Alexander Newman, P.E.

Strengthening Structural Steel Beams ASCE Web Seminar Presented by Alexander Newman, P.E.

Exponent Failure Analysis Associates [email protected] Tel.: 508-652-8533 Copyright © 2008-2009 Alexander Newman

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Agenda     

General issues Code provisions for renovation of steel structures Investigating existing conditions Allowable stresses in early steel beams Strengthening methods - Replacement - Passive vs. active methods - Shortening span - Adding members - Post-tensioning (external prestressing) - Enlarging section  Strengthening connections  Q&A

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Alexander Newman, P.E.

Acknowledgement Many thanks to American Institute of Steel Construction for a permission to reproduce parts of the AISC 2005 Specification, Commentary, Steel Construction Manual, Design Guides and other publications. The note “AISC” under any reproduced item indicates that it is Copyright © American Institute of Steel Construction, Inc. Reprinted with permission. All rights reserved.

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Some Publications for Further Reference

 A. Newman, Structural Renovation of Buildings, McGraw-Hill, 2001 (source of some illustrations here)  ASCE 41-06, Seismic Rehabilitation of Existing Buildings (based on FEMA 356, Prestandard and Commentary for the Seismic Rehabilitation of Buildings)  FEMA 274, NEHRP Commentary on the Guidelines for the Seismic Rehabilitation of Buildings (1997)  FEMA 547, Techniques for the Seismic Rehab. of Existing Buildings (2006)  AISC Design Guide 15: AISC Rehabilitation and Retrofit Guide: A Reference for Historic Shapes and Specifications 

Related ASCE Seminars - 2-day seminar Steel-Framed Buildings: Practical Issues in Design and Renovation - Webinar Renovation of Steel-Framed Buildings

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Alexander Newman, P.E.

Code Provisions for Renovation of Steel Structures 

Do We Even Need to Strengthen?  Don’t rush before careful code analysis

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IBC Chap. 34

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Renovations Codes May be More Lenient

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More on this in ASCE webinar Renovation of Steel-Framed Buildings

Code Provisions for Renovation of Steel Structures 

AISC Documents  AISC Spec. Appendix 5, Evaluation of Existing Structures: Basic approach to analysis, testing, reporting  AISC Design Guide No. 15, Chap. 4, Enhancement of Existing Structural Systems provides practical data for renovation; Appendix A1, Historical Review of AISC Specifications –1923 to 1999  AISC Shapes Database CD V13.0H  All are free downloads for AISC members

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Alexander Newman, P.E.

Investigating Existing Conditions 

Assessing Building Condition  SEI/ASCE 11-99, Guideline for Structural Condition Assessment of Existing Buildings

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Field Investigation  The first walk-through  Exploratory demolition  Analyzing existing steel framing: Try simple methods first - Examine the system. - Analysis methods.

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 Detailed assessment

Investigating Existing Conditions 

Load Testing: Per Governing Code  When to use it.  Load test per IBC and others codes: - Twice the design load; keep in place 24 hrs. - Test is OK if design load deflection is within limit, and within 24 hrs after load removal 75% of max deflection is recovered, and no evidence of failure during or after the test.

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Investigating Existing Conditions 

Load Testing Per AISC Spec. ’05 App. 5, Sec. 5.4  Tested strength: 1.2D + 1.6L (can use Lr, S, R in lieu of L), but use more severe load combinations when code requires  After service load is reached and inelastic behavior begins, monitor deformations - Keep max. test load for 1 hr; deformations should increase < 10% - Also record deformations 24 hrs after test load is removed [but no limit on permanent set is given]

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Investigating Existing Conditions 

Material Testing  ASTM A 370: tensile, bend, impact, hardness properties  Can test in situ materials with smaller specimens, but scale down dimensions proportionally  AISC Spec. App. 5 suggests destructive testing needs Requires determination of an appropriate ASTM standard and using Fy allowed by it, not just measured strength*  FEMA 356 requires testing of Fy , Fu , carbon equivalent of both base and connection material if design drawings with ASTM Spec. are not available

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*S. Zoruba, AISC, Answer in Steel Interchange, MSC, Nov. 2006

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Alexander Newman, P.E.

