Knee Braced Crane Girders-Problems and Solutions

Analysis of knee braced crane girders for a crane runway upgrade project without considering knee brace affects could create conditions for accelerated fatigue failure in one or more areas. The advantages and disadvantages ff two alternative approaches are examined.
 

Knee braced connections between columns and beams have been an important part of many different structures throughout the centuries. Portal-type knee braced structures, in different variations, can be found in old bridges, churches and castles. In old industrial buildings, knee braced connections were widely used in the roof truss to column connections in the lateral plane, between columns and struts in the longitudinal direction, and between columns and floor beams. In all of these cases, especially timber construction, knee braced connections provided a perfect engineering solution that increased stability of the structure, created rigid connections between elements and reduced the bending span for beams. Therefore, it is understood why the designers of early crane runway girders for overhead traveling cranes adopted this approach which had worked well in portal-type structures before.12 

Crane girders with knee braces are a typical feature in many mill buildings. The majority have survived in service for many years without any problems. However, some of them have experienced fatigue related damage.

The original intended function of the knee brace in crane runway design was to provide longitudinal stability of the crane runway against crane longitudinal forces and to help the columns deliver a horizontal shear to the foundations (Fig. 1).

The secondary effect of the knee brace, acting as an intermediate spring support for the girder, had not been considered. Girders were analyzed without considering this effect while knee braces were only analyzed for run­ way horizontal loads. In addition to the spring support function, knee braces provide a partial restraint to the crane girder support rotation resulting in a stress reversal in the girder and its supports. This stress reversal could lead to fatigue failure of the girder and/or girder to column connection details. It has been analytically proved that the magnitude of this stress reversal depends, to a great extent, on the relationship between stiffnesses of the girder, knee brace and column. Occasionally, this stiff­ ness relationship was established by the design in such a way that it resulted in a minor stress reversal which was successfully resisted by the structure, thereby, causing no fatigue failure. .

A computerized design approach is proposed in this article for the analysis of knee braced crane runway girders as a longitudinal frame with special modeling of the knee brace and crane girder supports. The proposed approach helps the design engineer determine the magnitude of the force reversal in the girder/column/knee brace system, check the fatigue stress range in these members and more accurately account for knee brace effects in crane runway modification projects.

Study Case Review

Knee braced crane girders can be found in steel mill buildings designed up to the late 1960's. In many steel mill buildings, knee braced crane girders are combined with X-bracing (Fig. 2). The introduction of X-bracing between crane columns has eliminated the original intended function of knee braces. Nevertheless, design engineers continued to design the runways with knee braced girders and X-bracing. The designers of this belt and suspenders system probably considered that extra tools for longitudinal stability would never decrease overall effectiveness, but it did. Numerous cases have been documented in which knee braced crane girders have developed cracks in the web of the girder in the support region, sheared web splice bolts and ruptured girder seat bolts.


In 1965, Mueller described knee braced crane girder problems as well as the failure modes of these girders.3 An exaggerated model presented by Mueller (Fig. 3), illustrated the possibility of knee brace buckling, in addition to web cracking, web splice plate distortion and uplift of the girder seat . However, Mueller indicated that he had not actually observed knee brace failure other than occasional loose rivets.

For the last 10 years, the authors of this article have observed many knee braced crane runway girders, from light 10-ton shipping cranes up to 150-ton melt shop cranes. Buckled knee braces were never observed. The majority of failure cases included sheared and ruptured bolts, broken web splice plates and girder to column diaphragms. Girder web failure has been rarely observed. At the same time, large numbers of knee braced crane girders without any sign of failure have been observed. The reason why some knee braced crane girders fail and others perform without problems is the relationship between the independent stiffnesses of the girder, knee brace and column. This relationship governs the magnitude of the knee brace axial load, reversible crane girder support reactions and vertical bending moment envelope for the crane girder. These forces define fatigue type stresses in the girder which could cause the observed failures of knee braced girder components.

