TxDOT has updated its policy on the Load Rating Statement form for assigned ratings to clarify that the following conditions must be met for assigned load ratings to be applicable:. For assigned load ratings, the coding for items 63 and Assumed load ratings can be utilized for concrete structures that do not have plans.
These types of ratings are based on field conditions of the structure and documented engineering judgement. The following conditions must apply in order for an assumed load rating to be applicable:. Load Factor is to be used for all on-system bridges, except for timber bridges. It is difficult to assign an ultimate strength to timber. Therefore, both on- and off-system timber bridges are rated using only AS methods. Load Factor may be used for all off-system bridges, but AS should only be used for timber and masonry off-system bridges.
A Load Rating Summary Sheet should be completed for all calculated load ratings. The first version of Supplement No. When a bridge was originally designed, the designer often had to select the next size of reinforcing bar, size of steel beam, or thickness of cover plate to meet the design stress criteria.
Sizes that were larger than the theoretically perfect size of member result in Inventory Ratings significantly higher than the design loading. However, the design loading and date of original construction are important parts of the bridge data since they often provide a basis for determining initial routing of overload permits. If the original design was made using an H-load, such as H or H, then the equivalent HS Inventory Rating will usually be significantly less numerically.
For example, an H design might rate at HS However, this difference means that the total inventory HS-load capacity is 43, lb two 19, pounds axles and one 4, lb axle totaling Determine the original design load from a review of the bridge plans if available. If the structure essentially matches an old TxDOT standard bridge, then the design load for that standard can be used for the Design Load Item Enter appropriate notation about this in the Electronic Bridge Record , and update the electronic Bridge Inventory File.
When AASHTO first introduced the use of Grade 60 reinforcing steel in the Interim Bridge Design Specifications, 28 the allowable of 24 ksi for Grade 60 was assigned based approximately on the ratio of the Grade 60 ultimate strength to that of Grade Thus, the AS procedures were still compatible in factor of safety for concrete members.
LF rating procedures usually assign a dead load factor of 1. The resulting stresses or bending moments are compared to the yield of steel members or the ultimate capacity of concrete members also considering appropriate phi strength reduction factors. Note that the value of 2. The load factor of 1. The factor of 1. Specific analysis of structures for over-weight loads, particularly superheavy permits over , pounds, is usually done with a load multiplier consistent with the restricted speed of the vehicle.
Commonly this factor is about 1. This procedure is explained more fully in Chapter 6, Routing and Permits. Do not consider temporary repairs for Inventory or Operating Ratings.
However, take temporary repairs into account when assigning the operational status code of Item 41 to the structure. Temporary repairs are to be considered for the operational status code only until a more permanent repair is made. Do not use temporary repairs for more than four years.
The Inventory Rating directly affects the Sufficiency Rating, so therefore do not assign any weight to temporary repairs in the Load Rating calculations. Use all field information and conventional analysis techniques when the design loading is unknown or deterioration exists. Even when the design loading is known, the only acceptable method for accurate load rating is to do calculations based on the plans and known field measurements.
A concrete bridge with unknown reinforcing details no plans need not be posted for restricted loading , provided that the following two considerations are met:. Ratings are assumed in the permanent Bridge Record, described in Chapter 8. This procedure is summarized in detail by Figure Three additional considerations for rating concrete bridges with unknown reinforcing are:.
A comparative original design rating can be used to estimate the amount of reinforcing in the main members. Normally, if the design was done prior to about and the above five considerations are met exist, then the amount of reinforcing can be estimated based on a percentage of the gross concrete area of the main beams if tee-beam construction , or depth of slab if slab construction. Anchor: igrtop Figure Load Ratings for Concrete Bridges without Plans.
Unusual bridges, such as those composed of old railroad flat cars, can be rated, but ensure that the critical rating component is considered. For instance, flat cars were originally designed for a maximum point load combined with a uniform load over the whole car. When used for traffic loadings, even though the main two-girder members may give a good equivalent HS load rating, the transverse stiffening members and floor beams often control the live load capacity.
Another unusual type of bridge in Texas is the continuous cast-in-place CIP flat slab. Most of these bridges were designed in the s and s with an H or H load pattern.
Unfortunately, the design negative moments were from the single truck load in one span. As a result, these bridges may be under-designed for HS-loadings and, as a consequence, may require a load restriction. Design procedures using an HS design load; use a lane load with two concentrated loads in adjacent spans for the controlling negative moment case for longer continuous bridges. For shorter, continuous bridges, an HS design uses two heavy axles of the HS load pattern at variable spacing in adjacent spans.
As a result, this type of bridge has greater strength for multiple trucks positioned in the middle of the bridge span. Some structural evaluators make live load distribution adjustments based on the number of lanes loaded for flat slab bridges. Exercise care and properly correlate it to two- or three-dimensional methods of analysis to use this procedure. Previously, all ratings were done with the equivalent H-truck and HS-truck shown in Figure At a minimum, confirmation of the governing HS20 or HL93 Operating Rating; show the factored components of the rating factor equation.
A more comprehensive check is recommended, especially when results significantly differ from the original Design Load increased to the Operating Level. The governing rating may be moment, shear, or occasionally service — but only one rating governs the Design Vehicle overall. It is helpful to show the internal components as well, the factors and such, but only the fully factored components are required. LRFD design strength criteria is sufficient.
How will segmental bridges be assessed? On a case-by-case basis. EV analysis for segmental bridges may be based upon the existing load rating, and amended to include EV ratings. Only consider mixed traffic EV and the governing legal load as a posting avoidance technique. When load rating the EV vehicles, are we to simultaneously consider other vehicles such as other Legal Loads in other lanes on the bridge?
However if the EV rating factor is less than 1. If other posting avoidance techniques are unsuccessful, then try the most difficult posting avoidance technique — mixed traffic.
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