Section 2: PermitsAnchor: #i1001657
OverHeight and OverWidth Permits
Many permits are for over-height or over-width loads. The routing of these loads usually depends on data contained in the electronic Bridge Inventory File. These types of loads do not normally require a structural evaluation of the affected bridges unless the weight and axle load distribution is such that an over-weight permit may also be required.
The electronic Bridge Record gives the values for available clearances as Items 51 (Roadway Width), 52 (Deck Width), 53 (Vertical Clearance over Roadway), 54.2 (Vertical Clearance Under Bridge), 55 (Lateral Underclearance on Right), and 56 (Lateral Underclearance on Left). These items taken together usually give sufficient information to define the limits for the passage of over-height and over-width vehicles.
The permit investigator, District Permit Officer, or District Bridge Inspection Coordinator can quickly access the electronic Bridge Record to determine if the proposed route is capable of handling the proposed overwidth or overheight load. Truss bridges are particularly of concern for both these types of loads since many are in the 18- to 22-foot width range, and vertical clearance to the portals is often less than normal current design clearances.
The electronic Bridge Inventory File gives vertical clearances to the least inch of clearance over the roadway, including shoulders rounded down to the nearest inch. The posted clearance signs are normally 3 inches less than this value. The clearance symbols maintained on the TxDMV permit maps are rounded down to the next 6 inches below the posted clearance. For instance, if the actual recorded clearance is 14-ft 2-in., the clearance sign is 13-ft 11-in., and the permit maps show the maximum available clearance as 13-ft 6-in. Occasional over-height loads can therefore be permitted for heights slightly over the limits given in the TxDMV permit maps provided there is close coordination between the district and the owner and pre-move specific measurements taken.
Normally, over-width permits are granted simply on the basis of available Roadway Width (the clear distance between curbs or railings). If the over-width load is configured such that the load will adequately clear bridge railings, then moves may be granted for loads significantly wider than the Deck Width. This requires the careful cooperation of all concerned parties including escort vehicles and traffic control. Damage and or removal of signs and delineators may occur for some over-width permits. TxDOT personnel should ensure that all such temporary changes are corrected immediately after the permit load has passed.Anchor: #i1001702
Over-weight Permit Loads
Misconceptions often arise about the relationship between Operating Ratings and Overweight Permit Loads. The primary difference is that overweight Permit Load analysis usually assumes only one load on the bridge, which, therefore, allows the use of single-lane load distribution. The Operating Rating is based on the standard AASHTO load distribution given in the current Standard Specifications for Highway Bridges3 for multi-lane distribution for bridges over 18 feet in width. This distribution implies two or more of the Operating Rating trucks being on the bridge side-by-side at the same time.
The other major difference is that Operating Ratings and Overweight Permit Loads use different load multipliers, resulting in Overweight Permit Load analysis being significantly more liberal than Operating Rating analysis. Review the current Operating and Inventory Ratings, the age and type of structure, the span lengths, and the Condition Ratings for any structure proposed on a permit route. For any Condition Rating of 4 or less, request more detailed information on the structure, including the written inspection comments. Reduced strength in a portion of a bridge can often be avoided by controlling the load path of the Overweight Permit Load across the bridge.Anchor: #i1001722
Over-weight Permit Loads are classified as Routine or Superheavy. Routine Overweight Permit Loads may be allowed in the regular traffic stream. An escort is required if the load is also over-length or over-width. Use the standard AASHTO load distributions since there may be a legal truck alongside the Routine Overweight Permit Load truck crossing a bridge at the same time.
The term Superheavy Permit Load designates total loads over 254,300 lbs gross. This load was determined cooperatively by the Motor Carrier Division and the Bridge Division in 1981 to represent the lower range of a typical superheavy load. It consists of a 14,300 lb steering axle followed by four groups of three axles, each totaling 60,000 lbs. Any configuration with multiple axles with a gross load of over 254,300 lbs is considered a Superheavy load and requires structural evaluation of individual bridges. Loads with individual axles or axle group weights that exceed the maximum permit weights are also considered to be Superheavy. Any load exceeding 200,000 lbs with a total overall length of less than 95 feet is also considered Superheavy.
