摘要：
吕利高架桥的创新设计提出了一种轻型结构和透明的三角交叉预制圆管，这种结构完全从节点区无线条作出。其结果是双空间桁架，以42.75米的典型跨径。每个横向三角形截面为2.9米和4.0米宽，是由一个细长桥墩支持。最大的直径和管壁厚度超过500毫米和70毫米，相差很大。第一期间的连接设计的主要困难是确定复杂交叉沿管周长的应力分布，并计算支点应力。几何计算，精密切割和边缘管的准备是必要的设计，桥身充满穿透焊缝。移动模架，这也确保了在混凝土浇筑，也需要特殊考虑的管状桁架的稳定。
本文介绍了从设计演变的项目，桁架加工和焊接，到施工现场的步骤。 引言
在瑞士A1高速公路，1000米长的吕利高架桥，空间管桁架制造，是一个工程师设计比赛的结果。该项目选择了由它的独创性和审美素质陪审团。在过去，没有人敢来建造与焊接动态应力管节点的道路桥梁。
设计争论 附近的村庄位于在弗里堡州，分为公路高架桥从东到西运行瑞士格中。穿越山谷的湿农村单位土地和树木所包围，这座桥将完成于伊华东之间与穆尔登公路连接。弗里堡州公路处，有3个项目之间的选择由选定的有经验的咨询公司提交的。
该设计必须遵循下列条件：
--桥梁总长度：约1000米
--桥面宽度：从原来的13.25米至16.00米（每个交通方向）
--为了维护原因，会议决定建立两个独立的道路
--纵倾角：2.9和3.6%之间的一个凹角与40000米半径圆弧
--水平曲线：2日期间过渡的3'000米半径圆
--桥高：4至15米
下列项目已提交给审核团的5名成员：
--一预应力的44.60米和一个2.50米，平均高度的连续混凝土箱形梁 --一预应力42.50米的大跨度混凝土箱形梁，平均这梁深度变化2至
2.45米
--之一，对42.75米和一个身高3.75米，平均跨度不断复合空间桁架 第三个项目是建议由它的“轻”，并允许它融入农村氛围。该建议是由负责当局批准的。该项目提交的咨询工程师小组迪爱生- DMA的（多纳Ingénieurs
Conseils SA和Devaud，蒙加蒂等Associées SA），接受了这一创新的设计挑战
概念设计：
周围的树木形状启发了笔者的概念设计[1]。此类样式，可与钢结构的元素小的地方可以很容易地改变和适应生活的承载能力[2]。
三种不同截面的考虑：
第一种选择是理想的设计。它没有超薄圆柱墩支撑，但不注重两个分开的道路维修情况。第三个解决方案已经设计，避免侵占周围的树木。这包括连接在桥墩上的两纵截面桁梁垂直。其结果是一个三维管状桁架支撑结构。
工程项目的说明
几何结构：
桁架的尺寸是根据等边三角形。相对于传统的箱形梁，桁架深度为50％以上。这个轻量级的上层建筑长细比约是13：20，而不是常规意义上的梁的长细比（图3）。
几何桁架首先考虑跨长以及可移动的最大因素。然后，管径作了初步的计算和考虑了下列因素：
对角线范围内的其它钢管的大小。初步分析，选择了267毫米和壁厚11至50毫米直径的对角线。再加上在节点的几何条件下，较低的最小直径为508毫米。 这有利于使用尽可能小的管道，以改善两者对角线较厚的墙壁的传力。低下弦和25至50毫米的各种管的厚度。在支持区域的厚度增加至50至70毫米。薄壁管的厚度有559毫米，以长2米的直径为中心。较难的加劲因此可避免。
这些节点上的弦角比较复杂（金形接头）。该管大小的选择取决于更多的超过实际的力量，这是最后阶段板坯主要的考虑因素。
1、节点交点在未考虑到具体的阻力时达到平衡。由于重叠的对角线，因此选择以对角线之间传输的垂直结构连接。
2、该管的直径必须足够大，以提供足够的剪力用于连接件的焊接，使其达到最小的空间和最小混凝土保护层。 遇到的困难之一是在空间管状结构制作中的管相贯焊接及周边焊根渗透的检查。即使结构不是太疲劳，但担心结构失衡或严重焊接质量差的紧张是可以理解的。基于这个原因，焊接支持的审核获得通过。这增加了内部焊趾的宽度。
支撑桁架是由较小的管建成，因为他们受力较低。其直径弦之间变化在219.1和323.9毫米之间，所有对角线的管状结构是168.3毫米直径管的。
桥面宽度在不等标高的12.0和14.65米之间。为了限制比较宽（4.0至5.33米）的悬臂的长期挠度，并尽量减少桥面板的重量，结构（600平方毫
米）使用了横向筋。甲板纵向预应力筋与其同一类型。纵向预应力筋已被选定，以保证在每个桥面板下的恒载（混凝土路面和部分压缩）。
设计：
这座桥的设计是根据瑞士标准新160。
拥有两种不同的结构模型，可用于空间桁架计算：
1、与结构安全设计的不断和弦棚对角线
2、疲劳和可维护性刚性节点设计
桥面已成为一个平面网格模型（空腹转换），两个纵向弦，并在与空间桁架
采用等效位移理论，以确定纵向弦刚度。甲板刚度，与上弦和圣维南扭矩阻力复合效应被认为是垂直成员属性。
接头：
对各部分和关键的结构安全进行了验证与铰链模型（讨论的内力[3]）。没有标准的建议可以检查、计算局部的抗疲劳性，或进行了实证方法并强调在圆形空心管交集。
下列标准获得通过后，委托方和专家工程师讨论
1、根据SIA标准的疲劳载荷160
2、内力计算距
--充分和破获组合截面距有n（= Esteel / Econcrete = 10）
--刚性节点
、
关键字： 美学，桥梁，组合结构桥梁，空间桁架，圆形空心管，疲劳应力，应
力分布，应力集中系数，热点应力，焊接
Abstract
The innovative design of the Lully viaduct proposes a light and transparent structure made of a triangularcross-section fabricated entirely from unstiffened circular tubes. The result is twin space trusses, with a typicalspan of 42.75 m. Each transversal triangular cross-section is 2.9 m high and 4.0 m wide, and is supported by
