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  • Sample of a rigid frame investigation



    RIGID FRAME INVESTIGATION

    Project Name: Rigid Frame #9999 General Investigation

    TABLE OF CONTENTS


    I. Introduction

    II. Background & History

    III. Frame Checks & Remarks

    IV. Proposed Frame Modifications & Considerations

    V. Conclusions

    VI. Appendix

    I. Introduction

    A preliminary engineering investigation has been undertaken to address the general
    design and overall structural integrity of MBMA manufacturer standaDr steel constructed
    built-up Rigid Frames. Other aspects of the MBMA manufacturer product are not addressed
    in this investigation.


    The objective of this engineering investigation was basically twofold in nature:

    1) To analyze and determine the extent of the structural soundness and adequacy of a
    metal building rigid frame relative to the specifications of the Ninth Edition of the
    AISC, Allowable Stress Design, in conjunction with standaDr Metal Building System
    rigid frame design considerations.

    2) To suggest structural modifications, changes, alterations or similar improvements, if
    any, to the existing frame structural design as it relates to code loads and forces
    and AISC requirements.

    II. Background & History

    The buildings in question are pre-engineered metal buildings manufactured by MBMA
    manufacturer.

    The primary frames are gabled rigid frames with widths of 48'-0" and 60'-0" and are spaced
    typically at 24'-0" on center with 3 different live load cases for each model width of
    12 psf, 21 psf, and 28 psf. The combinations of widths and loadings comprise the six
    unique standaDr buildings:

    Building purlins are 8" deep cold-formed zees at spacings of 5'-0" on center. Building
    girts are 8" deep cold-formed zees at spacings of 5'-0" on center. Building columns are
    at a slope of 4:12, reaching an elevation of 16'-0" over a lateral distance of 4'0" to
    define the building eave height. The frame columns are built-up tapered columns, with
    pinned bottom and fixed top connections. The frame rafters are at symmetrical gabled
    spans of 40'-0" in the 48' series and 52'-0" in the 60'-0" series, respectively with
    roof slopes of 3/8:12. The rafters are built-up tapered members with fixed ends at the
    sidewall columns.

    This report considers only one of the rigid frame models. It is designated as MBMA
    manufacturer #9999 and is a gabled, symmetrical structure measuring 48'0" wide by
    72'-0" long (assumed) having an eave height of 16'-0" and reaching 16'-7 1/2 " at the
    ridge, resulting in a roof slope of 3/8:12. MBMA manufacturer provided the detailed
    information for the frame design in a summary printed output from a frame
    analysis program which contained member properties, frame geometries, member loadings,
    final design forces and many of other bits of data which formed the basis of the
    investigation and the analysis. Along with this computer generated output, MBMA
    manufacturer also provided drawings and details of member parts and frame cross sections.


    III. Frame Checks & Remarks

    The investigation focused on the following aspects of the frame design:

    - Specific Member stress checks
    - Structural Connections check; baseplates, connection endplates, knee area
    - Load Cases & Load Combinations
    - Building Codes; e.g. MBMA versus BOCA wind loads, load combinations
    - Other considerations; e.g. serviceability
    - Member Stress Checks

    The results of the frame analysis for the MBMA manufacturer frame model #9999 with a 21 psf
    live load using the Program listed member forces were as follows:

    Frame Member*/Designation
    Actual Combined Percent
    Stress Ratio Overstressed

    Column 1 1.07 7

    Column 2 1.07 7

    Rafter 1:
    Section 2 1.14 14
    Section 3 1.03 3

    Rafter 2:
    Section 1 1.03 3
    Section 2 1.14 14

    (*Member number refers to MBMA manufacturer frame model member numbers.)

    See attached detailed section checks based on the Ninth Edition of the AISC.