The Superheavy Permit often requires that the load cross all bridges straddling a lane line in the case of four or more lanes on a two-way bridge, or straddling the center line for a two-lane bridge. This procedure ensures that other legal trucks will not be alongside the Superheavy load and also gives better load distribution. The AASHTO load distributions used for Superheavy loads are, therefore, usually single-lane. This allows higher Superheavy Permit gross loads to safely cross the bridge.
A printout of the proposed list of bridges to be crossed is reviewed by the TxDMV and the Bridge Division. Often, based on experience of the evaluator and other guidelines, it is necessary to structurally evaluate only a portion of the bridges on an extensive proposed Superheavy route. For any bridges on the route with a Deck, Superstructure or Substructure condition Rating of 4 or less, review the actual written Bridge Inspection Record. This bridge-by-bridge evaluation is one of the primary reasons that the data in the electronic Bridge Records must be accurate and up-to-date.
Superheavy Permit Loads are usually speed-controlled on bridges, sometimes as slow as a walk speed to minimize impact forces.
Many Superheavy Permit Loads also have greater than the usual 6-foot axle gage. The gages for Superheavy Permits can commonly be as much as 20 feet with 16 tires on each axle line. Methods of load distribution for these special carriers cannot directly use the customary AASHTO distributions, which are based on 6-foot axle gages with four tires on an axle line.Anchor: #i1001775
Other Differences Between Overweight Permits and Operating Rating
There are other major differences between Operating Ratings and Overweight Permit Loads.
The Operating Rating is usually based on Load Factor (LF) criteria, which use multipliers of 1.3 applied to both the dead and live loads. The live load has an additional allowance of up to 30 percent for impact. Note that Inventory Rating uses a significantly higher live load multiplier of 2.17. The result for either Operating Rating or Inventory Rating is compared to the yield or ultimate strength capacity of the members. A “phi” strength reduction factor (usually from 1.0 to 0.85) is also applied for concrete members.
Overweight Permit Load analysis usually assumes a factor of 1.0 applied to both the dead and live loads. Ten to 30 percent is added to the live load for impact, depending on the speed control and type of load suspension system. Stresses are compared to an allowable maximum of 75 percent of the yield capacity of steel members or 75 percent of the ultimate capacity for concrete members. The reciprocal of 75 percent is 1.33; thus it can be seen that Overweight Permit Load analysis with Allowable Stress (AS) methods has essentially the same factor of safety as an analysis using LF criteria. This result will be demonstrated below by a specific example comparison.Anchor: #i1001795
Overloads on Posted or Substandard Bridges
Occasionally a request is made for a Routine Overweight Permit or a Superheavy Overweight Permit to cross a load-posted bridge. TxDMV does not allow overweight permits for posted bridges.6 However, Section 623.0113 of the Texas Transportation Code allows TxDOT to issue weight tolerance permits for over-weight vehicles to cross load-posted bridges only when there is no other route.
Certain other bridges that are not load posted may not be capable of carrying Routine Overweight Permit Loads or Superheavy Permit Loads. Bridges that are in this category include but are not limited to continuous flat slabs with original H-15 designs. These bridges have short spans and were designed with the single H-load pattern truck placed along the span for maximum design conditions. Many of these bridges when rated with the now-required HS-load pattern, and even using LF analysis, will rate at significantly less capacity than other types of bridges designed with H-load patterns. These bridges, though not currently load posted, must be carefully evaluated when overload permits are considered. This is the primary reason that the original design loads given in the Coding Guide should be entered correctly. Often these bridges have been widened, and the widening design load has been incorrectly entered as the original design load.Anchor: #i1001815
Pre- and Post-Move Inspection
Another occasional responsibility of the District Bridge Inspection Coordinator is to inspect bridges before and after the passage of a particular overweight permit load. A representative of the owner-mover should be present at these types of inspections. Cast-in-place short span slab bridges, particularly those which have been widened from an original H-10 design to an H-15 or H-20 design, are susceptible to cracking by overloads.
Unusual bridges, such as arch spans, segmentally constructed post-tensioned spans, or long-span plate girder bridges, may also need special attention before, during, and after the move of an overweight permit load. It has been found that simple attention to the sounds made by a bridge when the load passes will call attention to possible broken diaphragm connections or lateral wind bracing connections that actually act as torsional bracing for curved and/or heavily skewed structures.