Based on the reported number of knee braced crane girder failures, the AISE Technical Report No. 13, Guide for Design and Construction of Mill Buildings does not recommend the knee braced crane girder design.4 Al­ though the authors agree with that recommendation, there are large numbers of existing knee braced crane girders already installed which raise the question whether they should be removed or kept. This question demands attention especially when a crane runway upgrade is being considered. It can be answered by analyzing the crane runway as a longitudinal frame consisting of crane girders with knee braces, columns and any existing X-bracing, prior to modifying of the runway girders. Computerized programs equipped with moving load design features (STAAD-Ill, STRUDL, etc) can be used to create force envelopes for the girder and knee braces under investigation. An increase of reversal stresses, which could cause a fatigue failure of the knee braced girder components arising from an increase in crane loads, can be evaluated from the proposed framing analyses.

In most cases, especially in girders with a design span up to 30 ft, analyses of knee braced crane girders as a longitudinal crane runway framing show a substantial reduction in the maximum positive bending moment for the girder in comparison with simple span girder analyses. A negative bending moment in the critical mid-span area of the girder could appear when the adjacent girder is loaded but the magnitude of the total moment fatigue range would still be less than the maximum positive moment for the simple span girder. In such cases, special attention should be paid to detailed analyses of knee braces and their connections to the column and girder. Inn all analyses, the designer should recognize the reversible character of stresses and check the members for compliance with fatigue design criteria.

An analysis of an existing crane runway without accounting for the knee brace effect provides an inaccurate solution. Reinforcement of an existing knee braced crane girder based on simple span girder analyses without sub­ sequent knee brace removal could create conditions for a low-cycle fatigue failure of elements, such as the girder web, seat bolts and web splice details, arising from cyclic stresses which will increase after the upgraded crane loads are applied. Removal of knee braces without girder support modifications could create an even more favorable condition for the fatigue failure of the girder web in a widely spread type of girder support with web splice plates.

In the case of a proposed crane load increase, there is a possibility that accounting for the knee brace effect could eliminate reinforcement of the crane runway girders. Reinforcement or replacement of knee braces is less expensive than reinforcement or replacement of crane girders.

Computer Modeling of Crane Girder/Knee Brace/Column Framing System

Longitudinal crane runway framing consists of knee braced crane girders supported on columns. This framing is a statically indeterminate system, in which simple span girders are transformed by knee braces into a type of continuous crane girder with partial rotational restraints at girder supports. The intersection detail between the crane girder, the knee brace and the column is the most important detail of this frame model.

Two major types of knee braced crane girder supports are shown on Fig. 4 and 5. Crane girders supported independently on the column are illustrated in Fig. 4. The girders are free to rotate at their supports. Crane girders with a bolted or riveted web splice over the column, which provides restrain t to the girder support rotation, are shown in Fig. 5. This type of girder has been utilized more often in the steel mill crane runways.

In this computer frame model, the crane girder is represented by members located at the girder's neutral line level. The points of the girder supports on the column and the knee brace are located below the girder neutral line at the bottom flange level. This feature should be included in the computer model. An example of the knee braced girder support computer model is shown in Fig. 6. Members l through 4 and 11 through 14 are crane girder, column and knee brace members, respectively. Members 5 through 10 are false rigid members with large flexural stiffness, representing the girder to column and knee brace connection offsets. The column member properties in this frame are represented by the properties of the crane shaft only.

The model of the crane gird.er support with end girder rotational restraints is similar to Fig. 6 with the exception of an additional member between the girder ends, which represents the web splice·.

Crane moving loads should include two cases, recommended by the AISE Technical Report No. 13:

  • Case 1-Vertical wheel loads from one crane with impact.
  • Case 2-As many cranes as can be positioned (from total number of cranes operating on the crane runway) to produce the most severe loadings without impact.

The following examples illustrate how use of the proposed computerized analyses assists in evaluating the knee braced runway condition under existing or upgraded crane loadings, while investigating the most economical method of crane runway modification.