Investigating Existing Conditions 

Early Structural Steel Beams  Bessemer converter (1856), open-hearth process led to iron with < 1% carbon (steel liquefied => slag rises to surface)  First steel I-beams, C, T, angles in 1884  Standard shapes agreed upon in 1896  First W in early 1900s; max. 15” (1884), 24” (1900), 36” (1927) 1927: First W’s by Carnegie Steel called CB-sections, became the basis of today’s

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Investigating Existing Conditions 

Allowable Stresses in Early Steel  Late 1800s: Each producer used own steel materials  ASTM formed in 1898 after frequent rail breaks  1900: ASTM A7 for bridges and A9 for buildings - ASTM A9-21: Min. Fu = 60 ksi, min. Fy = 30 ksi  Both consolidated in A7 steel in 1939 - A7 was primary steel until early 1960s (ASTM A36) - Exception: WWII (War Production Board issued National Emergency Specification)

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Alexander Newman, P.E.

Investigating Existing Conditions 

Allowable Stresses in Early Steel, Cont’d ASCE 41 for pre-1900 steel: Lower-bound Fy = 24 ksi, Fu = 36 ksi See AISC Design Guide No. 15 for an expanded table

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Investigating Existing Conditions  Steel Strength per ASCE 41 and FEMA 356 FEMA 356 Table 5.2 Lowerbound values (Part 1)

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Alexander Newman, P.E.

Investigating Existing Conditions  Steel Strength per ASCE 41 and FEMA 356, Cont’d FEMA 356 Table 5.2, lower-bound values (Part 2)

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Investigating Existing Conditions 

Translating Lower-Bound to Expected Strength FEMA 356 Table 5.3 AISC-05: Group numbers are gone… ASTM A6 now uses flange t > 2” to classify shapes as “heavy”

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Alexander Newman, P.E.

Strengthening Methods 

General Strengthening Strategies for Beams  Analytical and physical (which is less $??)

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Analytical Approach  Was the original LL less that required today?  1930-63 AISC Fb = 0.6Fy, now 0.66Fy  (10% increase in TL = larger % increase in LL)  LL reduction …

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Strengthening Methods 

Analytical Approach, Cont’d  Use today’s AISC standards – might be more lenient  Check if 50-ksi steel was delivered instead of 36-ksi specified (check SDs)  If IBC-06 applies, can use 15-psf partition load v. 20 In LRFD, check orig. code: BOCA, IBC considered it LL, while SBC, UBC considered it DL* Source: John Henry, The 2006 International Building Code, Structural Engineer, Nov. 2006

 Decrease DL Ceilings, cinder floor fill, replace exist. concrete with LW 18

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Alexander Newman, P.E.

Strengthening Methods 

Passive vs. Active Methods

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Strengthening Techniques  Shortening span  Adding beams  Replacement  Post-tensioning (external prestressing)  Enlarging section - Welding plates, WTs… - Introducing composite action

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Strengthening Methods 

Shortening Span  Add columns, girders  Add diagonal braces (not knee-braces) Check foundations or add new  Add walls with openings

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Alexander Newman, P.E.

Strengthening Methods 

Adding Beams  Deflection compatibility with existing framing  How to get the steel inside? (Use 2 halves?)  What if existing members are still overloaded?

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Strengthening Methods 

Adding Members in Steel-Joist Floors & Roofs  Roof/floor deck: keep or replace?  If needed, reduce bearing depth to fit (and shim) or supply joists without camber and bearing ends  Provide lateral support: weld to deck or provide extra bridging  Replacement may be economical…

From A. Newman, “Metal Building Systems: Design and Specifications,” 2nd ed., McGraw-Hill, 2004

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Strengthening Methods 

Reinforcing Beams by Post-tensioning  Can use ½” HS wire strands  Deflectors of bolted or welded pipes welded to flange or web  Consider eccentricities of brackets on member capacity  (+) Simple, ease of inspection, tendon replacement possible.  (-) Need protection from corrosion, fire, vandalism. (Encasing increases size)… Need access to bottom or sides

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Example in P. Kocsis, discussion on J. Miller’s art., AISC Engineering Journal, 3rd Q, 1997

Strengthening Methods 

Reinforcing Beams by Welding  FEMA 356: Pre-1970 weld Fu = 60 ksi (lower bound) is OK  Is the steel weldable? - Some pre-1923 high in carbon, phosphorus, sulfur - 1923-36 (ASTM A7) generally OK - 1936 -- OK. - Check for existing welding; test coupons or “fillet bend test” (AWS D1.1 Sec. 5) - Special procedures may be needed…

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Alexander Newman, P.E.

