Os membros estruturais de aço são conectados uns aos outros por meio de elementos de enchimento ou conectores que podem ser seções angulares, plates or even parallel flange channels (PFC). Essas são algumas das configurações comuns adotadas nos canteiros de obras, especialmente quando parafusos ou conexões aparafusadas são usados principalmente. For welded connections, os membros às vezes são conectados diretamente uns aos outros sem quaisquer elementos de preenchimento ou conectores intermediários. In this article we focus on bolts, cabeça hexagonal e escareada, e algumas das conexões aparafusadas comuns usadas.
Parafusos hexagonais e escareados
The hexagonal head bolt on the left is the normal structural fastener. The countersunk bolt on the right has a tapered head that sits flush with the surface of the ply, which matters where a protruding head would foul a bearing surface or a moving part. That taper is also what reduces its bearing capacity, as covered below.
Instituto Americano de Construção em Aço (AISC)
AISC usa principalmente unidades inglesas (polegada, libras) although the SI counterpart is also supported and available. One thing to take note of is that the dimensions in the English system are usually in increments of one eighth of an inch (⅛”), que podem não corresponder exatamente aos do SI.
Comparação do Diâmetro do Parafuso
| ½” | – |
| ⅝” | M16 |
| ¾” | M20 |
| ⅞” | M22 |
| 1” | M24 |
| 1 ⅛” | M27 |
| 1 ¼” | M30 |
| 1 ⅜” | – |
| 1 ½” | M36 |
These are nearest practical equivalents, not interchangeable sizes. A ¾” bolt is 19.05 mm against 20 mm for an M20, so the hole sizes and edge distances differ and the two cannot be substituted for one another on a drawing.
Parafusos de alta resistência
Specifications for bolt materials are covered by the American Society for Testing and Materials (ASTM), and AISC sorts them into two groups by strength:
Grupo A - ASTM F3125 Grades A325, A325M and F1852; ASTM A354 Grade BC; ASTM A449
Grupo B — ASTM F3125 Grades A490, A490M and F2280; ASTM A354 Grade BD
ASTM F3125 is a consolidated specification that absorbed the older standalone A325, A490, F1852 and F2280 documents, which is why those now appear as grades within it rather than as specifications in their own right. Older drawings and calculations still call them out the old way.
Besides differences in strength and other properties, these groups have different requirements for minimum pretension depending on bolt diameter.
Eurocódigo 3 (Anexo Nacional do Reino Unido)
The Eurocode is entirely in SI units, so bolts are designated in millimetres with the size prefixed by “M”, for example M16 meaning a 16 mm diameter bolt.
| M12 |
| M16 |
| M20 |
| M24 |
| M30 |
Bolt Classes and Strengths
Eurocode bolts are classified by a two-part designation that encodes both strengths. For a class written as a.b, the ultimate tensile strength is uma × 100 MPa, and the yield strength is b/10 of that value. So Class 4.6 has an ultimate tensile strength of 400 MPa and a yield of 240 MPa, while Class 8.8 has 800 MPa ultimate and 640 Rendimento de MPa.
Getting these two the right way round matters, because the bolt resistance formulas take the ultimate value. In calculating actual bolt resistances, the capacity is also affected by the preload, called pretension in AISC terms, which in certain cases increases the bolt carrying capacity if the right amount of tension is applied.
| Aula | Tração, fub (MPa) | Colheita, fyb (MPa) | Diameter range |
| 4.6 | 400 | 240 | M5 – M36 |
| 4.8 | 400 | 320 | M1.6 – M16 |
| 5.8 | 500 | 400 | M5 – M24 |
| 8.8 | 800 | 640 | M1.6 – M36 |
| 9.8 | 900 | 720 | M1.6 – M16 |
| 10.9 | 1000 | 900 | M5 – M36 |
| 12.9 | 1200 | 1080 | M1.6 – M36 |
Notas sobre parafusos hexagonais e escareados
The differences in geometry between hexagonal and countersunk bolts affect the bearing capacity, because the tapered head removes material from the ply it sits in. The effective thickness in bearing is the ply thickness less the depth of the countersink, which is what the figure below shows. (See AISC Manual 14th edn, Figura. 7-10.)