asingle slender pier. The largest diameters and thickness of the tubes are over 500 mm and nearly 70 mmrespectively. One major difficulty during the design of the connections was to define the stress distribution along the complex intersecting
perimeters of the tubes, and to calculate the hot spot stresses. Geometry calculations, precision cutting and edge preparation of the tubes were necessary for performing full penetration welds. The mobile formwork, which also ensures the stability of the tubular trusses during concrete pouring, also required special consideration.
This paper describes the evolution of the project, from design, truss
fabrication and welding, to construction on site.
INTRODUCTION
Located on the Swiss highway A1, the 1000 m long Lully viaduct, made of space tubular trusses, is the result of an engineer design contest. This project was chosen by the jury for its originality and aesthetic quality. Inthe past, no one dared to build a road bridge with welded tubular nodes due to the dynamic stress.
DESIGN CONTEST
Located near the village of Lully in the Canton of Fribourg, the viaduct is
incorporated into highway A1 running from the East to West of Switzerland. Crossing a rural flat valley surrounded by wet land and trees,this bridge will complete a highway link between Murten and Yverdon. The owner, the Fribourg Cantonal
Highway Office, had to choose between 3 projects submitted by selected experienced consulting firms.
The partcipants had to respect the following conditions:
Total bridge length : approximately 1'000 m
Width of the bridge deck : from 13.25 m to 16.00 m in each traffic direction For maintenance reasons, it was decided to build two separate roadways
Longitudinal inclination : between 2.9 and 3.6 % in a concave circular arc with a radius of 40'000 m
Horizontal curve : circle of 3'000 m between 2 transition radius
Height over the valley : between 4 and 15 m.
The following projects were submitted to the 5 members of the jury :
One prestressed concrete box girder with an average span of 44.60 m and a constant depth of 2.50 m.
One prestressed concrete box girder with an average span of 42.50 m. This girder depth varied between 2
and 2.45 m.
One composite space truss with an average span of 42.75 m and a constant height of 3.75 m.
The third project was recommended by the jury for its “lightness” and
transparency allowing it to integrate into the countryside (Fig 1). The
recommendation was approved by the responsible authority who accepted the challenge of innovative design. This project was presented by the consulting
engineers group DIC-DMA (Dauner Ingénieurs Conseils SA and Devaud, Mongatti et Associées SA).