    Built-up members of this frame used the same materials for flanges and webs, Fy = 50 ksi:

    - Flange width: 5.50 inches
    - Flange thickness: 0.1875 inches
    - Web thickness: 0.1345 inches

    The results indicate overstress conditions in some sections of the frame. The maximum
    overstress occurred in the rafter sections at the 15.50 inch depth near the haunch, having
    a 14% overstress. Purlins, girts and flange braces were considered to brace the rigid
    frame at the locations shown on the drawings.

    This rigid frame was also modeled using two other software analytical programs, including
    RISA. Similar, but different results were obtained. The differences were due to modeling
    assumptions and frame stiffness.

    - Structural Connection Checks

    In addition to the stress checks on the members, member connections including the knee
    area, connection bolts, plates, and welds must support and sustain design loadings.
    Knee Area - The rafter-column connection is a moment connection comprised of horizontal
    1/2 " thick endplates and 6- 5/8" diameter bolts, along with a pair of 2 1/2 " x 3/16"
    vertical stiffeners and a 0.1345 inch web section making up the knee area for this frame.
    For gravity loadings of dead load plus live load, the four bolt tension flange connection
    endplates and bolts were found to be adequate, based on the Program forces noted for
    column one. Also, the two 2 1/2 " x 3/16" vertical stiffeners were found to be adequate,
    based on the given forces.

    However, the 0.1345 inch web in this knee area was overstressed by a minimum of 47 percent;
    Fv = 6.58 ksi < fv =9.68 ksi, where Fv = allowable shear stress and fv = actual shear
    stress.

    Additional forces from various possible wind load combinations were not provided, and
    therefore reverse bending checks on the connection plate and bottom pair of bolts was not
    possible.

    Ridge Endplate Connection: The rafter-rafter ridge connection is a moment connection
    comprised of horizontal 1/2 " thick endplates and 6- 5/8" diameter bolts, similar to the
    haunch connection. For gravity loadings of dead load plus live load, the four bolt tension
    flange connection endplates and bolts were found to be adequate, based on the Frame forces
    noted for the rafters. In this case, the bottom flange is in tension for this loading and
    the four bolts are located at the bottom. Wind load combinations could reverse the bending
    moment and place the top flange in tension. Forces for this situation were not made
    available.

    Column Base Plates-The column base connection to the footings are pinned type connections.
    The base plates and anchor bolts were found to be adequate, based on the reactions provided.


    - Load Cases & Load Combinations

    The Program frame model provided four load cases: dead load, live load, wind load right
    and wind load left.

    The roof live load present in the frame data was ______ klf. Considering typical 24'-0
    bays @ 21 pounds per square foot live load, the live loading would be: 21 psf X 24' = 504
    plf, or 0.504 klf. Therefore, the roof live load value used in the program and the
    calculated loading do not agree.

    The wind loads used were equivalent to a single MBMA wind load case, with the same values
    applied in both directions, wind left and wind right.. Other MBMA wind load cases
    representing different magnitudes of force were not applied to the frame design.

    Three load combinations were applied to the frame:

    - Roof Dead Load + Roof Live Load
    - Roof Dead Load + Wind Load Left
    - Roof Dead Load + Wind Load Right

    Load combinations adding together percentages of live load plus wind load were not
    considered.

    - Building Codes

    The MBMA Low Rise Building Systems Manual appears to be the building code used in the
    frame design as noted in the Program frame data. The MBMA is a source for minimum loads,
    but does not equate unilaterally with all other codes, such as BOCA, ICBO and the Wisconsin
    Building Code.

    These codes require the following minimum load combinations for the design of a frame:

    1. Dead Load + Roof Live Load
    2. Dead Load + Roof Snow Load
    3. Dead Load + Wind Load
    4. Dead Load + Seismic
    5. Dead Load + 1/2 Wind Load + Snow Load
    6. Dead Load + Wind Load + 1/2 Snow Load
    7. Dead Load + Wind Load + Snow Load ---- Wisconsin Building Code

    In combinations involving wind or seismic loads, the allowable combined stress ratio can
    be increased by 33 percent.