Design Example No.1

An existing runway with knee braced crane girders was designed for a 10-ton crane with maximum wheel loads of 34 kips (4-wbeel crane, wheel spacing 13 ft-0 in.). It is proposed to upgrade the existing crane to 20-tcn lifting capacity with a maximum wheel load of 50 kips. The existing 21-ft span crane girder is made from a W27x85 with double angle knee braces of L3x3x3/8.

Two analyses are made to determine the appropriate modifications to the runway:

  • Simple span crane girder analysis.
  • Knee braced crane girder frame analysis.

The solutions are based on the following criteria: girder material, steel ASTM A7, FY= 33 ksi; and formula Fl -6 (AISC 89)5, allowable top flange bending stress, 15 ksi (compression). 

Simple span crane girder analysis- Analysis of the existing and proposed crane wheel loads indicate:

  • Existing crane wheel loads with impact factor, 1.25.
  • Mmax = 223 ft-kip, maximum bending stress = 12.4 ksi < 15 ksi allowable.
  • Proposed crane wheel load increase from 34 to 60 kips.
  • Mmax = 328 ft-kip, maximum bending stress = 18.2 ksi > 15 ksi allowable.


These data show that the existing girder should be re­ placed with a new girder, W27xl02 (ASTM A36) or the bottom flange of the existing girder to be reinforced with WT7x37.

Knee braced crane girder framing analyses - Because crane loadings in adjacent spans affect the analyzed girder forces, a model of four bays of framing is employed with the moving crane load through all bays. A computer model for vertical load analysis is shown in Fig. 7. The typical girder to be investigated is located in the second bay. Deflection and moment diagrams for the girder are illustrated in Fig. 8. An analysis shows:

  • Mmax = 245.2 ft-kip with impact.
  • Maximum bending stress = 13.6 ksi < 15 ksi allowable.
  • Bending moment fatigue range (no vertical impact included) = (+245.2- (-37.5))/1.25 = 226.2 ft-kip.
  • Bending stress fatigue range = 12.4 ksi < 16.0 ksi allowable for category B fatigue.
  • It is, thus, concluded that the girder does not need reinforcement.

  • Proposed crane wheel load increase from 34 to 60 kips.

  • Mmax = 328 ft-kip, maximum bending stress = 18.2 ksi > 15 ksi allowable.

These data show that the existing girder should be re­ placed with a new girder, W27xl02 (ASTM A36) or the bottom flange of the existing girder to be reinforced with WT7x37.

Knee braced crane girder framing analyses - Because crane loadings in adjacent spans affect the analyzed girder forces, a model of four bays of framing is employed with the moving crane load through all bays. A computer model for vertical load analysis is shown in Fig. 7. The typical girder to be investigated is located in the second bay. Deflection and moment diagrams for the girder are illustrated in Fig. 8. An analysis shows:

  • Mmax = 245.2 ft-kip with impact.
  • Maximum bending stress = 13.6 ksi < 15 ksi allowable.
  • Bending moment fatigue range (no vertical impact included) = (+245.2- (-37.5))/1.25 = 226.2 ft-kip.
  • Bending stress fatigue range = 12.4 ksi < 16.0 ksi allowable for category B fatigue.

It is, thus, concluded that the girder does not need reinforcement. 


Analyzing the knee bracing:

  • Knee brace maximum compression 58 kips, tension 4 kips.
  • Unbraced length, 4.25
  • Allowable compression force for two L's 3x.3 x3/8 77 kips > 58 kips allowable.

These data indicate that the knee braces are satisfactory but the connection between the knee brace, column and girder with the double-shear 3/4-in. dia rivets would fail. The rivets should be replaced with '4-in. dia, A325 bolts. (X-bracing is available in the crane shaft plane, therefore, negligible crane traction forces will be developed in the knee braces.)Analyzing 4¾-in.. dia, A325, crane girder seat bolts:

  • Cyclic uplift load per girder support from O to 25.4 kips.
  • Cyclic shear load per girder support from O to 40.5 kips.
  • Fatigue range per seat bolt, tension O to 6.4 kips, shear O to 10.l kips.