CONCEPTUAL DESIGN
The shape of the surrounding trees inspired the author for the conceptual
design [1]. A certain analogy can be made with the steel construction where the elements size can be easily changed and adapted to live load capacity [2].
Three different cross sections were considered :
The first alternative was the ideal design (Fig.2). It has slim cylindrical piers without bracing, but did not respect the maintenance condition of two separated roadways. The third solution has been design to avoid having piers dominating the surrounding trees. This included perpendicular truss connecting the two longitudinal girders at the cross section on the piers. The result was a three-dimensional tubular truss supporting structure.
PROJECT DESCRIPTION
Geometry
The dimensions of the trusses were based on equilateral triangles.
Compared to a traditional box girder, the truss depth is 50% higher. The slenderness (L/H) of this lightweight superstructure is approximately 13 instead of 20 as for regular beam girder (Fig 3).
The truss geometry was determined first by considering the span length as well as the maximum transportable element. Then the tube diameters were given by preliminary calculations and the following considerations:
The diagonals governs the size of other members. Preliminary analysis
leads to a diagonal diameter of 267mm and wall-thickness between 11 and 50 mm. Adding to this the geometric conditions at the nodes, thesmallest diameter for the lower chord was 508 mm.
It was beneficial to use the smallest possible tube in order to improve the
force transfer between diagonals due to the thicker walls . The thickness of the lower
chord tubes varied between 25 and 50 mm (Fig 4). In the support zone the thickness is increased from 50 to 70 mm. The thicker walled tube has a diameter of 559 mm and a length of 2 m centered on the bearing. Unsightly stiffeners could therefore be avoided.
The upper chords nodes are less complicated (K-shaped joint) (Fig 5). The choice of the tube size depended more on the considerations below than on the actual forces, which were carried mostly by the slab in the final stage.
1. Nodal forces had to be equilibrated without taking the concrete resistance into account. Overlapping of the diagonals was therefore chosen in order to transfer some of the vertical force directly between the diagonals.
2. The tube diameter had to be large enough to provide adequate space for the welded shear connectors and to allow minimum concrete cover.
One of the difficulties encountered in the fabrication of the space tubular
structure was the welding at the intersecting perimeters of the tubes and checking the penetration at the weld root. Even if the fatigue requirements are not too severe in road bridge construction, the fear of uncontrolled or bad quality weld roots in the tension tubes is understandable. For this reason, welding on backing shells was adopted. This increased the width of the inner weld toe.
The brace truss at the piers were built with smaller tubes (Fig 6 and 7), as
they are subject to lower forces. The diameter of their chords varied between 219.1 and 323.9 mm, all diagonals are made of 168.3 mmdiameter tube.
The deck width varied between 12.0 and 14.65 m. In order to limit the
long-term deflection of the relatively wide cantilever wings (4.0 to 5.33 m) and to minimize the weight of the deck, transversal tendons (600 mm2) were used. The concrete deck was prestressed longitudinally with the same type of tendons.
Longitudinally the prestressing force has been chosen to insure compression in every section under dead load (concrete and road surface).
Design
The bridge was designed according to the Swiss standard SIA 160.
Two different structural models were used for the calculation of the space
truss:
1. Hinged diagonals with continuous chords for structural safety design
2. Rigid nodes for fatigue and serviceability design
The bridge deck had to be converted into a plane grid model (Vierendeel), made of two longitudinal chord and vertical members placed at the intersection with
the diagonals of the space truss (Fig 8).
Equivalent displacement theory was used in order to determine the stiffness of the longitudinal chord. Deck stiffness, composite effect with upper chord and Saint-Venant torque resistance were considered in the vertical member properties.
Joints
The structural safety of the members and joints was verified with the
internal forces of the hinged model (discussed in [3]).
No standard recommendations could be found to check fatigue resistance, or an empirical method for computing local stresses at the circular hollow tube intersection, nor a fatigue category considering the backing shell.