    These additional combinations and any other code wind load case values were not considered
    in the frame data provided. Other code wind load values provide a sharp contrast in
    forces.

    Consider 80 mph exposures B & C wind loads, total pressure + suction frame forces:

    - MBMA total wind force on frame : 10.72 psf (used in frame design)
    - BOCA total wind force on frame : Exp. B= 13.2 psf / Exp. C = 22.82 psf
    - UBC total wind force on frame: Exp. B= 13.44 psf / Exp. C = 22.90 psf

    In locations with Exposure C wind and different building code values (other than MBMA )
    the frames would see more significant forces than those considered in this investigation.

    - Other Considerations

    Frame sidesway and rafter deflections and serviceability requirements are also pertinent
    and minimum allowable values must be set and met if code compliance is to be considered.
    StandaDr metal building rafter deflections are held to a value equivalent to: L/180, where
    L=span in inches. This L/180 deflection represents dead load and live loads. In this case,
    the maximum allowable frame deflection which was held was less than or equal to 3.2 inches
    at middle of the 48'-0 span. The Program value for this combination was ____ inches which
    indicates that the rafter deflection is acceptable.

    Frame sidesway for metal building systems is typically Eave Height/60 for a 10 year wind.
    In this case, the allowable frame sidesway would be 4.27 inches. The Program value for
    this combination was ____ inches which indicates that the rafter deflection is acceptable.


    IV. Proposed Frame Modifications & Considerations

    - Specific Member Overstresses

    There are two options which appear appropriate for those members which are overstressed:

    Option1: Increase the member depths to improve their section properties including moments
    of inertia and section modulus which would increase member allowable stresses to
    meet or exceed the actual stresses. An increase of 1-3 inches on at specific
    locations will eliminate the current overstressed conditions.

    Option 2: Increase the flange material to improve their section properties including
    moments of inertia and section modulus which would increase member allowable
    stresses to meet or exceed the actual stresses.


    - Structural Connections check; baseplates, connection endplates, knee area
    Knee Area-There are two options which appear appropriate for the overstressed web in the
    knee area, based on the provided frame forces:

    Option1: Increasing the web thickness to approximately 0.156 inches could eliminate the
    overstressed condition.

    Option2: Add web stiffeners to the knee area to reinforce the 0.1345 inch existing web.
    A pair of 2 1/2 " x 3/16" vertical, horizontal or diagonal stiffeners can be
    added. See attached sketch showing proposed location of stiffeners.

    Endplate Connections- The present orientation of the 6-bolt connections is appropriate for
    only the load combination of Dead Load + Live Load where the tension flange in each case
    has four bolts. Increase wind load forces and additional load combinations combining wind
    and snow loads might present different ranges of forces. With the availability of different
    forces to evaluate, the endplates could be more thoroughly investigated and checked for any
    overstressed conditions.


    Column Base Plates - With the availability of different forces to evaluate, the base plates
    and anchor bolt capacities could be more thoroughly investigated and checked for any
    overstressed conditions.


    - Load Cases & Load Combinations & Building Codes

    The present frame does not represent a solution for the full spectrum of forces and stress
    reversals and stress shifts possible due to load cases and combinations which would meet
    most building codes, such as BOCA and UBC.

    It is suggested that the applied load cases and load combinations be expanded to include
    the standaDr loads and combinations represented by the building codes in the locations where
    the frames are going to function.

    - Other considerations

    The current performance of the frame in terms of serviceability is adequate, based on
    standaDr metal building deflections. All future frame changes, reflecting revised loadings
    and forces will require continued monitoring of frame member and system displacements.

    V. Conclusions

    There are two primary issues with the present rigid frame and loadings.

    First, locations of possible overstress need to be looked at, reviewed and checked over
    and modified if deemed necessary. These areas included specific member stresses and the
    web in the knee area.

    Secondly, frame load cases and load combinations could be revised and expanded to address
    specific national building codes.



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