Strengthening Methods 

Example of Special Welding Procedures  Project: Conversion of 4-story Chicago warehouse c. late 1800s into a courthouse  Coupons strength: Average Fy = 47.2 ksi, Fu = 66.9 ksi Steel ~ A36, except high phosphorus, sulphur  Welding: Low-heat input procedure. Inspect welds after 48 hrs. If cracks found, 2 options: - E7018 welding (low-hydrogen) with Atom Arc electrodes produced by ESAB - SS (309L) electrodes and procedures

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Source: Flora Calabrese, “Holding Court,” Modern Steel Construction, July 2006

Strengthening Methods 

Adding Members: Minor Reinforcement

Rafter reinforcement at tips to help with torsion, flexure (& to keep welds away from thin web). Sandman Consulting PC, Moorhead, MN 26

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Alexander Newman, P.E.

Strengthening Methods 

Procedure: Method of AISCM 13th ed. p. 3-30

Intermittent welds OK except in high-cycle fatigue applications 27

Strengthening Methods 

Major Reinforcement

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Reinforcing with WT Section  Check connections, deflection compatibility with other beams  What if the beam is distorted?

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Strengthening Methods 

Welding Under Load  Elevated temperatures reduce E, Fy, Fu  How much temporary reduction in properties occurs?  Some say: For thin members, consider strips 1” wide x thickness temporary ineffective… compute “effective” S  Shore loaded beams to prevent weakening by welding, if possible  Can relieve DL by jacking beam up

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AISC Design Guide 15

Strengthening Methods 

Welding Under Load Using ASD: Differing Views  Can existing stresses be relieved by added pieces?  Some (Blodgett and AWS D1.1, for DL stresses > 3 ksi), limit stress in added pieces to the difference between the old allowable and actual in existing steel. (If allow. was 18 ksi and exist. 8 ksi, max. stress in cover plate < 10 ksi.)  More common: Limit stresses in exist. beam and new plate to today code’s values. (A 1949 beam is designed for 0.66 x 33 ksi = 22 ksi, the new A36 plate for 24 ksi)…

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Alexander Newman, P.E.

Strengthening Methods 

In LRFD, Initial Stresses Don’t Matter  As in composite construction, where strength of section doesn’t differ whether shored or unshored…  One exception: If local buckling is a concern, strength may be affected by initial load  Also, deflection for combined section starts from the shape of the original cross-section

Source: C. Carter, Composite Beam Reinforcing, Structure, Winter 2000, p. 54 31

Strengthening Methods 

Weld Design: Full-Length Plates  AISC-05, Sec. F13.3, Cover Plates: Welds/bolts transfer total Vhor from bending… longitudinal distribution ~ intensity  Blodgett: Design welds in full-length plate by traditional F = V A y/(2I), where F = force in weld, V = shear at the point, A = plate area 2 = number of welds, I = I of whole section y = distance from c.g. plate to c.g. of whole section  Max. spacing (Sec. J3.5): 24tpl or < 12” except weathering steel  Also, design weld for any forces applied directly to plate 32

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Strengthening Methods 

Weld Design: Partial-Length Plates  End welds required by AISC-05 Sec. F13.3 (& ASD-89 Sec. B10)  Must develop cover plate’s portion of flexural strength in the beam at theoretical cutoff point by 2 cont’s welds within in the length a’. Per Blodgett: F = M A y/(2I), where M = moment at theoretical cutoff point (Or, conservatively, develop plastic capacity of plate F = A Fy)  If a continuous fillet weld exists across the end… a’ = w (plate width) if tweld > ¾tpl exists a’ = 1.5w if tweld < ¾tpl  a’ = 2w if no weld exists across the end

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Strengthening Methods 

Weld Design: Partial-Length Plates, Cont’d  Intermittent welds needed to develop Vhoriz: f = V A y/(2I), where f = shear flow (force in kips/in) V = shear at theoretical cutoff  Weld size, length and spacing: trial and error Spacing < that for compression or tension members (AISC-05 Sec. E6 or D4)

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Alexander Newman, P.E.