The following criteria were adopted after discussion with the owner and the expert engineers
1. Fatigue load according to SIA 160
2. Calculations of internal forces withfull and cracked composite section (with n = Esteel/Econcrete = 10)rigid nodes
Key words
Aesthetic, bridge, composite bridge, space truss, circular hollow tubes, fatigue, stress distribution, stress concentration factor, hot spot stress, weld
摘要：
吕利高架桥的创新设计提出了一种轻型结构和透明的三角交叉预制圆管，这种结构完全从节点区无线条作出。其结果是双空间桁架，以42.75米的典型跨径。每个横向三角形截面为2.9米和4.0米宽，是由一个细长桥墩支持。最大的直径和管壁厚度超过500毫米和70毫米，相差很大。第一期间的连接设计的主要困难是确定复杂交叉沿管周长的应力分布，并计算支点应力。几何计算，精密切割和边缘管的准备是必要的设计，桥身充满穿透焊缝。移动模架，这也确保了在混凝土浇筑，也需要特殊考虑的管状桁架的稳定。
本文介绍了从设计演变的项目，桁架加工和焊接，到施工现场的步骤。 引言
在瑞士A1高速公路，1000米长的吕利高架桥，空间管桁架制造，是一个工程师设计比赛的结果。该项目选择了由它的独创性和审美素质陪审团。在过去，没有人敢来建造与焊接动态应力管节点的道路桥梁。
设计争论 附近的村庄位于在弗里堡州，分为公路高架桥从东到西运行瑞士格中。穿越山谷的湿农村单位土地和树木所包围，这座桥将完成于伊华东之间与穆尔登公路连接。弗里堡州公路处，有3个项目之间的选择由选定的有经验的咨询公司提交的。
该设计必须遵循下列条件：
--桥梁总长度：约1000米
--桥面宽度：从原来的13.25米至16.00米（每个交通方向）
--为了维护原因，会议决定建立两个独立的道路
--纵倾角：2.9和3.6%之间的一个凹角与40000米半径圆弧
--水平曲线：2日期间过渡的3'000米半径圆
--桥高：4至15米
下列项目已提交给审核团的5名成员：
--一预应力的44.60米和一个2.50米，平均高度的连续混凝土箱形梁 --一预应力42.50米的大跨度混凝土箱形梁，平均这梁深度变化2至
2.45米
--之一，对42.75米和一个身高3.75米，平均跨度不断复合空间桁架 第三个项目是建议由它的“轻”，并允许它融入农村氛围。该建议是由负责当局批准的。该项目提交的咨询工程师小组迪爱生- DMA的（多纳Ingénieurs
Conseils SA和Devaud，蒙加蒂等Associées SA），接受了这一创新的设计挑战
概念设计：
周围的树木形状启发了笔者的概念设计[1]。此类样式，可与钢结构的元素小的地方可以很容易地改变和适应生活的承载能力[2]。
三种不同截面的考虑：
第一种选择是理想的设计。它没有超薄圆柱墩支撑，但不注重两个分开的道路维修情况。第三个解决方案已经设计，避免侵占周围的树木。这包括连接在桥墩上的两纵截面桁梁垂直。其结果是一个三维管状桁架支撑结构。
工程项目的说明
几何结构：
桁架的尺寸是根据等边三角形。相对于传统的箱形梁，桁架深度为50％以上。这个轻量级的上层建筑长细比约是13：20，而不是常规意义上的梁的长细比（图3）。
几何桁架首先考虑跨长以及可移动的最大因素。然后，管径作了初步的计算和考虑了下列因素：
对角线范围内的其它钢管的大小。初步分析，选择了267毫米和壁厚11至50毫米直径的对角线。再加上在节点的几何条件下，较低的最小直径为508毫米。 这有利于使用尽可能小的管道，以改善两者对角线较厚的墙壁的传力。低下弦和25至50毫米的各种管的厚度。在支持区域的厚度增加至50至70毫米。薄壁管的厚度有559毫米，以长2米的直径为中心。较难的加劲因此可避免。
这些节点上的弦角比较复杂（金形接头）。该管大小的选择取决于更多的超过实际的力量，这是最后阶段板坯主要的考虑因素。
1、节点交点在未考虑到具体的阻力时达到平衡。由于重叠的对角线，因此选择以对角线之间传输的垂直结构连接。
2、该管的直径必须足够大，以提供足够的剪力用于连接件的焊接，使其达到最小的空间和最小混凝土保护层。 遇到的困难之一是在空间管状结构制作中的管相贯焊接及周边焊根渗透的检查。即使结构不是太疲劳，但担心结构失衡或严重焊接质量差的紧张是可以理解的。基于这个原因，焊接支持的审核获得通过。这增加了内部焊趾的宽度。
支撑桁架是由较小的管建成，因为他们受力较低。其直径弦之间变化在219.1和323.9毫米之间，所有对角线的管状结构是168.3毫米直径管的。
桥面宽度在不等标高的12.0和14.65米之间。为了限制比较宽（4.0至5.33米）的悬臂的长期挠度，并尽量减少桥面板的重量，结构（600平方毫
米）使用了横向筋。甲板纵向预应力筋与其同一类型。纵向预应力筋已被选定，以保证在每个桥面板下的恒载（混凝土路面和部分压缩）。
设计：
这座桥的设计是根据瑞士标准新160。
拥有两种不同的结构模型，可用于空间桁架计算：
1、与结构安全设计的不断和弦棚对角线
2、疲劳和可维护性刚性节点设计
桥面已成为一个平面网格模型（空腹转换），两个纵向弦，并在与空间桁架
采用等效位移理论，以确定纵向弦刚度。甲板刚度，与上弦和圣维南扭矩阻力复合效应被认为是垂直成员属性。
接头：
对各部分和关键的结构安全进行了验证与铰链模型（讨论的内力[3]）。没有标准的建议可以检查、计算局部的抗疲劳性，或进行了实证方法并强调在圆形空心管交集。
下列标准获得通过后，委托方和专家工程师讨论
1、根据SIA标准的疲劳载荷160
2、内力计算距
--充分和破获组合截面距有n（= Esteel / Econcrete = 10）
--刚性节点
、
关键字： 美学，桥梁，组合结构桥梁，空间桁架，圆形空心管，疲劳应力，应
力分布，应力集中系数，热点应力，焊接
Abstract
The innovative design of the Lully viaduct proposes a light and transparent structure made of a triangularcross-section fabricated entirely from unstiffened circular tubes. The result is twin space trusses, with a typicalspan of 42.75 m. Each transversal triangular cross-section is 2.9 m high and 4.0 m wide, and is supported by
