Abstract
A representative one-tenth scale model of an open-deck three-span timber trestle bridge was constructed and subjected to load testing in the laboratory. The scaled timber trestle bridge incorporated a realistic wooden pile foundation in sandy soil. A computer-based analytical model was created with AxisVM software. The analytical model was used to predict the behavior of the physical model.
The three-span complete timber trestle bridge model was constructed out of common dimension Douglas Fir. Each span was 1176 mm (48 in) long and utilized two semi-continuous bridge chords. Peeled pine poles were used in the pile foundation. This foundation type was used to create support motions similar to those observed in previous field testing. Wood crossties were also included in the physical specimen.
Observed support motions of the physical specimen were similar to motions observed in field bridge tests. The AxisVM model was successful in predicting the behavior of the physical specimen. Typically, predicted deflections were within 5% to 10% of the measured values. The support motion created by the pile-soil interaction was also modeled successfully by using a linear spring approximation.
1. Introduction
1.1 Timber Bridges
Timber was widely used as the primary construction material of bridges through most of history due to its availability and ease of use in construction. Only recently have wood's structural qualities been evaluated [Troitsky, 1994]. More than one third of the United States is considered forestland, which is capable of producing large amounts of structural timber each year [Smith, 1999]. The availability of timber made it the most logical option for construction during the industrial revolution and western expansion of the United States well into the middle of the twentieth century. There are more than 71,000 highway and non-highway timber bridges in the United States [Ritter, 1990]. There are more than 2,900 km (1,800 miles) of timber railroad bridges in the United States [Ritter, 1992] and many of the timber railroad bridges have a trestle configuration.
1.2 Background
Timber trestle bridges consist of longitudinal stringers supported by intermittent bents that are typically supported by piles. The bridge deck is attached to the stringers. The stringers are the primary supporting structure of trestle bridges and are designed as simple span beams under the recommended loading [AITC, 1994]. Many timber trestle railroad bridges have been in services for more than 50 years. Some have been performing successfully for nearly 100 years [Byers, 1996]. During such long operating lives these bridges have endured material deterioration and increases in the train loads. The ability of these bridges to perform successfully under such conditions stems, in part, from the conservative nature by which the bridges were designed. In addition, the wood used in newer timber trestle bridges is typically chemically treated with creosote. This treatment helps reduce the rate of wood weathering and decay.
Increased loads are a basis for ongoing consideration of a major increase in the minimum design load requirement. Consequently, in the early 1990's, the Association of American Railroads (AAR) initiated an extensive program to evaluate the strength of existing bridges, whether comprised of timber or of other construction materials. Also the AAR has been pursuing methods to rehabilitate in-situ bridges to increase bridge strength and stiffness up to the impending standards. One approach to rehabilitate the timber trestle bridges is to add additional members and/or replace deteriorated ones. Addition of intermediate supports to bridges has also been considered.
1.3 Description of an Open Deck Timber Trestle Bridge
The design of timber trestle bridges is specified and controlled by the American Railway Engineering and Maintenance Association (AREMA) via its design manual [AREMA, 1995]. This manual also specifies the construction, maintenance and inspection procedures for timber railroad bridges. Figure 1.1 below illustrates the general configuration of a standard "open deck timber trestle railroad bridge." This bridge is composed of two distinct sections, that of a "superstructure" and a "substructure."
The superstructure is the portion of the bridge that includes the traveling way. The substructure of a bridge is comprised of the foundation system that supports the superstructure. Timber trestle bridge substructures generally use a series of timber piles connected to each other by bracing and a pile cap above, this arrangement being referred to as a "bent." Generally three to six round piles approximately 355 mm (14 in.) diameters are used. The cross bracing is connected to the segments of the piles that are above ground and is used to prevent lateral movements. The pile cap is spiked on top of the piles and has typical dimensions of 381 mm (15 in.) wide and up to 508 mm (20 in.) deep with lengths slightly greater than the total width of the superstructure (to facilitate connections).
The superstructure of timber trestle bridges generally consists of two bridge "chords," "crossties" and the steel train "rail." Each chord is comprised of multiple "plies" of timber stringers. These plies are placed side by side and are either "packed" together with no space between them or "spaced" with a clear space of 51 mm (2 in.) to 102 mm (4 in.) between them. The stringers are typically 178 mm (7 in.) to 254 mm (10 in.) wide and approximately 355 mm (14 in.) to 508 mm (20 in.) deep with lengths dictated by the available size of timber. Stringers act as the primary supporting members of the superstructure as they span the distances from bent to bent. Stringers can have a length in excess of 6.1 m (20 ft), which allows bridge spans on the order of 3.6 m (12 ft) to 4.6 m (15 ft) long, with 4.6 m (15 ft) being most common. Figure 1.2 depicts the general configuration of the stringers and their staggered pattern of placement.
The stringers in each chord are arranged using staggered single and two span members, the members are arranged so that one stringer is continuous over a bent while the neighboring two stringers are butted end to end over the bent. This pattern repeats for the total number of stringers used in the superstructure. The number of stringers in a bridge is divided so half are in one chord and half in the other; the stringers in each chord are centered under the train rail.
Transverse wood crossties are attached above the stringers and typically have a 203 mm (8 in.) square cross-section. The crossties act to distribute the loads applied to the rail down to the stringers. The AREMA manual specifies that the spacing between the crossties cannot exceed 203 mm (8 in.) [AREMA, 1995]. This spacing of crossties is the primary reason that a bridge is classified as an "open deck" bridge. If there is no space between the crossties the bridge is classified as a "closed deck" bridge. The rail rests upon rail platens that transfer the trainloads onto the crossties, then down to the stringers and finally to the foundation and ground below.
Each length of rail is spiked to the cross ties and connected to each other by a series of bolts and connection plates. The crossties are connected to the stringers by a combination of long bolts and spikes. Generally every third crosstie is bolted to the outer most stringer of each chord, and the intermediate two crossties are spiked to a timber "tie rail" that lies above the crossties above each chord. The intermediate crossties are not physically connected to the underlying stringers. The stringers are bolted to each other by horizontal bolts near the ends of and at mid-span of each span. The stringers are bolted horizontally with spacers (approximately 51 mm (2 in.) wide) between them if the stringers are not "packed." The outer most stringer is then bolted down through the pile cap, which connects the superstructure to the substructure. This connection configuration results in the majority of stringers lacking vertical connection devices to both the crossties and the pile caps. They are held in place by the horizontal stringer bolts and by friction from bearing between the crossties and pile caps. This configuration allows for irregular gaps to exist between the stringers, crossties and pile caps.
1.4 Background to the Research
The AAR has a conducted cooperative research with several universities to investigate the condition of timber bridges in their inventory. Colorado State University (CSU) has been involved in this research since 1995. CSU's involvement began with the field-testing of three timber trestle bridges in Colorado, [Uppal et al. 2002] under static and moving trainloads, as well as ramp loads. These tests investigated the load paths of each bridge. A subsequent laboratory study of a full-scale timber trestle railroad bridge chord was conducted at CSU to further investigate the load paths through the chord [Doyle et al., 2000]. Complications occurred due to the uplift of the ends of the chords of the multi-span specimens. Later, improved tests were conducted to eliminate the uplift.
This report presents the results of the most recent phase of laboratory research. The effects of support movements observed in the field test were examined by physical laboratory testing and computer-based structural modeling using AxisVM software [Inter-CAD. Kft, 2004]. The goal of this research was to begin developing analytical tools to help predict the performance capabilities of in-situ bridges with support motions included.