Strengthening Methods 

Design Example: Partial-Length Cover Plate

35 Modified from Example 3.2 in A. Newman, “Structural Renovation of Buildings,” McGraw-Hill, 2001 (pp. 132-135)

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Since no weld across the end is used, a’= 2w =18”. Size the weld to develop plastic capacity of plate F = A Fy = 3.375 x 36 = 121.5 kips Use two ¼” welds with a plastic capacity of 5.57 kip/in (force II to welds LRFD design strength = 0.6x70x¼x0.707x0.75 = 5.57 k/in) Required length = 121.5/(5.57 x 2) = 10.9” but use Lweld = a’ = 18” to fill a’  Use two ¼” weld 18” long at each end. Also, specify nominal welding along the length of plate, such as ¼” weld 2 @ 12” o.c. The actual cutoff points are 9.37’-1.5’ = 7.87’, say 7.75’ and 15.63’ + 1.5’ = 17.13’, say 17.25’ The total plate length is 17.25 – 7.75 = 9.5’ centered between supports 39

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Strengthening Methods 

Reinforcing Beams by Adding Composite Action  Composite steel-concrete beam construction used since 1894  1930 NYC Code: Increased Fb for comp. beams (20 v. 18 ksi)  Steel encased with concrete or shear connectors  Encased construction per AISC ASD 9th ed.: 2 in. of concrete min. + mesh (top of beam 2” above bottom of slab) - No comparable provisions in AISC-05  Useful if bottom is blocked  Design by trial and error

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Strengthening Methods  Composite Action with Shear Studs  AISC-05 Sec. I3 revised shear capacities of connectors  Shear connectors: Use AISCM-05 Table 3-21 ¾” max. studs for steel deck Partial vs. full comp. LRFD vs. ASD  May include parallel slab bars in section properties  For 1 “weak” ¾” stud per rib, NW conc., f’c = 3 or 4 ksi Qn = 17.2 k 42

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Alexander Newman, P.E.

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Design Example: Adding Composite Action

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Modified from Example 3.3 in A. Newman, “Structural Renovation of Buildings,” McGraw-Hill, 2001 (pp. 143-144)

Since AISCM 13th ed. includes only tables for composite W shapes of 50-ksi steel, use AISC LRFD Manual, 1st ed. for 36-ksi beams

AISC LRFD Manual, 1st ed. Partial Table on p. 4-23 44

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Alexander Newman, P.E.

AISC LRFD Manual, 1st ed. Partial Table on p. 4-49

Use Qn = 17.2 k for 1 “weak” ¾” stud per rib, NW conc., f’c = 3 ksi Reduce needed shear capacity by 184.4/215 = 0.858 45

n = 95.4k x 0.858 x 2/ 17.2k = 9.5, say 10 studs total Check max. stud spacing per AISC-05 Sec. I3.2d(6) : 25’ x 12”/(10 + 1) = 27.3” < 40” (5 x tslab) < 36” OK Min. stud length (1.5” above deck) = 2 + 1.5 = 3.5” Max. length (0.5” below TOC) = 5 – 0.5 = 4.5” => use (10) ¾” studs 3.5” long Check beam shear per AISC-05 Sec. G2.1(a) Vn = 0.6FyAwCv where Fy = 36 ksi Aw = (web area) = 15.9 x 0.295 = 4.69 in2 Cv = 1 and Φv = 1.0 Vu = 2.36(k/ft) x 25’/2 = 29.5 k < 1x 0.6 x 36 x 4.69 x 1 = 101.3 k OK 46

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Alexander Newman, P.E.

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Strengthening Composite Beams Use a combination of approaches in two examples.

See also John P. Miller, Strengthening of Existing Composite Beams Using LRFD Procedures, AISC Eng. Journal, 2nd Q, 1996 47

Case Study  Case Study: Strengthening Beams for Column Removal*  A club house at a golf club in NJ  Previous work: 2nd story banquet room added, short-span steel beams and columns replaced ext. wall

48 *Case study courtesy Edward P. Ryan Consulting Engineering. E-mail: [email protected]

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Alexander Newman, P.E.

Case Study 

Case Study, Cont’d  New challenge: Remove those columns, open up space  Investigation: Beams slightly oversized…One column could be removed, w/o decreasing headroom

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Reinforcing Beam Connections 

Early Steel Fasteners  Rivets  Bolts  Welds

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AISC bolt and rivet designations (AISC Manual)

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Alexander Newman, P.E.

Early Fasteners 

Rivets  First to be used; hot vs. cold-formed; clamping effects for hot rivets debatable…bearing or friction?  Sizes: ½” to 1 ½”; most common ¾” and 7/8” with button head.  Identifying size: Per AISCM 5th ed. (1961)… Driven head dia. = 1.5D + 1/8” (1 ¼” for ¾”, 1 7/16” for 7/8”) Manuf. head = 1.5D + 1/32” (1 5/32” for ¾”, 1 11/32” for 7/8”)

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Early Fasteners 

Rivets, Cont’d  Determine allow. rivet values from current AISC spec. as for high-strength bolts.  1923 AISC spec. allow. values: 10 ksi shear or hand-driven, 13.5 ksi for power-driven, no tension stress given.  1949 AISC spec. allow. values: 15 ksi shear, 20 ksi tension

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Alexander Newman, P.E.

