portal frames, including their advantages, anatomy, analysis, and design, with references to BCSA and SCA publications.

Here's a summary of the key topics:

• Popularity and Advantages of Portal Frames:

    ◦ Portal frames constitute 50% of constructional steel used in the UK and 90% of the single-story market, making them very popular.

    ◦ They are highly efficient, utilize steel's excellent strength-to-weight ratio, are well-documented and optimized, and allow for large, open, unobstructed spaces, making them easily repurposed or extended.

    ◦ Their lightweight design often leads to minimal foundations, and a competitive market exists for proprietary products.

• Anatomy of a Portal Frame:

    ◦ Portal frames rely on continuity and stiffness through moment-resistant connections at the eaves (haunch) and ridge (Apex).

    ◦ The primary frame provides support for both vertical (gravity) and lateral loads.

    ◦ Bracing typically provides resistance in the longitudinal direction, or a portalized bay can be used.

    ◦ Secondary steelwork, such as purlins and side rails, supports the building envelope (cladding), which transfers wind loading to the frame.

    ◦ Other elements include gable bays (often portalized for future extension), roof bracing (transferring wind loads to foundations and providing stability), and variations like multi-span portals with valley beams, hip-end roofs, parapets, mezzanine floors, and prop portal frames. Considerations for overhead traveling cranes and Mansard-type arch frames are also available in MasterPort.

• Loads and Actions:

    ◦ Vertical Actions/Permanent Loads: Self-weight of roof, secondary steelwork, cladding, and services typically range from 0.3 to 0.5 kN/m².

    ◦ Variable Loads: Imposed loads for maintenance are around 0.6 kN/m², along with snow loads, wind loads, crane loads, and equivalent horizontal forces for frame imperfections.

    ◦ Snow Loads: Determined by location and altitude (Eurocodes UK National Annex) with three considerations: undrafted/uniform, drafted/asymmetric, and drifted exceptional snow loads.

    ◦ Wind Loads: A significant factor, determined by Eurocode 4 based on wind velocity, altitude, terrain, distance from sea, and direction to derive peak velocity pressure and external/internal pressure coefficients. Dominant openings require special consideration for uplift.

    ◦ Load Combinations: Eurocodes combine gravity actions with varying factors, considering both wind and snow as leading variable actions, which can generate numerous load combinations.

• Structural Behavior and Stability:

    ◦ Elastic-Plastic Design: Most economical, considering elastic analysis for service loads/deflections and plastic analysis for ultimate loads (collapse due to plastic hinges).

    ◦ Bending Moment Diagrams: Show how moments distribute in elastic and plastic states, with plastic hinges forming at haunches and then potentially the Apex under ultimate loads.

    ◦ Second-Order Effects (P-Delta): Crucial for relatively flexible portal frames, these effects can add up to 15% to design moments. They include imperfections (equivalent horizontal forces) and flexibility of the frame (P-Delta effects) and members (P-small delta effects). A frame stability check (Alpha Crit value) is performed to assess their significance.

    ◦ Member Verification: Columns and rafters require checks for major/minor buckling and lateral torsional buckling. Plastic hinge locations, typically at the end of the rafter (haunch) and Apex, require additional restraint to the compression flange (e.g., stays) to prevent buckling failures.

• Typical Details:

    ◦ Haunch Connections: Increase bending resistance and stiffness at high moment locations, reducing deflection. They are usually cut from the rafter section, often double its depth and about 10% of the span in length.

    ◦ Apex Connections: Angled splice connections, typically sized for bolting rather than resisting major moments.

    ◦ Bases: Generally pinned in the UK to simplify design, though 10% fixity for stability and 20% for serviceability can be assumed for more realistic deflections. Moment-resisting bases are rare due to complexity, size, cost, and reinforcement needs, mainly considered near site boundaries for overturning resistance in fire scenarios.

    ◦ Horizontal Forces: Wind loads on lightweight structures can cause issues with base kicking out; relying solely on passive earth pressure or friction may be insufficient. Tying the base back to a ground-bearing floor slab is an alternative.

• Serviceability:

    ◦ Primarily concerns deflections, especially for deflection-sensitive details like sheeting or masonry panels.

    ◦ Special attention is needed for differential deflections between the gable bay (stiffer) and the first adjacent bay.

• MasterPort Software Demo:

    ◦ User Interface: Simplified, Wizard-style interface for generating 2D and 3D portals, including symmetrical/asymmetrical, mono-pitch, non-symmetric, propped frames, and lintels.

    ◦ Load Generation: Automatically generates gravity, wind, snow, and crane loads, creating ultimate and service load cases.

    ◦ Design Codes: Defaults to Eurocode 3 (with options for 6.10a/b or 6.10) but can also use other standards.

    ◦ Initial Setup: Users define spans (e.g., 30m), rise (e.g., 6.5m), column height (e.g., 7m), haunch/Apex lengths and depths, and material grades. It can model mono-pitch or Mansard roofs.

    ◦ Wind/Snow Load Input: Snow zones and altitude (e.g., 0.5 zone, 100m altitude) are used to calculate snow loads. Wind loads are derived from a MasterKey wind analysis model where users can select a site location on a map (e.g., Birmingham), specify wind direction (e.g., 60 degrees to north), and the program calculates pressure coefficients and wind zones.

    ◦ Analysis and Design Workflow:

        1. Elastic Analysis: Performed initially to check deflection shapes and bending moments.

        2. Auto Design: Sizes members as an elastic portal first, optimizing based on restraint locations.

        3. Space Stability Check: Crucial for Alpha Crit values. If below 10, P-Delta (second-order) analysis is required.

        4. Detailed Design: Optimizes purlin/cladding rail positions and determines optimum torsion stability stays. It performs Appendix BB stability checks and identifies dominant failure modes (e.g., lateral buckling). The software can auto-restrain groups and individual members and auto-size sections if failures occur.

        5. Plastic Analysis: Confirms if the frame remains fully elastic as intended.

    ◦ 3D Building Generation: A 2D frame can be easily extended into a 3D structure by defining bay spacing (e.g., 6.5m) and the number of bays (e.g., 8).

    ◦ Advanced Features: Supports multiple spans (e.g., 24m, 30m), varying column heights, lintels, canopies, and mezzanine floors with supporting columns. It also allows for defining gable posts with specified dimensions and various bracing arrangements (roof, internal, external), including dynamic bracing generation or manual adjustment of leg positions.

The webinar concludes by encouraging attendees to try the software and provide feedback.