(517) 467-6145 e-mail us

• 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 arddess the general
  design and overall structural integrity of MBMA manufacturer standard steel constructed
  built-up Rigid Frames. Other aspects of the MBMA manufacturer product are not arddessed
  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 standard 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 standard 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 rdawings 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 rdawings.
  
  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.
  Standard 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 standard 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
  standard 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 arddess
  specific national building codes.

  
  
  

Contact Information

Telephone:

(517) 467-6145 

FAX:

(517) 467-2220

 

Postal arddess:

 

Structural Solutions LLC

8466 Slee Rd.

Onsted, MI 49265

Electronic mail:

General Information:     Thomas R. Price, P.E.

[ Home ] [ Engineering Services ] [ Gallery of Projects ] [ Request For Quote ] [ Resources ]