Figura 1. Espessura efetiva no rolamento de um parafuso escareado
Especificações de conexão e furos de parafuso
As conexões aparafusadas podem ser especificadas como bem apertadas, pré-tensionado ou antiderrapante. These conditions play a large role in the choice of bolt hole used. Hole types are standard (Força de rotação no membro, in EN terms), oversized, or slotted short or long, and they are adopted according to the connection specification. The RCSC specification sets out the requirements for the snug-tight condition. If the connection is designed for anything other than snug-tight, that condition must be clearly identified on the drawings, whether it is an AISC pretension or an EN preloading requirement.
The distinction is not paperwork. A snug-tight bolt resists load by bearing once it has slipped into contact, while a slip-critical connection is designed so that the clamping force stops the plies moving at all.
Slip-critical connections matter wherever slip has to be prevented, and different factors are applied in the calculation of slip resistance depending on the hole type, since an oversized or slotted hole gives the ply more room to move before it bears. Na maioria dos casos, standard or normal bolt holes are adopted together with a specified level of pretension or preload. These variables can be modelled and analysed using tools like Análise e projeto de estrutura de aço SkyCiv, which helps engineers account for connection behaviour in the wider structural system.
Figura 2. Ângulo parafusado, um tipo de conexão de cisalhamento
Conexões aparafusadas
Bolted connections can be used both in shear plate connectors such as web plates, camisetas, and web and seat angles, and in moment connections (rígido, fully restrained) que comumente usam enchimento, placas de continuidade e emenda. Using bolts means adding an intermediate connector or connecting element of the kind just mentioned.
Figura 3. Conexão da placa de flange aparafusada
In theory there are many ways to combine connection configurations. Na prática, buildability and fabrication cost mean certain connection types are favoured over others and are therefore more widely adopted. Here are some of the common connection types that use bolts, with brief descriptions.
Cisalhamento (Simples) Conexões
- Placa Simples/Dupla (tab or fin) uses a plate or plates to connect a beam web to the supporting member, a girder or a column. One side usually needs a weld at the edge of the plate, while bolts connect it in lap to the beam web.
- solteiro/Ângulo Duplo is similar to plates, except that angles can be bolted on both sides of the connection.
- Ângulo do assento, como o nome sugere, uses an angle section to bear the flange of the supported member (or the flat wall for a closed section) so the beam can sit on it. Bolts can be used throughout this connection type.
Momento (Restringido) Conexões
- Placas de flange, used alongside a shear connection, provide continuity to flanges framing into the columns of a moment-resisting frame, or more generally in member splices.
- Collar and Through Plates are found on HSS columns (RHS ou CHS) forming part of a moment-resisting frame. A through plate cuts through the column, while a collar plate is fitted around it, worn much like a collar on a shirt. Both are expensive to fabricate because of the cutting and fitting involved.
Visite nosso artigo sobre Tipos de conexões de aço aprender mais, compare fasteners in bolts vs welds vs rivets, or try our calculadora de resistência ao cisalhamento do parafuso. A bolts documentation covers how bolt grades, hole types and pretension are entered in the connection design module.
Try it now
Pick a bolted connection, set the bolt grade, diameter and hole type, and every bolt check comes back with its utilization ratio and the clause behind it. Ele roda no navegador, sem nada para instalar.
Referências
[1] Instituto Americano de Construção em Aço (AISC). (2016). correspondente às especificações para aço estrutural. Chicago, A 60601-6204
[2] Instituto Americano de Construção em Aço (AISC). (2011). Manual de construção em aço. 14ª Ed. Chicago, A 60601-6204
[3] O Steel Construction Institute e a British Constructional Steelwork Association. (2014). Juntas em Construção em Aço Juntas Simples ao Eurocódigo 3. Ascot SL5 7QN; Londres SW1A 2ES, Reino Unido
[4] O Steel Construction Institute e a British Constructional Steelwork Association. (2014). Juntas em Construção de Aço Juntas Resistentes ao Momento para Eurocode 3. Ascot SL5 7QN; Londres SW1A 2ES, Reino Unido
