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Creating Efficient Structural Grids in Mass Timber Buildings

Although a mass timber solution may work economically on grids created for other materials, a few modifications can increase efficiencies related to member sizing and manufacturer capabilities.

Mass timber products such as cross-laminated timber (CLT), nail-laminated timber (NLT) and glue-laminated timber (glulam) are at the core of a revolution that is shifting how designers think about construction. At no time has materials selection been such an integral aspect of the building designer’s daily responsibilities. In addition to its sustainability and light carbon footprint, mass timber has benefits that include enhanced aesthetics, 施工速度快,重量轻, 所有这些都会对成本产生积极影响. 然而, to convince building owners and developers that a mass timber solution is viable, the structural design must also be cost competitive. This requires a full understanding of both material properties and manufacturer capabilities.

Mass timber is commonly seen in projects such as offices, 学校和高层混合用途建筑, which often have assumed structural grids. Intended to meet the need for tenant flexibility, these “default” grids align with the capabilities of materials historically used—i.e.钢筋混凝土. When it comes to laying out a structural grid for mass timber, the square peg/round hole analogy is pertinent. Although a mass timber solution may work economically on many grids conducive to steel/concrete framing, 一些网格修改可能是有价值的. Trying to force a mass timber solution on a grid laid out for steel and concrete can result in member size inefficiencies while negating opportunities related to manufacturer capabilities. As such, it is critically important to design a mass timber building 作为一个大型木结构建筑 从一开始. This requires a thorough understanding of how to best lay out the structural grid, 不牺牲空间功能, to optimize member sizes—but there’s more to cost efficiency than column spacing.

The following considerations are based on a post-and-beam frame for occupancies such as offices; however, many also apply to bearing wall-supported systems in other occupancy types.

网格选择

Simplistically, there are two main grid options for mass timber buildings: square and rectangular. In deciding which to use, there are a number of factors to consider.

确定有效的网格间距, it is important to understand possible span ranges for mass timber floor panels. 因为它们的重量相对较轻, allowable spans for these panels are often governed by vibration and deflection rather than bending or shear capacity. 另外还有面板的振动设计, vibration performance of the framing system as a whole, 包括梁, 应该考虑吗. The table below illustrates example ranges based on panel size, assuming stiff supports. (每个项目的具体跨度, 加载和支撑条件, as well as manufacturer-specific design properties, should be accounted for when selecting panel thickness.) For more details on the structural design of mass timber floor panels, contact your local WoodWorks区域总监 或发邮件给WoodWorks帮助台 help@amnahclinic.com. 有关其他信息,请参见 结构 杂志的文章, Cross-Laminated Timber Structural Floor and Roof Design钉层压木材:美国.S. 设计施工指南.

Chart demonstrating the relationship between panels and floor span ranges
Albania Yard is built on a 20x20-ft grid with one intermediate beam in each bay and 3-ply CLT panels spanning 10 ft between Glulam beams

Based on completed buildings in the US, square grids tend to be in the range of 20×20 to 30×30 ft. Although a mass timber panel may be able to span the 20-ft distance between support beams in a 20×20-ft grid, an alternate method would be to include one intermediate beam within each bay to reduce the span of the mass timber floor panel. 例如, a 20×20-ft grid could have one intermediate beam so 3-ply CLT floor panels spanning 10 ft can be used. 此场景用于 阿宾娜院子里 office building in Portland, OR (pictured above). Larger square grids such as 28×28 or 30×30 ft with one intermediate beam can also be used. This typically results in the use of 5-ply CLT or 2×6 NLT floor panels, spanning 14 or 15 ft. 这个场景用于 粘土创意也是在波特兰(见下图). In general, thinner floor and roof panels may result in lower material costs. 然而, lower horizontal panel costs may be offset by higher beam (and perhaps column) costs, additional intermediate beams also need to be coordinated with MEP systems. As such, a cost analysis for thicker floors and fewer beams vs. thinner floors and more beams may be necessary.

粘土创意, designers chose a 30x30-ft grid with one intermediate beam in each bay and 2x6 NLT panels spanning 15 ft between Glulam beams

Going much beyond a 30 or 32-ft span with glulam girders starts to require fairly large (deep) beams. It can be done, but economics and headroom issues may outweigh the benefits of longer spans. The image below illustrates several square grid options and associated member sizes.

table showing square grid options and associated member sizes

Rectangular grids are usually in the 12×20-ft to 20×32-ft range. The main difference with a rectangular grid is that intermediate beams tend not to be used, often simplifying the approach to accommodating MEP. The narrower grid dimension is typically based on the span capability of the floor panel (see table of span ranges above). The larger grid dimension is based primarily on programmatic layout, while taking into account economical spans for glulam. Projects that have used this scenario include the 第一科技联邦信用合作社 in Hillsboro, OR, which used a 12×32-ft grid with 5-1/2-in. CLT面板横跨12英尺,和 东大会堂111号 in Des Moines, IA, which used a 20×25-ft grid with 2×8-ft DLT panels spanning 20 ft. 两个项目如下图所示.

The 第一科技联邦信用合作社 includes a 12x32-ft grid with 5-1/2-in. CLT panels spanning 12 ft between Glulam beams

There are several reasons to eliminate the intermediate beam, but the one often cited by design teams is easier MEP coordination. Since exposing the mass timber floor panels on the ceiling side is desired in most mass timber buildings, 在管道系统方面有一些创造性, sprinkler lines and other MEP services are accommodated is required. For more information on this topic, read this 向专家请教 Q&A. 如果没有中间光束, the main MEP trunk lines can be run around a central corridor with branch lines extending into each bay. A benefit to this approach is that no intermediate beams means no or minimal penetrations through glulam purlins or girders to coordinate, 减少, 设计, 等.

制造商的输入

When selecting grid dimensions, another important consideration is manufacturer capabilities. Most North American CLT manufacturers certified to the PRG-320 Standard for Performance-Rated Cross-Laminated Timber are capable of producing panels between 8 and 12 ft wide and between 40 and 60 ft long. Minimizing the amount of waste from each panel is key to maximizing efficiency. 例如, a grid with 20-ft increments could be very efficient; it could use 40-ft-long panels or 60-ft-long panels (if the manufacturer is capable of producing those sizes). 另一方面, a 24-ft grid may not be as efficient since it would either require 48-ft-long panels (for double spans) or 减少ting 16 ft from 40-ft-long panels. Both options increase waste and reduce efficiency. When considering especially long panels, trucking logistics should also be taken into account.

阿肯色大学 Adohi大厅 student residence hall project in Fayetteville, AR utilized a 20-ft grid increment for a 60-ft-wide building. The CLT manufacturer provided 40-ft-long panels, resulting in the use of one full length and one half-length panel to achieve the full 60-ft building width. Because each half-length panel was simply a full-length panel 减少 in half, efficiency was high and waste was minimized.

虽然制造商的能力不同, it is possible to create grids that are efficient for several manufacturers. An important step in mass timber building design is to consult with manufacturers to determine the most efficient panel layouts for their capabilities. Most North American mass timber manufacturers are WoodWorks 国家 Partners. Information on each can be found by clicking the logos on 这个网页.