Since the fatigue strength of high-strength bolts loaded in tension in T-type connections is significantly affected by the preload in the fastener and by prying action, it is recommended that the permissible tensile forces in this type of connection not exceed 0.75 allowable tension/bolt for more than 500,000 cycles. For 3/4 in. d..ia, A325 bolts,

0.75 T allowable = O. 75 x 19.4 = 14.5 kips > 6.4 kips.

Fatigue shear in the bolts, 10.1 kips, exceeds the 7.5 kips allowable for a slip critical connection. Changing from slip-critical to a bearing type of connection in the joints subjected to force reversal could cause excessive movement of the connected parts and eventual failure of the seat bolts. The recommendation is to replace the existing seat bolts with 1-in. dia, A325 bolts.

In summary, analyses of the knee braced crane girder as part of crane runway framing make it possible to increase crane wheel loads from 34 to 50 kips (approximately 47%) without reinforcement of the crane girder. Only modification of bolted connections (girder to column, and knee brace to girder and column) are required. The column combined axial and bending stress level should be checked.

Design Example No.2

An existing crane runway with knee braced crane girders was designed for a 35-ton crane with maximum wheel loads of 40 kips. It is proposed to install a new crane with a 50-ton lifting capacity with a maximum crane wheel load of 54 kips.

The existing 24 ft-0 in. span crane girders made from W36x l 35 with two L's 4x3x3x3/8 knee braces.

Two analyses are made to determine, as in example No. 1, the appropriate modifications to the runway:

  • Simple span crane girder analysis.
  • Knee braced crane girder framing analysis.

The solutions are based on ASTM A36 steel with an allowable bending stress of 22 ksi.

Simple span crane girder analysis - Analysis of the existing and proposed crane wheel loads indicate:

  • Existing crane wheel loads with impact factor, 1.25.
  • Mmax = 694 ft-kip, maximum bending stress = 19 ksi < 22 bi allowable.
  • Proposed wheel load increase from 40 to 54 kips.
  • Mmax = 937 ft-kip, maximum bending stress = 25.6 bi> 22 bi allowable.

These data indicate that girder replacement or reinforcement is required.

Knee braced crane girder framing analysis - Three bays of the longitudinal crane runway framing are modeled in Fig. 9. The girder being investigated is located in the second bay. The girder web splice over the support is represented by the member shown in the enlarged girder support model. The presence of this splice plate creates a substantial restraint to the girder support rotation.

The first run of the computer analyses showed that a horizontal shear and negative bending moment of large magnitudes are developed in the girder web splice plate, which would cause a failure of this member. In reality, the failure of the web splice can be observed in the form of bolt and/or plate failure, or development of slots in the web upper bolt holes, which releases the girder from the rotational restraint at the support.

The second run of the computer analyses was performed for the runway model without crane girder web splices. For the crane girder, considering vertical bending moments:

  • Mmax = +742 ft-kip to Mm.in -120 ft-kip (vertical impact included).
  • Maximum bending stress = 20.3 ksi < 22 ksi allow­ able.
  • Bending moment fatigue range (no vertical impact included) = [742-(-120)]1. 25 = 690 ft-kip.
  • Bending stress fatigue range = 18.9 k.si vs 18 ksi for category B fatigue loading 3.

The 18.9-ksi bending stress fatigue range represents an acceptable 4.7% theoretical overstress.

For the knee braces:

  • Knee brace maximum compression 79.6 kips, tension 2.1 kips.
  • Unbraced length, 6.1 ft.
  • Allowable compression force for two L's 4x3x3/8=87.5 kips.

The data indicate that the knee braces are satisfactory but the connection between the knee brace, column and girder with three double shear 7/8-in. dia, A325 bolts would fail since the allowable shear, 61.2 kips, is less than 79.6 kips. The recommendation is to replace existing bolts with 1-in. dia, A325 bolts.