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Bolts

Early Fasteners

 First “unfinished” bolts: weak and loose; revisited in 1930s because of rivet noise, fire, $  Common (rough, unfinished, unturned, ordinary) bolts: square nuts, similar to hand rivets

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Early Fasteners 

Bolts, Cont’d  Turned (high-strength) bolts (c. 1950s) ~ similar to power rivets; replaced rivets by mid-1960s  For both types of bolts c. ’50s & ’60s, use tensile-strength table values w/ “stress area” = 0.785(D-0.9743/n)2, where D = nom. bolt size, n = number of threads per in.

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Welding  Developed around 1915, structural use since 1920s  Poor quality in first welds  Discussion follows…

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Alexander Newman, P.E.

Reinforcing Simple Connections 

Common Types of Simple Connections  Types: Framing, seated, partial-depth end-plate  Check by current AISCM, adjust for strength of steel

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Methods of Strengthening  Replacing fasteners  Improving connection design

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Reinforcing Simple Connections 

Check Older AISC Manuals if Designation Known  A, H, HH use 7/8” rivets;

B, K, KK use ¾” rivets

Manual includes design capacities

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AISCM 5th ed. p. 151

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Alexander Newman, P.E.

Reinforcing Simple Connections 

Replacing Fasteners  Replace exist. fasteners with HS bolts (ream holes if needed) Slip-critical A 325 and A 490 bolts can share load with exist. rivets (AISC-05 J1.9)… ignore existing ordinary bolts  Remove rivets by chiseling w/ pneumatic hammer or burning off the heads and pushing shanks out w/ punch and hammer (safety hazard)

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Source: AISC Design Guide No. 15

Reinforcing Simple Connections 

Improving Connection Design  Add welds at the perimeter of connection With existing bearing-type bolts, design welds for total load* With exist. rivets or slip-critical HS bolts, can design welds only for add’l required strength (see AISC-05 Sec. J1.8)  Add welds to properly cleaned existing welds *AISC-05 Sec. J1.8 allows an exception for bolts in shear conn.’s in std holes and shortslotted perp to load that are allowed to share load with longitudinally loaded fillet welds, but the avail. strength of the bolts shall be taken as < 50% of bearing-type bolt strength

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AISC Design Guide No. 15

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Alexander Newman, P.E.

Reinforcing Simple Connections 

Improving Connection Design, Cont’d  Add angles or plates to convert single to double-shear connections  Can weld exist. 1-side L or plate to web before replacing fasteners

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AISC Design Guide No. 15

Reinforcing Simple Connections 

Improving Connection Design, Cont’d  Weld an extension to exist. L or shear plate, adding welds or bolts to connection

 Use new seated L 64

AISC Design Guide No. 15

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Alexander Newman, P.E.

Reinforcing Simple Connections 

Improving Connection Design, Cont’d  For an existing unstiffened seated L connection, can: - Weld framing L or shear plate - Add web stiffener plate (check beam web for crippling and yielding)

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AISC Design Guide No. 15

Reinforcing Connections 

Reinforcing Connections at Diaphragm Chords

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TI 809-05

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Alexander Newman, P.E.

Q&A Alexander Newman, P.E. Exponent Failure Analysis Associates [email protected] Tel.: 508-652-8533 Copyright © 2008 Alexander Newman 67

Bonus Material 

Making Openings in Existing Beams  AISC Design Guide No. 2, Steel and Composite Beams with Web Openings; AISC WEBOPEN program; ASCE 23-97, Specification for Structural Steel Beams with Web Openings  Use same approach as for new, no shortcuts for field work  Critical for success: Require experienced contractors

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Bonus Material: Openings in Existing Beams 

Making Openings in Existing Beams, Cont’d  Place reinforcement before cutting holes  Use thermal cutting (plasma ark cutting faster than oxy-fuel*); can also drill holes at corners, grind to reduce notches  Deflections can increase => avoid cuts where shear is high  Fire watch?

*Source: D. Ricker, MSS Steel Interchange, 12/00 69

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