asingle slender pier. The largest diameters and thickness of the tubes are over 500 mm and nearly 70 mmrespectively. One major difficulty during the design of the connections was to define the stress distribution along the complex intersecting
perimeters of the tubes, and to calculate the hot spot stresses. Geometry calculations, precision cutting and edge preparation of the tubes were necessary for performing full penetration welds. The mobile formwork, which also ensures the stability of the tubular trusses during concrete pouring, also required special consideration.
This paper describes the evolution of the project, from design, truss
fabrication and welding, to construction on site.
INTRODUCTION
Located on the Swiss highway A1, the 1000 m long Lully viaduct, made of space tubular trusses, is the result of an engineer design contest. This project was chosen by the jury for its originality and aesthetic quality. Inthe past, no one dared to build a road bridge with welded tubular nodes due to the dynamic stress.
DESIGN CONTEST
Located near the village of Lully in the Canton of Fribourg, the viaduct is
incorporated into highway A1 running from the East to West of Switzerland. Crossing a rural flat valley surrounded by wet land and trees,this bridge will complete a highway link between Murten and Yverdon. The owner, the Fribourg Cantonal
Highway Office, had to choose between 3 projects submitted by selected experienced consulting firms.
The partcipants had to respect the following conditions:
Total bridge length : approximately 1'000 m
Width of the bridge deck : from 13.25 m to 16.00 m in each traffic direction For maintenance reasons, it was decided to build two separate roadways
Longitudinal inclination : between 2.9 and 3.6 % in a concave circular arc with a radius of 40'000 m
Horizontal curve : circle of 3'000 m between 2 transition radius
Height over the valley : between 4 and 15 m.
The following projects were submitted to the 5 members of the jury :
One prestressed concrete box girder with an average span of 44.60 m and a constant depth of 2.50 m.
One prestressed concrete box girder with an average span of 42.50 m. This girder depth varied between 2
and 2.45 m.
One composite space truss with an average span of 42.75 m and a constant height of 3.75 m.
The third project was recommended by the jury for its “lightness” and
transparency allowing it to integrate into the countryside (Fig 1). The
recommendation was approved by the responsible authority who accepted the challenge of innovative design. This project was presented by the consulting
engineers group DIC-DMA (Dauner Ingénieurs Conseils SA and Devaud, Mongatti et Associées SA).