Analyzing conditions for the 4¾-in. dia, A325 crane girder seat bolts:

  • Maximum cyclic uplift, 43 kips or 10.8 kips/bolt, in cyclic tension is acceptable (see Example o. 1).
  • Maximum cyclic horizontal shear is 59.2 kips or 14.8 kips/bolt.

The maximum cyclic horizontal shear/bolt exceeds the allowable 10.2 kips/bolt for slip critical joints. Changing from a slip-critical to a bearing type of connection in the joints subjected to force reversal is not recommended (see example No. 1 for explanation). Therefore, the seat bolts should be replaced with 11/r in. dia A325 bolts.

In summary, the 35% crane wheel load increase will not require crane girder reinforcement if the knee braced crane girder is analyzed as a part of the crane runway framing. Bolts in fatigue sensitive connections should be replaced with stronger bolts to satisfy fatigue design requirements.

Summary

Knee braced crane girders are fatigue sensitive structures. Most of the observed cases of failure include shear of girder seat bolts, cracking of girder web splice plates over supports or shearing of bolts at those splices and cracking of girder to column diaphragms. Less frequent modes of failure include girder web cracking in the support area and bolt or rivet shea r at the knee brace to girder or column connections.

Analyses of knee braced crane girders for a crane runway upgrade project, without considering knee brace effects one or more of the areas mentioned in the previous paragraph.

Two alternate approaches for a knee braced crane run­ way upgrade can be considered: 

  • Crane girders are analyzed and modified into simple span girders. In this case, knee braces have to be removed, girder support. on columns modified to as­ sure rotational freedom and the crane girder rein­ forced as required by simple span girder analyses. Crane girders are analyzed as parts of the crane runway frame while accounting for the knee brace effect.  In many cases, analyses show that knee braced girders can carry crane loads approximately 30% larger than the original crane loads without exceeding design criteria. In such cases, minor modifications of connections can be expected.  If these analyses show that a reinforcement of the crane girder is still required, the first approach to girder modification should be considered.

The design engineer and the plant owner should consider both. The advantages and disadvantages of each approach before making a final 

decision on which approach to use. The first approach provides the plant owner with a trouble-free crane runway, but the modification project is substantially more expensive than the second approach and requires more downtime to perform the crane runway modification.

The second approach provides the plant owner with a quick, concise  method   of increasing  the  crane runway loads  with   minimum  expenses  but, on   the other hand, results in a fatigue sensitive system which requires periodic monitoring during the life of the structure. This approach can also be considered as a temporary measure, giving the plan t owner an opportunity to upgrade the crane and temporarily postpone the crane runway upgrade project.

In both cases, modification of the girder seats and/or girder ties to the column is expected. In each case, those details shall be designed to better satisfy the particular girder design model.

REFERENCES

I. Ketchum, M. S.• Th• Design of Steel Mill Buildings. Fifth Edition, 1932.

  1. . Dunham, C. W., Planning Industrial Structures, 1948 .
  2. Mueller, J . E., "Lessons from Crane Runways." AJSC Engineering Journal, Jan. 1965.
  3. AJSE Technical Report No. 13, Guide for Design and Construction of Mill Building s , 199 1 Edition .

Specification for Structural Building, Allowable Sires. Design , AISC, 1989.

New AISE Subcommittee on Lubrication and Fluid Power Technology

A new Applied Engineering Subcommittee on Lubrication and Fluid Power Technology has been approved. An organizational meeting will be held at 9:00 a.m. on May 7, 1993, at AISE headquarters in Pittsburgh.

Employees of steel companies involved in lubrication and fluid power may apply to attend the meeting and can apply for subcommittee membership. Consultants and vendors may also apply. All attendees must have a supervisory or other responsible direct role in lubrication and fluid power. Sales personnel are not eligible. A membership balance between steel company employees and others will be maintained.

Persons who are regularly active in the areas described above and wish to attend the organizational meeting may obtain more information by contacting: Association of Iron and Steel Engineers, Three Gateway Center, Suite 2350, Pittsburgh, PA 15222, Phone: (412) 281-6323 or Fax: (412) 281-4657