Los Angeles Earthquake Retrofit
The Los Angeles City Website reports that from past earthquakes, multi-story buildings with weak and/or open front wall lines creating a “soft-story” (i.e. buildings with tuck-under-parking) performed poorly and collapsed.
The goal of the mandatory retrofit program, under
is to reduce structural deficiencies by the most economical and feasible method.
Without proper strengthening, these vulnerable buildings may be subjected to structural failure during and/or after an earthquake. Learn More ===> ladbs.org/soft-story-retrofit-program
What are the Different Retrofit Methods?
There are various methods to retrofit a wood-frame building. These methods include
2) cantilever columns and
3) moment frames.
Each of these techniques has their own specifications. The difference between shear-walls, cantilever columns, and moment frames are provided below. The figures show the overall geometry of each retrofit method. The following table shows the comparison between the different systems of retrofit.
Estimated retrofit cost without having engineering plans can vary a lot among different companies depending on the method of contract fulfillment, experience of the engineering and efficiency of the contractor.
The selection of retrofit method depends on various items including the geometry of the buildings, location of walls and direction of existing beams, number of stories on top of the parking area, the depth of the tuck under parking area, and material of the building elements (wood or steel beams/columns/posts). This is the reason that LA RETROFIT provides a free inspection to be able to investigate the most appropriate solution for soft-story properties.
Different Types of Soft-Story Retrofit Methods and Their Relative Cost
There are two general methods among Soft-Story retrofit companies
- The first method is separating the engineering design and construction into two individual contracts (similar to city’s break-down of the process).
- The second method is doing the engineering and construction under one single contract (design-build). The first method which engineering and construction are contracted separately are preferred among many as it gives property owner the freedom to get accurate construction bids after engineering plans being approved by the City. The second method that contracts engineering and construction in one contract appeals to certain group of property owners who don’t mind paying an extra cost that retrofitting companies add to the overall cost because of uncertainties involved in the non-existence of engineering plans at the time of their bid.
How Important is the Engineering Phase?
Engineering aspects of the retrofit work certainly plays a major role in the overall cost of the project. Often, it is the case that methods of retrofit for a project are exchangeable. It is the duty of an engineer to not only make sure of the safety of his design, but also to balance safety and economy of the project. An experienced engineer who is equipped with sophisticated engineering tools is able not only to model the existing capacity of the building and design only for the short-comings, but also to try different retrofit methods in a reasonable amount of time and choose the most economical one. aoausa.com *
Soft Story Buildings
Model – Prototype demonstration
The parking level is referred to as a soft story because the upper levels are supported on very narrow walls, or oftentimes on posts, to provide wide openings for cars to enter.
To demonstrate the structural shortcomings of a soft-story building in an earthquake, visualize a tabletop with wobbly legs supporting a lot of weight. The table will support the weight, but shaking it side to side will cause the legs to buckle and collapse. The effect on soft-story structures can be the same. The narrow walls were designed for supporting vertical loads, the weight of the upper floors, but will fail when lateral loads are applied.
Code-evaluated structural solutions and materials for soft-story retrofits are widely available. There is not a single solution for all buildings nor is there a single solution within a particular building. For some conditions,
conventional plywood shear walls may be appropriate; in others,
pre-fabricated structural-shear-wall systems or
manufactured moment frames will be necessary.
But for maintaining parking areas beneath multifamily, soft-story buildings, retrofit moment frames are frequently the best solution.
Moment frames consist of columns and a beam joined with a “moment connection.” Loads are resisted by these frames through bending in the beam and columns. In addition to resisting lateral and vertical loads, the moment connection transfers rotational movement at the top of the column where it connects to the beam. jlconline.com *