CONCEPTUAL DESIGN
The shape of the surrounding trees inspired the author for the conceptual
design [1]. A certain analogy can be made with the steel construction where the elements size can be easily changed and adapted to live load capacity [2].
Three different cross sections were considered :
The first alternative was the ideal design (Fig.2). It has slim cylindrical piers without bracing, but did not respect the maintenance condition of two separated roadways. The third solution has been design to avoid having piers dominating the surrounding trees. This included perpendicular truss connecting the two longitudinal girders at the cross section on the piers. The result was a three-dimensional tubular truss supporting structure.
PROJECT DESCRIPTION
Geometry
The dimensions of the trusses were based on equilateral triangles.
Compared to a traditional box girder, the truss depth is 50% higher. The slenderness (L/H) of this lightweight superstructure is approximately 13 instead of 20 as for regular beam girder (Fig 3).
The truss geometry was determined first by considering the span length as well as the maximum transportable element. Then the tube diameters were given by preliminary calculations and the following considerations:
The diagonals governs the size of other members. Preliminary analysis
leads to a diagonal diameter of 267mm and wall-thickness between 11 and 50 mm. Adding to this the geometric conditions at the nodes, thesmallest diameter for the lower chord was 508 mm.
It was beneficial to use the smallest possible tube in order to improve the
force transfer between diagonals due to the thicker walls . The thickness of the lower
chord tubes varied between 25 and 50 mm (Fig 4). In the support zone the thickness is increased from 50 to 70 mm. The thicker walled tube has a diameter of 559 mm and a length of 2 m centered on the bearing. Unsightly stiffeners could therefore be avoided.
The upper chords nodes are less complicated (K-shaped joint) (Fig 5). The choice of the tube size depended more on the considerations below than on the actual forces, which were carried mostly by the slab in the final stage.
1. Nodal forces had to be equilibrated without taking the concrete resistance into account. Overlapping of the diagonals was therefore chosen in order to transfer some of the vertical force directly between the diagonals.
2. The tube diameter had to be large enough to provide adequate space for the welded shear connectors and to allow minimum concrete cover.
One of the difficulties encountered in the fabrication of the space tubular
structure was the welding at the intersecting perimeters of the tubes and checking the penetration at the weld root. Even if the fatigue requirements are not too severe in road bridge construction, the fear of uncontrolled or bad quality weld roots in the tension tubes is understandable. For this reason, welding on backing shells was adopted. This increased the width of the inner weld toe.
The brace truss at the piers were built with smaller tubes (Fig 6 and 7), as
they are subject to lower forces. The diameter of their chords varied between 219.1 and 323.9 mm, all diagonals are made of 168.3 mmdiameter tube.
The deck width varied between 12.0 and 14.65 m. In order to limit the
long-term deflection of the relatively wide cantilever wings (4.0 to 5.33 m) and to minimize the weight of the deck, transversal tendons (600 mm2) were used. The concrete deck was prestressed longitudinally with the same type of tendons.
Longitudinally the prestressing force has been chosen to insure compression in every section under dead load (concrete and road surface).
Design
The bridge was designed according to the Swiss standard SIA 160.
Two different structural models were used for the calculation of the space
truss:
1. Hinged diagonals with continuous chords for structural safety design
2. Rigid nodes for fatigue and serviceability design
The bridge deck had to be converted into a plane grid model (Vierendeel), made of two longitudinal chord and vertical members placed at the intersection with
the diagonals of the space truss (Fig 8).
Equivalent displacement theory was used in order to determine the stiffness of the longitudinal chord. Deck stiffness, composite effect with upper chord and Saint-Venant torque resistance were considered in the vertical member properties.
Joints
The structural safety of the members and joints was verified with the
internal forces of the hinged model (discussed in [3]).
No standard recommendations could be found to check fatigue resistance, or an empirical method for computing local stresses at the circular hollow tube intersection, nor a fatigue category considering the backing shell.
The following criteria were adopted after discussion with the owner and the expert engineers
1. Fatigue load according to SIA 160
2. Calculations of internal forces withfull and cracked composite section (with n = Esteel/Econcrete = 10)rigid nodes
Key words
Aesthetic, bridge, composite bridge, space truss, circular hollow tubes, fatigue, stress distribution, stress concentration factor, hot spot stress, weld